CA1123726A

CA1123726A - Explosive fracturing of deep rock

Info

Publication number: CA1123726A
Application number: CA000205542A
Authority: CA
Inventors: David L. Coursen
Original assignee: EI Du Pont de Nemours and Co
Current assignee: Explosives Technologies International Canada Ltd; ETI Explosive Technologies International Ltd
Priority date: 1973-07-26
Filing date: 1974-07-24
Publication date: 1982-05-18
Also published as: US3902422A; GB1482023A; GB1482024A; AU7158674A

Abstract

ABSTRACT OF THE DISCLOSURE
Producing a fracture network in deep rock, e.g., in an ore body, by detonating explosive charges sequentially in separate cavities therein, the detonations producing a cluster of overlapping fracture zones and each detonation occurring after liquid has entered the fracture zones pro-duced by previous adjacent detonations. High permeability is maintained in an explosively fractured segment of rock by flushing the fractured rock with liquid, i.e., by sweep-ing liquid through the fracture zones with high-pressure gas, between sequential detonations therein so as to entrain and remove fines therefrom. Ore bodies prepared by the blast/flush process with the blasting carried out in sub-stantially vertical, optionally chambered, drilled shot holes can be leached in situ via a number of holes previously used as injection holes in the flushing procedure and a number of holes which are preserved upper portions of the shot holes used in the detonation process. In the leaching of ore, fines are removed from fractures therein by inter-mittent or continuous flushing of the ore with lixiviant and high-pressure gas, e.g., air, using, in the case of the in situ leaching of an explosively fractured ore body, a lateral and upward flow of lixiviant from zones that have been less severely, to others that have been most severely, worked by multiple detonations in the ore body.

Description

3'7~;

The present inventlon relates to the production of a network of fractures in an underground rock by means of explosives, e.g., to prepare deep ore bodies for in situ leaching.
Processes for fracturing deep rock are becoming increasingly important as it becomes necessary to tap deep mineralized rock masses, e.g., ore bodies or oil or gas reservoirs located from about 100 feet to about a few thou-sand feet beneath the earth's surface, in order to supple-ment or replace dwindling energy sources and mineralssupplies. Numerous deposits of ore, for example ore con-taining copper, nickel, or silver, lie too deep to mine by open-cast methods or are too low in grade to mine by under-ground methods. Open-cast methods incur both the costs and the environmental impact associated with moving large quan-tities of earth and rock. Underground methods incur unus-ually high costs per unit volume of ore mined, as well as difficult safety problems. In contrast, the leaching of ore in place circumvents these difficulties and therefore can be a preferred technique for winning values from some ores that are unsuitable, or marginally unsuitable, for working by traditional mining methods.
Usually however, ore that is favorably situated for leaching in place has such a large fragment size and such low permeability to leaching solutions that the leach-ing rate would be too low to support a commercial leaching operation.
According to the present invention there is provided a process for producing a network of fractures in deep rock, as hereinafter defined, which comprises 1~.237~b;

sequentially detonating a plurality of spaced explosive charges in said rock so as to produce a cluster of overlap-ping fracture zones, the detonation of each charge after the initial one being delayed until liquid is present in fractured rock in the region of the charge to be detonated.
The term "deep" rock is used herein to mean a subsurface segment of rock which lies at a depth at which detonation of the explosive charges causes no significant change in the overlying topography, i.e., the surface does not swell significantly on detonation of the charges. Nor-mally, this implies rock which lies at a depth of at least 100 feet below the earth's surface. The process of the invention would not generally be practiced in rock lying at a depth below about 3,000 feet. "Fractured rock" and "fracture zones" denote herein rock and zones of rock in which new fractures have been formed, or existing fractures enlarged, by the detonations. The term "fracturing" in-cludes reducing the size of rock fragments or forming fissures therein or misaligning such fragments.
The process of the invention can be employed in an oil or gas-bearing rock formation to increase the over-all drainage area exposed to the bore of a well penetrating the formation and thus increase the rate at which hydrocar-bon fluids drain toward the well. The invention therefore extends to the recovery of hydrocarbon fluids using the process of the invention described above.
A more important aspect of the invèntion however is the in situ leaching of mineral values from ore bodies.
The invention therefore includes a method for in situ leaching mineral values from an ore body which has been fractured by detonating explosive charges in separate cavities therein, said process comprising introducing lix-iviant for the ore into the ore body through a plurality of in~ection holes therein and driving said lixiviant through the ore body to a plurality of recovery holes by injecting fluid into the ore body, said lixiviant being driven laterally and upwardly from zones which have been less severely fractured, to others which have been more severely fractured, by the detonations, whereby mineral values are entrained by said lixiviant and removed from the ore body therewith. Preferably in carrying out the leach-ing method of the invention, the ore body is fractured by the process of the invention set forth above.
The leachability of a fractured ore body depends on the size of the ore fragments, and on the permeability of the intact ore as well as of the fracture system sepa-; rating the fragments. The permeability of the fracturesystem separating the fragments, which is variable and generally much higher than the permeability of a single fragment, is determined by a network of wider, open frac-tures (determining the permeability of the ore body as a whole), and a network of narrower, open fractures (deter-mining the irrigability of individual particles to be leached). Therefore, in explosively fracturing a segment of an ore body to prepare it properly for in situ leaching, the ob~ective is not simply an indiscriminate reduction in the fragment size of the ore body. Smaller-size, well-irrigated fragments have a higher leaching rate than larger-size fragments, but fragment-size reduction by means of blasting processes heretofore known to the art, when 1~.;Z3726 applied to deep ore, tends to leave large unbroken frag-ments of rock, or to create a network Or fractures that are largely closed or plugged with fines. The process Or the invention makes it possible to reduce the larger frag-ments to a size that will leach at an economically accept-able rate through a network of open fractures throughout the blasted ore permitting good irrigation by a leach liquid.
When the cavities ~ormed are substantially verti-cal drill holes, some of the holes in the assemblage pref-erably are left uncharged with explosive, and these holes employed as a set of passageways within the fracture net-work from the earth's surface, generally to substantially the bottom of the blasted rock, e.g., for the introduction of liquid and/or gas to (or removal thereof from) the frac-ture network. The uncharged holes preferably are drilled and provided with support casing prior to the det-onation of charges in adjacent holes. The sections of substan-tially vertical shot holes located in the overburden that overlies the rock segment to be fractured preferably sur-vive the blasting process and serve as an additional set of passageways, leading from substantially the top of the blasted rock to the earth's surface, also for liquid and/or gas passage.
In a preferred explosive fracturing process, liquid is driven through the fracture zones produced by the sequential detonation of explosive charges in a plurality of,cavities in a segment of rock, in a manner such as to entrain the fines found in the fractur~ zones, and the fines-laden liquid removed from the rock. This flushing of the blasted rock is achieved by sweeping or driving llquid at high velocity through the fracture zones by in-Jecting gas into said æonesat high pressure, the liquid moving laterally and upwardly through the blasted rock, passing into the fractures, for example, from the passage-ways formed by uncharged substantially vertical drill holes and out of the fractures into passageways formed by pre-served sections of substantially vertical detonated holes located in the overburden. Best results are achieved when substantially each detonation is followed by a flushing step applied to the fracture zone thereby produced, before the next detonation in an adJacent cavity occurs, and this is preferred. In the leaching of a mass of ore, e.g., in the in situ leaching of an explosively fractured ore body or in dump leaching, fines also preferably are flushed out of fractures therein by sweeping the lixiviant therethrough at high velocity by high-pressure gas.
In the process accor~ding to the invention explo-sive charges are detonated sequentially in separate cavi-tie~ in a segment of mineralized rock to be fractured, eachdetonation in the sequence producing a zone of fracture in the rock and being delayed until liquid is prësent in the fractured rock around the cavity containing the charge to be detonated, especially in fracture zones produced by the previous detonation of charges in cavities adJacent th~eto.
Thus, the detonations occur while fractures in the sur-rounding rock are filled with liquid, or the rock is in a flooded, or liquid-soaked, condition. The cavities, e.g., drill holes or tunnels, containing the explosive charges are spaced sufficiently close together, and the charges are 3';'26 sufficiently large, that the fracture zones produced by the detonatlons thereln overlap one another. Thus, each fracture zone is wlthin the region of influence of other detonations and is subJect to the cumulative effect of a succession of detonations of explosive charges in a group of adjacent cavities. This cumulative effect permits the fragment slze-reductlon and disorlentation needed to en-hance leachability to be obtained readily from the avail-able explosive energy. The degree of overlapplng of the fracture zones, which are generally cylindrical in shape, is at least that required to locate all of the rock, in the segment of rock to be fractured, within the fracture zone produced by the detonation of at least one of the charges.
The cavities in the assemblage in which explosive charges are to be detonated (i.e., blast cavities) can be substantially vertical holes (shot or blast holes) drilled lnto the segment of rock from the surface or from a cavity in the rock, or substantially hor~zontal cavities such as tunnels, driven in the rock, e.g., from a hillside or shaft. Whether the cavity volume is provided by tunnel driving techniques such as are employed in coyote blasts, for example, or drilling techniques, possibly associated with chambering procedures, will be largely a question of economics, although technical practicability depending on such factors as topography, compressive strength of the rock, etc., will influence the selection of the method.
Substantially vertical drill holes are preferred in many cases since the preserved sections of the shot holes can be used subsequently as passageways to or from the frac-tured rock, reducing the number of holes needed to be f 37;?d~

