Claims
- 1. In a process for leaching a fracture-containing solid mass of ore by introducing a lixiviant for the ore and a gas into the solid mass through a plurality of holes therein, the improvement which comprises driving liquid lixiviant laterally and upwardly through the fractures in the solid mass to a plurality of recovery holes by injecting said gas, and allowing it to exit from said recovery holes with said lixiviant, at a rate sufficient to produce a surging flow of lixiviant in said recovery holes, whereby fines present in the fractures in the solid mass are entrained by said lixiviant and are removed from the solid mass of ore therewith.
- 2. A process of claim 1 wherein said gas is injected into said mass at a pressure in excess of the lithostatic pressure.
- 3. A process for the in situ leaching of an ore body which has been worked by detonating explosive charges in separate cavities therein to produce in the ore body immediately adjacent to the site of each detonation a fracture zone comprised of a most severely fractured core portion surrounded by a less severely fractured outer portion, comprising introducing a liquid lixiviant for the ore into the ore body through a plurality of injection holes in the less severely fractured portions and driving said lixiviant laterally and upwardly through the ore body to a plurality of recovery holes in the most severely fractured portions by injecting gas into the ore body and allowing said gas to exit from said recovery holes with said lixiviant at a rate sufficient to produce a surging flow of lixiviant in said recovery holes, whereby fines are entrained by said lixiviant and removed from the ore body therewith.
- 4. A process of claim 3 wherein said lixiviant is injected into said ore body at a pressure which is at least as high as the lithostatic pressure, and said gas is injected and said fines are removed intermittently.
- 5. A process of claim 3 wherein the ore body has been worked by the detonation of chemical explosive charges.
- 6. A process of claim 5 wherein the injection holes extend from the earth's surface to substantially the bottom of the ore body, and the recovery holes extend from the earth's surface to substantially the top of the ore body.
- 7. A process of claim 6 wherein the recovery holes are preserved upper portions of shot holes in which explosive charges have been detonated.
- 8. A process of claim 3 wherein said gas is an oxidizing gas.
BACKGROUND OF THE INVENTION
This is a Division, of application Ser. No. 382,845, filed July 26, 1973 and now U.S. Pat. No. 3,902,422.
The present invention relates to the production of a network of fractures in a deep underground segment of 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 thousand feet beneath the earth's surface, in order to supplement or replace dwindling energy sources and minerals supplies. Numerous deposits of ore, for example ore containing copper, nickel, or silver, lie too deep to mine by open-pit methods or are too low in grade to mine by underground methods. Open-pit methods incur both the costs and the environmental impact associated with moving large quantities of earth and rock. Underground methods incur unusually 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 suitable, 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 leaching rate would be too low to support a commercial leaching operation. In such cases, it becomes necessary to prepare the ore for leaching, by fragmenting it in a manner such as to provide the necessary permeability and leachability. The use of explosives to fracture underground segments of mineralized rock to create areas of high permeability has often been suggested. In an oil- or gas-bearing formation the fracturing is required to increase the overall drainage area exposed to the bore of a well penetrating the formation, and thus increase the rate at which hydrocarbon fluids drain toward the well. In an ore body the fracturing is required to increase the surface area of ore accessible to an injected lixiviant, and thus increase the leachability.
The use of nuclear explosives has been proposed for fracturing large-volume, deep ore bodies for subsequent in situ leaching. Also, the use of multiple chemical explosive charges in deep reservoir rock has been described in a method for stimulating hydrocarbon-bearing rock, e.g., in U.S. Pat. 3,674,089. However, if a deep ore body, i.e., one lying at depths of about from 100 to 3000 feet from the surface, is to be effectively leached in place, and the ore prepared for leaching by blasting, i.e., blasting in the absence of a free face for the ore to swell toward, it becomes necessary to employ special blasting and associated techniques which will provide and maintain the type of fracture network required for efficient leaching.
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 separating the fragments. The permeability of the fracture system 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 fractures (determining the permeability of the ore body as a whole), and a network of narrower, open fractures (determining 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 objective 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 applied to deep ore, tends to leave large unbroken fragments of rock, or to create a network of fractures that are largely closed or plugged with fines. An explosive fracturing process is needed which reduces the larger fragments to a size that will leach at an economically acceptable rate, and that will result in a network of open fractures throughout the blasted ore that will permit it to be well-irrigated with leach liquid.
This invention provides a process for producing a fracture network in a deep subsurface segment of rock, e.g., in an ore body, comprising (a) forming an assemblage of cavities, e.g., drill holes or tunnels, in the segment of rock; (b) positioning explosive charges in a plurality of the cavities in the sections thereof located in the segment of rock to be fractured, e.g., in sections of drill holes which have been previously chambered, such as by an explosive springing procedure; (c) providing for the presence of liquid in the segment of rock, e.g., by virtue of the location of the segment of rock below the water table so that water naturally is present in, or flows into, fractures therein, or by introducing liquid into one or more cavities therein; and (d) detonating the charges sequentially in a manner such as to progressively produce a cluster of overlapping fracture zones, the detonation of each charge in the detonation sequence producing a fracture zone which is subject to the cumulative effect of a succession of detonations of explosive charges in a group of adjacent cavities, and the detonation of the charge in each cavity being delayed until liquid is present in fracture zones produced by the previous detonation of charges in cavities adjacent thereto, as determinable by measuring the hydraulic potential, e.g., the liquid level, in the cavity, or in a cavity adjacent thereto.
When the cavities formed are substantially vertical drill holes, some of the holes in the assemblage preferably are left uncharged with explosive, and these holes employed as a set of passageways within the fracture network 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 fracture network. The uncharged holes preferably are drilled and provided with support casing prior to the detonation of charges in adjacent holes. The sections of substantially vertical shot holes located in the overburden that overlies the rock segment to be fractured preferably survive 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 fracture zones, and the fines-laden liquid removed from the rock. This flushing of the blasted rock is achieved by sweeping or driving liquid at high velocity through the fracture zones by injecting gas into said zones at high pressure, the liquid moving laterally and upwardly through the blasted rock, passing into the fractures, for example, from the passageways formed by uncharged substantially vertical drill holes and out of the fractures into passageways formed by preserved 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 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.
The term "deep" as used herein to describe a subsurface sgement of rock denotes a depth at which the detonation causes no significant change in the overlying topography, i.e., the surface does not swell. As a rule, "deep" rock as described herein lies at a depth of at least 100, and usually not more than 3000, feet. "Fracture zones" and "fractured rock" herein denote zones and rock in which new fractures have been formed, or existing fractures opened up, by the detonations. "Fracturing" denotes herein a treatment which reduces the size of, and/or misaligns, rock fragments.
US Referenced Citations (5)
Divisions (1)
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Number |
Date |
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382845 |
Jul 1973 |
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Patent Grant
3999803
