1. Field of the Invention
The present invention relates to a method of stemming boreholes, i.e., to a method of blocking the exit of holes which contain an explosive charge to prevent the rapid escape of the high-pressure gases produced when the charge is exploded and therefore allow more effective utilization of the explosive energy.
2. Description of the Prior Art
A well-known method of fracturing rock involves drilling a hole in the rock, placing an explosive charge in the drilled hole, and firing the charge. The effectiveness of the explosion in extending long, gas-pressured fractures out into the rock surrounding the hole is increased by blocking the exit of the hole so that the high-pressure gases cannot rapidly escape through the collar of the hole. This procedure increases the duration of high gas pressure in the hole and thereby allows more high-pressure gas to enter fractures in the rock so as to extend and widen them. The undesired escape of gas from the collar of the hole usually is prevented by filling at least part of the empty portion of the hole between the explosive charge and the collar with clay, sand, gravel, drill cuttings, or water. This material is called "stemming" and its emplacement is called "stemming the hole".
In the blasting of deep ore bodies with explosive charges preparatory to the leaching of the ore in place, a shot hole may have neighboring holes that are not charged with explosive. The effectiveness of an explosion in such a shot hole may be reduced by the escape of explosion gases from the neighboring holes, and thus it may be desirable to stem them as well as the shot hole.
In some situations, for example when gas, oil, or water wells are being stimulated with explosives, or deep ore is being blasted preparatory to being leached in place, it is necessary to remove the stemming after the blast so that another, larger charge may be placed in the cavity produced by the previous charge, or so that oil, gas, water, copper, uranium, or other mineral values may be extracted from the hole, or so that lixiviant solutions may be pumped down the hole to extract mineral values from the surrounding rock. In other cases, removal of the stemming may be desired in order to allow a failed charge to be reprimed.
Solid stemming generally is removed by drilling it out or by blowing it out with compressed air. Such an operation may be potentially hazardous if an unshot charge lies below the stemming, and, in any case, it is expensive. Also, the driller may not stop drilling when he has reached the base of the stemming, but may continue drilling into the hole interval that has been stimulated by the explosion. This can plug the fractures generated by the explosion and thereby damage the well.
U.S. Pat. No. 2,707,436, issued to H. D. McCool, describes a method of fracturing a subsurface formation with an explosive charge wherein a solid plug of frozen fluid is to be formed in the borehole above the charge to provide stemming. A fluid such as water or a drilling mud is introduced into the hole, and thereafter is to be cooled and frozen to form a solid columnar plug. The explosive charge is to be detonated while the plug is solid. Removal of the plug is accomplished simply by allowing it to melt by absorbing the heat resulting from the explosion and the heat from the formation itself. This technique avoids the necessity of post-explosion drilling but suffers from several disadvantages. First, the stemming required may be on the order of about 30 meters in length. A large amount of fluid must be introduced into the hole and some of this undoubtedly will be absorbed into the surrounding formation, with possible deleterious effect on the fluid conductivity of the fractures produced by the explosion when the rock contains clays that can swell by absorbing the fluid introduced for plug formation. Such swelling of the rock and the associated softening and decrepitation thereof can cause closure or plugging of the fractures. Such effects are common when water is the fluid.
Also, depending on the location and nature of the formation, the ambient temperature of the rock surrounding the hole may well be in the range of about from 30.degree. C. to 50.degree. C. It may be very difficult to cool the long column of fluid down to the vicinity of its freezing point, to freeze the entire column solid, and to keep it in the frozen condition for several hours as is often desirable before firing the charge. If a layer of the fluid at its interface with the warmer wall of the hole should remain unfrozen, or become so, before the explosive charge is fired, the explosion gases may eject the entire plug from the hole, obviously destroying its effectiveness. Furthermore it is relatively difficult to control and predict the condition of such stemming over time because its state is determined first by a freezing process and then a melting process.
According to the aforementioned U.S. Pat. No. 2,707,436, an endothermic reactant, preferably dry ice, is introduced into the fluid material standing in the borehole, in an amount sufficient to freeze the fluid material. The endothermic reactant is used as a coolant in the process of the plug's formation, and escapes as a gas during the freezing process. The patentee stresses the importance of (a) having an impervious solid plug which fills all cavities and irregularities in the borehole wall, and (b) forming the solid plug by freezing the fluid material in the borehole, which is indicated to give good friction hold due to the expansion of the fluid on freezing.