drilled solely to provide passageways for liquld ln~ectionor e~ection.
- Although the blast cavities need not form a regular pattern, and regularity of pattern actually may not be desirable or practical, a somewhat regular pattern is indicated in a formation of reasonably uniform contour, structure, and physical strength to assure a high degree of uniformity in the fracture network produced. In some cases, as core tests reveal unpredictable changes in the rock occurring during the sequential blasting process, it may be desirable to deviate from a regular pattern, e.g., to use one or more additional blast cavities where needed to provide the required overlapping of fracture zones.
Nevertheless, substantial regularity of pattern generally will be provided in the arrangement of most of the blast cavities. It will be understood, of course, that in the case of substantially vertical drill holes the actual pat-tern of the holes within the segment of rock to be frac-tured may approach, rather than match, the hole pattern at f20 the surface, inasmuch as the available drilling equipment may not be counted on to produce parallel holes at depths of the order considered herein.
Regardless of the blast cavity pattern employed, the distance between explosive charges (and, also there-fore, between cavities) of a given composition and size is such that a cluster of overlapping fracture zones is pro-duced by the detonation of adJacent charges. Although it may not be possible to delineate the fracture zones with precision, the extent or radius of the fracture zone that can be expected to result from the detonation of an ~3~

explosi,ve char~e of a glverl composltion, density, shape, and size under a given amount of confinement in a given geological mass can be appl~oximated by making some experi-mental shots and studying the fracture zones surrounding the blast cavities by using one or more geophysical methods. Such methods include ~1) coring, (2) measure-ments in satellite holes of compressional and shear wave propagation, o~ permeability, and of electrical conductiv-ity, and (3) acoustic holography. Based on these studies, the cavities are spaced close enough together to provide the required overlapping of fracture zones.
"Adjacent" blast cavities or explosive charges, as described herein, are blast cavities or explosive charges which, although spaced from one another, are imme-diate or nearest neighbors to one another, as contrasted to blast cavities or explosive charges which are more dis-tant neighbors or separated from one another by one or more other blast cavities or explosive charges.
Although it is not intended that the invention be limited by theoretical considerations, the delaying of each detonation until liquid is present in the fracture volume surrounding the cavities is believed to have two beneficial effects. ~irst, the liquid can lubricate the fractures so that opposing faces can move suddenly in shear more easily, thereby enhancing fragmentation of the surrounding rock, which is no longer supported by the relatively high resis-tance of a dry fracture to transient shear. Secondly, liquid-filled fracture volume cannot be rammed shut by the suddenly applied pressure of an explosion. This incom-pressible behavior, together with the low resistance of the ~.;Z3726 liquid-filled fractures to sudden small displacements in shear, is believed to cause disorientation Or individual rock fragments and dilation and swelling of the bed of fragments as a whole. Each detonation creates a misalign-ment or disarrangement of fragments with an accompanying increase in void volume. Therefore, when the fracture zones produced by the successive detonations in ad~acent cavities partially overlap, the fracture zone around each cavity thereby being sub~ect to additional fracturing and~
disorientation produced by the detonations in the ad~acent cavities, and previously produced fracture zones are flooded, each fracture zone will be swelled in increments, i with each detonation jacking it to larger volume, and higher permeability~ against the pressure of the surround-ing rock. The present process makes use of the lubricating effect and incompressible behavior of the liquid in the fractures, and does not require the use of high liquid pressures, e.g., of the magnitude needed to lift the over-burden and enlarge the fractures before blasting. A liquid pressure in the fractures at the time of blasting equal to the head of liquid above the blast zone is sufficient.
Also, any readily available, relatively cheap liquid, e.g., water or water mixtures, can be used to flood the rock. If leaching of ore is performed in the course of the detona-tion sequence, a lixiviant can be used as the flooding liquid. For reasons of economy as well as because of the safety risks associated with the use of explosives which are sensitive enough to detonate in extremely small diame-ters, the use of~ explosive liquids in the fracture zones is not contemplated. Any fluid explosive which may be used ~ 237~6 ln the present process will be gelled to a viscosity that will hinder any appreciable loss thereof from the blast cavities to the surrounding fracture zones, and in any case will not be sufficiently sensitive to be detonated in said zones. Thus~ while small amounts of the explosive charges may escape into the fracture zones, such material will behave as a nonexplosive liquid therein. Accordingly, the flooding liquid is nonexplosive.
A preferred blast cavity pattern for use in the present process is one in which substantially all of the internal cavities, i.e., cavities not located at the edge of the pattern, are surrounded by at least four adjacent blast cavities, e.g., a pattern in which the blast cavities are at the corners of ad~acent polygons, which are either quadrangles or triangles and which are as close to equi-lateral as permitted by wander of the cavities.
Although all of the blast holes in a group of ad~acent substantially vertical drill holes can be drilled prior to the sequential detonation of the charges, this procedure is not preferred inasmuch as it could be neces-sary to apply a support casing to the as-yet undeto~ated holes in the sections thereof located in the segment of rock to be fractured to prevent them from collapsing as a result of detonations in ad~acent holes. Casing of the shot holes in these sections usually would be considered economically unsound because the casing would occupy volume that could otherwise be loaded with explosive and because casing in these sections of the holes is not needed in subsequent leaching operations. Therefore, it is preferred that in a group of ad~acent drilled shot holes the 37.~

detonation of each charge takes place before ad~acent shot holes are drilled. In practice, one migh~ drill and, ir desired, chamber (as described later) one shot hole of a group of ad~acent holes, load the hole or chamber with explosive, allow water to enter the formation surrounding the hole or chamber, and detonate the charge, and then repeat the sequence of steps with adjacent holes. In each successive sequence of steps, the entrance of water into the formation can occur prior to, or during, any of the other steps, however. The avoidance of the presence of drilled shot holes during detonations refers to holes in a group of adjacent holes, e.g., a central hole and four to six surrrounding holes. However, shot holes farther re-moved from the detonations can be predrilled.
The total amount of drilling needed for vertical-cavity blasting can be reduced by drilling one or more branch or off-set holes by side-tracking from one or more points in the preserved upper portion of a trunk hole which extends to the surface. ~Each off-set hole is drilled after the charges in the trunk hole and other off-set holes thereof have been detonated. Such holes will be inclined at small angles to one another.
Most of the ore bodies and other mineralized formations to which the present process is expected to be primarily applicable will be located below the water table, and in such a case, unless the section to be blasted rises locally above the water table, or the rock surrounding this section is so impermeable that flooding of the frac-ture zone does not occur by natural flow, the section will be naturally flooded, or water-soaked, before the sequential z~

blasting begins, and after a certain period of time has elapsed after each detonation to allow the water to flow naturally into the newly formed fractures. If natural flooding is incomplete or absent, water or some other liq-uid can be pumped into the catity to be shot after the explosive charge has been emplaced therein, and also into any available nearby uncharged cavities, at a sufficiently high flow rate to cause the rock to be blasted to be in a flooded condition at the time of detonation.
As stated previously, liquid is present in the rock around each cavity prior to the detonation of the charge therein. This means that liquid is present in any ; preexistent fractures in the zone which will become a frac-ture zone as a result of the detonation of the charge in that cavity, and in fractures produced by previous detona-tions in cavities adjacent thereto. This condition permits the above-described incremental swelling of overlapping fracture zones to take place. In the case of substantially vertical drill holes, the liquid level in the rock around the hole should be at least as high as the top of the ch~rge in the hole, thereby assuring the presence of liquid throughout the height of the formation where fracturing will occur. With horizontal cavities, the liquid level in the rock around the cavity should be at least as high as the radius of fracture to be produced by the detonat~on of the charge therein. When the segment of rock to be frac-tured is located below the water table, the position of the water table above it will conform to the water levels in undisturbed holes, and may be inferred at other loca-tlons by interpolation between the elevations of the water 1~.23726 ; levels in undisturbed holes. As a practlcal matter, the water table will almost always be sufficiently horizontal that the first charge can be detonated when the elevation of the liquid level in any nearby hole is at least as high as the elevation to be reached by the top of the charge (or radius of fracture in the horizontal cavity case). If the liquid level is measured in the cavity in which the charge is to be detonated, the level before loading of the explosive into the cavity should be the level measured.
After the detonation, the liquid level in cavities within the resulting fracture zone drops in proportion to the new fracture volume produced, the expulsion of liquid from the immediate vicinity of the charge by the gaseous products of detonation, and the drainage of liquid into the cavity - created by the detonation. The detonation of the next charge in the sequence in a cavity adjacent to the first is delayed until the liquid in the formation around the next cavity (including the new fracture volume produced by the previous detonation in an ad~acent cavity) returns to its required level. It is understood, however, that explosive charges in blast cavities elsewhere in a section of the formation that is not strongly influenced by a previous detonation (i.e., where the liquid level has not dropped below the required elevation as a result of the previous detonation) can be detonated at any time after the previous detonation. The delay to allow flooding applies to detonations in cavlties which are adJacent to previously detonated cavities, where the previously formed fracture zones will be sub~ect to the effect of the next detonation.

' l~.fZ37Z6 As was stated previously, some of the holes in an assemblage of substantially vertical holes preferably are left uncharged with explosive, these holes providing passageways to the fractured rock to allow the introduction ~ of gases and/or liquids thereto, e.g., in a subsequent leaching operation. These holes, which can thus be looked upon as in~ection holes (although they may serve as ejec-tion or recovery holes depending on the required flow pat-tern), are also useful in preparing the ore body for leaching, as will be described more fully hereinafter, and it is preferred, on the basis of ease of drilling, that they be drilled prior to the sequential detonation process in holes surrounding them. Predrilled in;ection holes are provided with a support casing, e.g., unperfo-rated pipe grouted to the upper part of the hole wall, at least in the section thereof located in the segment of rock ~; to be fractured, and ungrouted perforated pipe or a well-screen in the bottom section of the hole, in order to prevent hole collapse as a result of the detonations. Inas-much as full-length casing will be required for subsequent leaching operations, however, the full length of the injec-tion holes usually will be cased prior to blasting.
Damage to the in~ection piping is minimized in the present blasting process owing to the sequential, long-delay char-acter of the multiple detonations.
The location and pattern of the in~ection holes are selected on the basis of their intended function during the fracturing and leaching processes, which will be de-scribed in detail hereinafter. The overall purpose of these holes usually is to provide a means for introducing 1~.iZ3726 gases and/or liquids into the fracture network produced, or being produced, and therefore the inJection holes should be distributed throughout the segment of rock among the blast cavities in a manner such that they lie within the fracture zones produced by the detonations. After the detonation of the charge in a substantially vertical shot hole, the resulting fracture zone permits communication between a neighboring injection hole and the portion of the shot hole remaining in the overburden. The shot hole remnants thereby act as passageways to complete the liquid circuit through the fractured rock.
If injection holes are present in the formation during the sequential detonation process, an in~ection hole lying within the fracture zone produced by a previous detonation in a cavity ad~acent to a cavity to be shot can be employed to determine whether the liquid level in the rock surrounding the cavity to be shot has recovered sufficiently to flood the section to be blasted. Whenever a hydraulic potential (e.g., a liquid level) measurement is required after a blast cavity has been loaded with explosive, a nearby in~ection hole can be used. When the segment of rock to be blasted is at least partly above the water table, liquid is introduced into the rock in the cavlty to be shot, in previously detonated cavities ad~a-cent thereto, and/or in nearby injection holes. Flooding via multiple cavities is preferred. Liquid is run into a blast cavity after the explosive charge has been emplaced therein (if the charge is stable in the presence of water), and liquid level measurements, if required, are made in nearby in~ection holes. It should be understood that, in 1~.237.~6 practice, hydraulic potentlal measurements, e.g., pressure measurements made with a piezometer, or llquid level measurements, will not be requlred after each detonation, inasmuch as the experience gained in determining the neces-sary delay times to permit recovery of hydraulic potential between a few of the early detonations in the sequence wlll usually allow the practitioner to select with confi-dence suitable delay times to be used between subsequent detonations.
Although the exact delay required depends on the size of each blast, the void volume to be filled, the eleva-tion of the segment to be blasted relative to the water table, and the hydraulic transmissibility of the surround-ing rock, delays on the order Or hours or days generally will be needed As a practical matter, the time required for a shot hole to be drilled, or a tunnel to be driven, and loaded with explosive usually will be more than suffi-cient for the hydraulic potential around the cavity and the previously detonated adjacent cavities to recover to the minimum required level either by natural influx of ; water from the surrounding rock or by introduction through cavities made in thq formation. In general, delay times between detonatio'ns of at least about one hour, and typi-cally in the range of about from 4 to 24 hours, are suffi-cient for flooding to take place, although much longer delays, e.g., in the range of about from 4 to 30 days, may - be employed in order to prepare the next blast cavity for blasting. It will be understood that these delays refer to the time between detonations of adJacent charges, and that one or more charges whose zones of fracture are ~,' 3~72ti nonadjacent (i.e., whose regions of influence are mutually exclusive) can be detonated at much shorter delay times or even slmultaneously.
It has been found that when sequential blasting is carried out in less competent, broken, or clayey rock, the permeability of the rock may be decreased, although the fracture volume is increased, by the blasting. Lost perme-ability can be restored by flushing of the fractured rock, i.e., by sweeping or driving liquid through the fractures at high velocity and removing the fines-laden liquid from the rock, preferably after each detonation. The flushing procedure appears to remove from the fractures the clogging fines that prevent free irrigation around the rock frag-ments. Such fines are present in the form of existing clays and rock crushed or abraded during blasting.
The flushing can be accomplished by the pressure injection of liquid and gas into the fractured rock through one or more injection holes, and removal of the fines-laden liquid from the fractured rock by bringing it to the surface through one or more detonated shot holes, in the preserved sections of the latter which pass through the overburden to the surface. Liquid and gas, e.g., water or other aqueous liquid and air or oxygen, can both be in-jected; or gas alone can be injected so as to sweep ahead the liquid already present in the fractures. Alternatively, a liquefied gas, such as air, nitrogen, oxygen, can be introduced into the injection holes and allowed to vaporize therein and thereafter drive the liquid through the frac-tures. Inasmuch as there is a two-phase flow in a general~
upward direction and laterally in the direction of the 37.~

detonated shot holes, the circulation Or the liquid is powered by gas lift such that the gas chases the liquid upward and outward through the broken formation, and fines are driven toward the zones of severest fracture, where their concentration ls heavlest, from which zones they are ejected with the liquid. This direction of sweep is pre-ferred inasmuch as the reverse direction drives the fines more deeply into the less severely worked zones of the formation away from their point of heaviest concentration and can cause an intensified clogging of the fractures.
The surging high-velocity flow which develops with the upward two-phase flushing system removes fines that prevent free irrigation around the fragments, If necessary to achieve the required lateral circulation of liquid between injection hole and e,~ection hole throughout the length of the fracture zones being flushed, two or more verti-cal~y separated in,jection zones in a given in,jection hole can be employed, one substantially at the bottom of the fractured rock and one or more others above it.
The buoyancy of the pressurized gas alone can be sufficient to raise the fines-laden liquid to the surface of the ground when the water table is relatively close to the surface. When the water table is so deep that the buoyance is insufficient, the liquid can be pumped up the~
collar of the shot hole.
At the start of flushing, the gas injection pressure should be higher than the ambient hydrostatic pressure at the position in the injection hole where in-jection occurs, and preferably higher than the lithostatic pressure at this position. The minimum gas pressure 3~

required for flushing i5 highest at the start of the opera-tion and falls as gas in,;ection proceeds.
Although there can be much variation in the number of fracture zones bein~, flushed out at any given time, and the nature and number Or other operations which can be performed during flushing, it is preferred that a detonation in any given cavit~ be followed by detonations in no more than two or three ad,lacent cavities, and most preferably by a detonation in no ad,~acent cavity, before the fracture zone produced b~y the detonation in the given cavity has been flushed out as described. In some forma-tions, if a given fracture zone is sub3ected to a number of subsequent detonations without the intervention of flushing, restoration of permeability by a later flushing becomes difficult because the fractures may have become plugged up too tightly with fines. Therefore, a cyclic blast/flush/blast/flush, etc. process is preferred, One or more fracture zones can be flushed at the same time, and flushing of the same zone can be repeated, if desired.
An already flushed zone can be left untreated during the flushing of adjacent zones by plugging the ejection hole in that zone. Flushing of one or more zones can be carried out while adjacent blast cavities are being drilled and loaded.
In the present process, the detonation of the charges in sequence permits the preservation of the sec-tions of substantially vertical shot holes that pass through the overburden (the strata overlying the rock segment being worked), and these sections of the shot holes can serve as e~ection holes in the flushing process, as 37Z~

described above. The reduced fragment size and unclogged fracture network achieved after all of the charges have been detonated, and the detonations followed by a flushing procedure, produce, in the case of an ore body, an ore which is well prepared for in situ leaching.
The present invention also provides a leaching process wherein fines are flushed out of a mass of ore by driving lixiviant through the mass by means of high-pressure gas, e.g., in a specific circulation pattern.
According to one embodiment of the present leaching proc-ess, an ore body which has been prepared for leaching by detonating explosive charges in separate cavities therein, e.g., according to a process of this invention, is leached in situ by introducing lixiviant for the ore into the pre-pared ore body through a plurality of in;ection holes therein and intermittently or continuously driving the lixiviant through the ore body to a plurality of recovery holes by means of high-pressure oxidizing gas, the lixiv-iant moving laterally and upwardly from zones th~t have been less severely worked, to others that have been most severely worked, by the detonations, whereby fines are removed from the ore body. When the ore body has been prepared for leaching by means of the above-described blast/flush process,the lixiviant for the ore can be in-Jected into the ore body through in~ection holes which have previously been used in the flushing steps, and fines-laden pregnant leach solution recovered from the ore body through the preserved upper portions of shot holes, piping havlng been grouted lnto all holes used to circulate lixi-vlants and pump8 provlded as necessary to in~ect llxlvlants ~ 'Z~7~

ln one set of holes and remove prcgnant liquor from another ffet Or holes. The bottom ends o~ the pipos and any other posltlons along the plpes where ll~lvlants are to be lnJected or collected are provlded wlth perfora-tlons or wellscreon~.
me llxivlant (e.g., sulfurlc acld/water or sulrurlc acld/nitrlc acid/wnter ror ores whose acid con-sumptlon 18 wlthln tolerable level~, or NR40H/water rOr ores havlng a high acid con~umption), which is a liquld, and a gas, usually an oxidizing gas, prererably oxy~en, ~ir, X0x, or mixture~ thereof, are in~ected lnto the bass of the prepared ore body at hiKh pres~ure. AB in the case Or rlushing between blasts, thls type of inJectlon give~ a clrculatlon powered by gas liM such that the ga8 cha8 the llquld through the broken rock. Even wlth constant rlow rate~ of gas and llquid at the in~ection hole~, a surging, high-velocity flow develops in the rock which 18 believed to be benericial in (1) removing fines around the oro fragment6 (such ~ine~ being created durlng the leach-ing process in rorm~ such as decrepltated ore sllmes andpreclpitated iron salts), (2) lncreaslng the leaching rate as a result Or the cycllc squeezing of the ore frag~ents from the pressure fluctuatlons assoclated with the surglng ~low, and (3) working the ore gently so as to collapse wide openlngs among the rragments that may develop during the leaching process and can cause channelling of leachlng solution. Sweeplng the lixlvlant laterally toward collec-tlon polnts ln the more severely worked fracture regions Or tho ore body, and from in~ectlon polnts ln the leRs severely worked reglons reduces the chances that a more intense clogglng Or the ore body with rlnes will occur.

37~6 The process of producin~ a network Or rractures in deep rock will be illustrated with reference to the accompanying drawlngs in which:
Figure 1 is a schematic representation showing in plan view a subsurface segment of rock which has been fractured in accordance with the invention and the liquid circulation pattern between holes therein, Figure 2 is a schematic representation in sec-tional elevation showing the surface-to-surface liquid circulation pattern through the segment of rock shown in Figure 1, Figure 3 is a schematic representation of a typi-cal shot hole pattern, and Figure 4 is a plot showing the effect of repea~d detonation and flushing operations on the permeability of a fracture zone produced with the shot hole pattern shown in Figure 3.
In Figure 1, the holes designated by the letter S are substantially vertical shot holes. Within the blasted segment of rock, these holes are destroyed by the ._ . ., .~ ,,, ~ , .. . .
detonations which have taken place therein in the fractur-ing process and are rep`laced by the ad~acent, overlapping fracture zones shown in the upper half of the figure, and also denoted by the letter S, to indicate a previous shot hole. The shot holes rather than the fracture zones are shown in the lower half of the figure so that liquid circu-lation lines can be indicated clearly. It should be under-stood, however, that upon completion of the entire blast sequence all shot holes are surrounded by fracture zones (as depicted in the upper half of the figure) in the sec-tions thereof located in the rock segment that was blasted.

~.Z3~7~

In the sections overlying the blasted segment, the shotholes remain substantially intact and in these sections all shot holes appear as they are shown in the lower half of the figure. The preserved upper sections of the shot holes are ejection holes in the flushing steps of the blasting process, and recovery holes in the leachlng process. In the hole arrangement illustrated in FIG. 1, the shot holes are arranged in a trigonal pattern wherein lines between adjacent holes form substantially equilateral triangles.
The holes designated I are injection holes.
These holes are uniformly distributed among the shot holes as shown. The arrows indicate the direction of flow of liquid from injection holes Il, I2, I3, I4, I5 and I6 to the preserved upper section of shot hole Sl; and from in~ection holes I4, I5, I7, Ig, and two other undepicted injection holes to the preserved upper section of shot hole S2. The preserved upper section of shot hole S3 is plugged off while shot holes Sl and S2 are being used for flushing or as recovery holes for pregnant leach solution. At the same time, liquid injected into these injection holes is being driven to other oPen shot holes.
In FIG. 2, piPin~ in in,~ection hole I and shot hole S is shown as it passes through overburden 1 to the fractured rock segment 2. Piping 6 in injection hole I
leads from the earth's surface 3 to substantially the bot-tom of rock segment 2. Piping 7 in shot hole S leads from the earth's surface 3 to the top of rock segment 2. Frac-ture zone 4 has been produced by the detonation of an explosive charge in shot hole S, which before the detona-tion led to substantiall,~ the bottom of rock se~ment 2.

- 24 ~

~237~

Piping 7 terminates in well screen 5, and piping 6 is pro-vided with perforations vertically spaced along the length thereof located in rock segment 2. In the flushing steps of the fracturing process, and in the leaching process, liquid is in;ected into fractured rock segment 2 through the perforations in piping 6, then is driven by pressur-ized gas through the fractured rock as indicated by the arrows~ and leaves the top of the rock segment through pip-ing 7. Lateral as well as upward flow occurs from the less severely worked zone around hole I to the most severely worked zone, i.e., fracture zone 4.
Regulation of the rate at which gas and liquid lixiviant are injected and collected at the various injec-tion and collection holes allows a high degree of control of the in situ leaching process. By the operation of control valves, the injection and collection pressures can be regulated to obtain a relatively uniform flow throu~
the ore body in spite of variations in permeability from place to place. Shifting the injection or collection from one set of holes to another will change the direction of flow through the ore and can be used to frustrate channel-ling. The regulation of pressures and flow rates at the various holes can be used to maintain a net flow of ground water toward the operation under conditions that might otherwise result in the escape of leach solution. Leakage of the leach solution is also reduced in the present proc-ess as a result of the carriage of some of the fines away from the area of gas agitation where they settle out and plug the leak. In leaching, the gas/liquid pressure inJec-tion can be intermittent or continuous, depending upon the 1~.23~

degree to which the ore tends to plug up, and the fre-quency wlth which flow patterns are changed to obtain uni-form and complete leaching throughout the ore.
When lixiviant is introduced into an injection hole simultaneously with gas, its injection pressure should be equal to that of the gas, i.e., higher than the ambient hydrostatic pressure at the injection point, and preferably higher than the lithostatic pressure at this point. In some cases, especially at greater depths, the injection of lixiviant and oxidizing gas at sufficient pressure to exceed the lithostatic pressure may be necessary in order to get sufficient flow rate through the ore. If, in some or all of the inJection holes, there are periods of time `when lixiviant alone is introduced into the ore, this in-' troduction preferably is done at a pressure at least ashigh as the lithostatic pressure at the in~ection position.
; That is, the pumping pressure preferably is at least as high as the lithostatic pressure minus the heads of fluid in the piping leading from the pump to the in~ection position.
According to the present invention, permeability can be increased also in ore masses such as mine waste dumps by driving lixiviant through fractures therein by means of gas at sufficiently high pressure that the lixiv-iant is swept through at a rate sufficiently high to entr~n fines present in the fractures, and removing the fines-laden lixiviant from the ore mass.
In a preferred embodiment of the present process, the sections of substantially vertical shot holes which are located in the segment of rock to be fractured are first - 26 ~

~ ~37Z6 chambered to larger diameter, and the explosive charges positioned in the chambered portions. In this procedure, drilling costs are reduced by drilling widely spaced-apart shot holes Or smaller diameter than is required to accommo-date the size of explosive charges to be employed, and enlarging or "springing" the lower parts of the shot holes to produce chambers having the volume required to hold the explosive charge. The sections of the holes in the rock segment are chambered either by drilling them out, e.g., with an expansion bit, or by detonating explosive charges therein. The chambering method is not critical, the pre-ferred method generally being the one that results in the lowest overall cost per unit of chamber volume for the particular rock segment in question. In the present proc-ess, explosive charges used for springing may be 20 feet or more in length. If rock fragments tend to fall from the walls of an explosively sprung hole and thus to occupy some of the volume required for the explosive charge sub-sequently to be used in producing the fracture zone, the hole to be sprung can be drilled deeper so that the bottom of the hole is located below the bottom of the formation.
In this manner, any loss in volume that is to be available for explosive loading is minimized since a portion of the chamber volume below the segment of rock to be fractured can hold the fallen rock fragments.
The advantage of chambering the shot holes before loading them with the charges which will be detonated to produce the fracture network becomes evident when it is considered that an explosively sprung hole typically will hold about ten times as much explosive as an unsprung hole.

- ~7 -~.237~

Thus, for example, a pattern of 30-inch-diameter charges on 100-foot spacings (center-to-center) typically can be achieved by drilling 9-inch-dlameter holes on 100-foot spacings.
Although the blast/flush process has utility in deep underground blasting with explosives of all types, the use of chemical explosive charges is much preferred for several reasons. The many technical as well as civil (legal, political, public relations) problems associated with the undertaking of nuclear blasting are self-evident.
Vibration effects and redioactivity are the two maJor roots of these problems. A nuclear blast which is large enough to be economically feasible must be set off at sufficient depth, e.g., preferably appreciably deeper than 1000 feet, in order to be safely contained and not release radioactiv-ity to the atmosphere. Many potentially workable ore bodies will not be located as deep as the safe containment depth. Furthermore, the extreme magnitude and concentra-tion of the energy produced in a nuclear blast imply that it will be difficult, if not impossible, to achieve (a) a high degree of uniformity in explosion-energy distribu-tlon and ore breakage, (b) close hydraulic control of the flow of lixiviants without an appreciable amount of addi-tional drilling to increase the number of lnjection and extraction polnts, and (c) a close match of the broken volume with the outline of the ore body~ particularly for small or irregular ore bodies, such a match resulting in economics in the use of the available explosive energy and in the consumption of lixiviants.
While single explosive charges generally will be 3L~oZ37Z6 detonated in sequence to produce the fracture zones, the ~ charges also can be multi-component charges positioned in ; separate cavities and detonated substantially simultane-ously as a group to produce each fracture zone, each detona-tion in the sequence of detonations in such a case being a group of detonations.
The following example illustrates specific embodiments of the process of the invention.
The formation to be fractured was a bedded series Or shales and silt stones, dipping about 45, located at a depth of 70 to 90 feet below the surface, and therefore sub~ected to a lithostatic pressure of about 70 to gO
p.s.i. The water table was at a depth of about 15 feet below the surface. A 3-inch-diameter hole was drilled into the formation to a depth of 100 feet. This hole was used as a core-sampling and permeability-testing hole, and also i as an in~ection hole for purposes of fIushing surrounding shot holes. A core test revealed a competent silty shale at the 70-90 foot depth. A well screen was installed in the hole at the 70-90 foot level, and piping to the well screen was grouted to the hole. Cement filled the hole below the well screen.
- The pattern of shot holes used is shown in FIG.3.
Three shot holes (SH 1, SH 2, and SH 3) were located 16.25 feet from the in~ection hole I, their centers lying on 120 radii from the center of hole I and the lines joining them forming an equilateral triangle. The distance be-tween these shot holes was 28 feet. Three shot holes (SH 4, SH 5, and SH 6) were located 32.5 feet from hole I, their centers also lying on 120 radii from the center of . . . ~ .

Z37~'~

hole I, and the lines ,~oinirlg them (also rorming an equi-lateral triangle) beinK biscet;ed by the centers of holes SH 1, SH 2, and SH 3. The dlstance between holes SH 4, SH 5, and SH 6 was 56 feet. It is seen that in this arrangement the lines ~oining adJacent (i.e., nearest neighbors) shot holes formed equilateral triangles. SH 1, SH 2, and SH 3 each had ~our shot holes ad~acent thereto (SH 2, SH 3, SH 4, and SH 5 for SH l; SH 1, SH 3, SH 5, and SH 6 for SH 2; and SH 1, SH 2, SH 4, and SH 6 for SH 3), and SH 4, SH 5, and SH 6 each had two shot holes adjacent thereto (SH 1 and SH 3 for SH 4; SH 1 and SH 2 for SH 5;
and SH 2 and SH 3 for SH 6).
Shot hole SH 1 was drilled first. The hole was 5 inches in diamater and 91 feet deep and was loaded with 255 pounds of an aluminized water gel explosive having the fol-lowing composition: 18.9% ammonium nitrate, 10. 5% sodium nitrate, 29 . 6% methylamine nitrate, 30% aluminum, and 11%
water. The explosive column was 21.7 feet high, and was covered by a layer of water which naturally flowed into and filled the remainder of the hole and stemmed the explosive charge. The water level in the injection hole was above the level of the top of the explosive charge in the shot hole, indicating that the rock surrounding the shot hole was properly flooded. Before the explosive charge was ini-tiated, the permeability and sound velocity of the rock surrounding the injection hole were measured. The perme-ability was determined by slug tests, in which the perme-ability is inferred from the rate at which the head of liq-uid subsides toward the ambient level in a hole after the rapid introduction of a slug of liquid therein (see 3o Z3~6 Ferrls, J. G., et al., "Theory of Aquifer Tests", U.S.
Geological Survey, Water-Supply Paper 1536-E, 1962). The sound velocity, measured at depths of 74.5 feet to 85 feet between the in~eotion hole, shot hole SH 1, and a test hole collared 13 feet on the opposite side of the in~ection hole, was 3970 meters per second.
The ex~losive charge in shot hole SH 1 in the flooded formation was detonated, whereupon the water level in the in~ection hole dropped to below its predetonation level, as a result of the formation of a new fracture volume around shot hole SH 1, the chasing of water from the rock fractures by the gaseous detonation products, and the flow of water into the cavity created by the explosive charge. After partial recovery of the water level in the in~ection hole, the second shot hole (shot hole SH 2~ was drilled to the same size as shot hole SH 1, and the rock surrounding shot hole SH 1 was then flushed with water by (a) blowing compressed air into the bottom of the open in~ection hole, (b) injecting water through a packer in the in~ection hole, and tc) three long air injections, and then (d) 18 short air injections through a packer in the in~ection hole. The total flushing time was about 4 hours.
Silt-laden water was e~ected from shot hole SH 1 (but not shot hole SH 2) during the flushing, indicating the preser-vation of the top of shot hole SH 1, the circulation of the water from the bottom of the rock segment (bottom of the in~ection hole) laterally and upward through the fracture network to the top of the rock segment (bottom of shot hole SH 1), and the removal of fines from the fractures. The permeability was measured in the in~ection hole (as 37~

described above) before and after the flllshing operations.
Shot hole SH 2 and subsequently drilled shot hole SH 3 were loaded and the charges therein detonated as de-scribed for shot hole SH 1.
The water level in the injection hole returned to its predetonation level, above the level of the top of the explosive charge in shot hole SH 1 before detonation, in about 18 hours. Thereafter, the charge in shot hole SH 2 was detonated, whereupon the water level in the injection hole again dropped to below its predetonation level. The rock surrounding shot hole SH 2 was flushed with water, and silt-laden water eJected from shot hole SH 2, by sealing off shot hole SH 1 and (a) injecting air through a packer in the injection hole, (b) blowing compressed air into the bottom of the open injection hole, and (c) injecting air through a packer in the injection hole, followed by water through the packer while blowing compressed air into the bottom of shot hole SH 3. The total flushing time was about 11 hours. The permeability was again measured before and after the flushing operations.
After the water level in the injection hole had returned to its predetonation level, the charge in shot hole SH 3 was detonated, and the surrounding rock flushed by (a) air injection through a packer in the injection hole, followed by sealing off shot hole SH 1 and blowing air down shot hole SH 2 and shot hole SH 3 to drive water to each shot hole in turn until water was exhausted from the broken rock; and (b) two air injection flushings, each followed by water injection. The total flushing time was about 7 hours. The permeability was again measured before ~.~ Z 37 '~d and after the flushing operat-Lorls.
The remaining shot holes, SH 4, SH 5, and SH 6, were drilled, loaded, and detonated in the same manner as holes SH 1, SH 2, and SH 3, with the detonations occurring after the return of the water level in the injection hole to its predetonation level. Between the shooting of shot holes SH 4 and SH 5, the rock surrounding hole SH 4 was flushed by three air in~ections in the injection hole, each followed by water injection; between the shooting of shot holes SH 5 and SH 6, the rock surrounding hole SH 5 was flushed by injecting air into the in~ection hole, and blow-ing air down hole SH 6 (unshot), separately and simultane-ously; and after hole SH 6 was shot, the rock surrounding it was flushed by alternately ~njecting air into the in-jection hole and blowing air down the surviving section of hole SH 6.
The permeabilities measured by slug tests in the injection hole before the blast/flush process began and after each blast and flush operation at each of the six holes are plotted in FIG. 4 as a function of the operation performed, the permeabilities being presented in milli-darcys on a logarithmic scale as the ordinate. Nineteen points are shown, including those obtained after the four flushing procedures (a, b, c, and d) described above after the shooting of hole SH l; three flushing procedures (a, b, and c) after the shooting of hole SH 2; and two flush-ing procedured (a and b) after the shooting Or hole SH 3.
Each point denotes the average permeability measured after a given operation.
The plot shows that the permeability of the rock ~ 33 ~

~ ~ Z37Z~

was increased considerably (from 500 to over 2000 milli-darcys) by the total six-cycle blast/flush process, and that variations in permeability occurring during the cycli-cal shooting and flushing tend to decrease as the rock is broken and swelled. The plotted experimental values also show that the rapid flow of water to the remnant of a shot hole achieved by means of air injection through another hole or by strong pumping from a shot hole (by blowing air ~nto the bottom of an open shot hole, for example) in-creases the permeability after blasting, best results hav-ing been achieved when both air injection at the injection hole and strong pumping at a nearby shot hole was used.
While blasting was found generally to decrease the perme-ability, permeability which had previously been reduced by the injection of water (alone or as a final flushing step after blasting) was increased by blasting.
The degree of dilation produced in the rock by the first three of the above-described detonations in flooded rock was estimated from calculations of porosity based on sound velocity measurements. The sound velocity around the inJection hole after the three blasts was 3650 meters per second at a 12 foot radius from the hole, and 2530 meters per second through paths ~n the blasted rock running from the shot holes in to the inJection hole (com-pared with 3970 meters per second in the same prism of rock before blasting). The total porosity in the rock (~) was calculated from the following empirical equation for the sound velocity (~, in m/sec) as a function of porosity, for flooded ocean sediments having various degrees of lithification:

~.Z37;~

~ = -50.748 ln~ + 432.23.
Total porosity be~ore blasting: 11.7%
Total porosity after blasting: (12-ft.
radius) 16.0%
Total porosity after blasting: (center to shot holes) 34.6%
These porosities imply that the fracture volumes caused by the blasting were 4.3% t12-ft. radius) and 22.9% (center to shot holes).

- 35 ~

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for producing a network of fractures in a deep segment of mineralized rock to prepare said segment for the in situ recovery of mineral values therefrom comprising a) forming an assemblage of cavities in said rock seg-ment;
b) positioning chemical explosive charges in a plural-ity of said vacities in the sections thereof located in said rock segment;
c) allowing liquid to fill the fractures existing in the rock around the cavities in the sections thereof located in said rock segment as can be evidenced by a liquid level which is at least as high as the top of said rock segment; and d) detonating said explosive charges sequentially, each detonation in the sequence producing a zone of fracture in said segment and a drop in the level of said liquid, as measurable in the cavity which contained the detonated charge or in a cavity adjacent thereto, said charges be-ing sufficiently large and spaced sufficiently close to-gether that the fracture zones produced in said segment by the detonations of charges in adjacent cavities over-lap,the detonation of the charge in each cavity being delayed until after the level of the liquid, as measur-able in the cavity containing the charge to be detonated or in a cavity lying within the zone of fracture pro-duced by a previous detonation in a cavity adjacent thereto, has ceased to drop and is at least as high as the top of said rock segment.

2. A process of Claim 1 wherein said liquid is water.

3. A process of Claim 2 wherein said water is naturally occurring in said rock.

4. A process of Claim 1 wherein, in groups of adjacent cavities in which explosive charges are to be detonated, the deton-ation of the charge in each cavity takes place before charges are positioned in cavities adjacent thereto.

5. A process of Claim 1 where in the time elapsing be-tween any two consecutive detonations in a group of adjacent cav-ities is at least one hour.

6. A process of Claim 5 wherein said liquid is intro-duced into at least one of said cavities and enters said rock by permeating the cavity walls.

7. A process of Claim 1 wherein the cavities in which explosive charges are to be detonated form a pattern in which each internal cavity of the pattern is surrounded by at least four ad-jacent cavities.

8. A process of Claim 1 wherein said cavities are sub-stantially vertical drill holes.

9. A process of Claim 8 wherein the sections of the holes which are located in said rock segment to be are chambered to larger diameter, and the explosive charges are positioned in the chambered sections.

10. A process of Claim 9 wherein said chambered sections are produced by the previous detonation of springing explosive charges in the holes.

11. A process of Claim 10 wherein the springing ex-plosive charges employed to produce the chambered sections of said holes are at least about 20 feet long and the bottoms thereof are located below the bottom of said segment of rock.

12. A process of Claim 8 wherein a plurality of holes of said assemblage are drilled from the earth's surface and are left uncharged with explosive, said holes forming passageways within the fracture network from the earth's surface to substan-tially the bottom of said segment of rock.

13. A process of Claim 12 wherein said plurality of un-charged holes are drilled and their walls provided with support casing, at least in the portions thereof located in the said seg-ment of rock, prior to the detonation of the explosive charges in holes adjacent thereto.

14. A process of Claim 13 wherein liquid is injected into said segment of rock through said uncharged holes.

15. A process of Claim 8 wherein, in groups of ad-jacent holes in which explosive charges are to be detonated, the detonation of the charge in each hole takes place before adjacent holes are drilled.

16. A process of Claim 8 wherein said substantially vertical holes are drilled into the segment of rock from the sur-face, and branch holes are drilled into the segment of rock from said substantially vertical holes at small angles thereto starting at locations overlying said segment of rock.

17. A process for producing a network of fractures in a deep segment of mineralized rock to prepare said segment for the in situ recovery of mineral values therefrom comprising a) forming an assemblage of cavities in said rock seg-ment;
b) positioning explosive charges in a plurality of said cavities in the section thereof located in said rock segment;
c) detonating said explosive charges sequentially to produce a plurality of fracture zones;
d) driving liquid through said fracture zones by in-jecting gas therein at sufficiently high pressure that fines present in said fracture zones as a result of the crushing or abrading of existing clays or rocks are en-trained by said liquid; and e) removing fines-laden liquid from said rock whereby the permeability of said rock segment is restored.

18. A process of Claim 15 wherein liquid is driven through, and fines-laden liquid removed from, the fracture zone produced by the detonation of an explosive charge in a given cav-ity prior to the detonation of explosive charges in cavities ad-jacent thereto.

19. A process of Claim 15 wherein an assemblage of sub-stantially vertical holes is drilled into said rock from the earth's surface; explosive charges are detonated in the sections of a plurality of said holes in said segment of rock in a manner such that the sections of said detonated holes located in the over-burden that overlies said segment of rock survive the detonations and form a first set of passageways from the earth's surface to said segment of rock, said first set of passageways leading sub-stantially to the top of said segment of rock; a remaining plur-ality of holes of said assemblage are left uncharged with explo-sive and form a second set of passageways from the earth's sur-face to said segment of rock, said second set of passageways lead-ing substantially to the bottom of said segment of rock; and liquid is driven through said fracture zones laterally and upwardly from said second set of passageways to said first set of passageways and is brought to the surface through said first set of passage-ways.

20. A process of Claim 17 wherein said uncharged holes are drilled prior to the detonation of said charges, and have their walls provided with support casing at least in the sections thereof located in said segment of rock.

21. A process of Claim 19 wherein liquid present in the fracture zones is driven therethrough by gas injected therein through said holes which form said second set of passageways.

22. A process of Claim 21 wherein said liquid is water, and said gas is selected from the group consisting of air, oxygen, and nitrogen.

23. A process of Claim 19 wherein said segment of rock is an ore body, and, after the detonation of said explosive charges, a lixiviant for said ore is driven through the fracture zones in said ore body from said holes which form said second set of passage-ways to said holes which form said first set of passageways, and is brought to the surface through said first set of passageways.

24. A process for producing a network of fractures in a deep segment of mineralized rock to prepare said segment for the in situ recovery of mineral values therefrom comprising a) forming an assemblage of cavities in said rock seg-ment;
b) positioning chemical explosive charges in a plural-ity of said cavities in the sections thereof located in said rock segment;
c) allowing liquid to fill the fractures existing in the rock around the cavities in the sections thereof located in said rock segment as can be evidenced by a liquid level which is at least as high as the top of said rock segment;
d) detonating said explosive charges sequentially, each detonation in the sequence producing a zone of fracture in said segment and a drop in the level of said liquid, as measurable in the cavity which contained the deton-ated charge or in a cavity adjacent thereto, said charges being sufficiently large and spaced sufficiently close together that the fracture zones produced in said segment by the detonations of charges in adjacent cavi-ties overlap, the detonation of the charge in each cav-ity being delayed until after the level of the liquid, as measurable in the cavity containing the charge to be detonated or in a cavity lying within the zone of frac-ture produced by a previous detonation in a cavity ad-jacent thereto, has ceased to drop and is at least as high as the top of said rock segment;

e) driving liquid through said fracture zones by injecting gas therein at sufficiently high pressure that fines present in said fracture zones as a result of the crushing or abrading of existing clays or rocks are entrained by said liquid; and f) removing fines-laden liquid from said rock whereby the permeability of said rock segment is restored.

25. A process of Claim 24 wherein liquid is driven through, and fines-laden liquid removed from, the fracture zone produced by the detonation of an ex-plosive charge in a given cavity prior to the detonation of explosive charges in cavities adjacent thereto.

26. A process according to Claim 1, in which the network of fractures is formed in oil or gas-bearing strata so as to increase the rate at which hydrocarbons drain towards the bore of a well.