A New Perspective on Well Shooting-Behavior of Deeply Buried Explosions and Deflagrations

Abstract

Summary Explosive charges sometimes are detonated in wellbores in efforts to stimulate production of gas or oil; often these efforts meet with little or no success. In this paper, the physical phenomena that control the dynamic responses of rock to deeply buried charges are discussed and reasons for the lack of success in some explosive stimulation operations are identified. The assessment of these physical phenomena has provided a basis for defining alternative dynamic well stimulation techniques. Introduction This paper describes and summarizes the results of nuclear containment studies that are not readily available to petroleum engineers and discusses the significance and implications of these results to well shooting techniques. No attempt is made to review the vast literature of explosives, well shooting experiments, or actual results from explosive stimulation operations.Oil and gas wells have been stimulated with high-energy explosives since the late 1800's. It appears, however, that the term "well shooting" originated many years before when a water well sometimes was rejuvenated by pointing a rifle down the well and pulling the trigger. This method of "shooting" a well is used even today, and presumably the concussion helps to break out encrustation. Well shooting, as discussed here, refers to any rapid release of energy from a chemical reaction in a wellbore for the purpose of stimulating production, presumably by fracturing the reservoir rock. This includes use of explosives (solid, liquid, and gas), which detonate, as well as propellants, which deflagrate. The distinction between these two energy-release mechanisms is noted again when alternative well shooting techniques are discussed later. In a broad sense, well shooting has been applied in several geotechnical fields - e.g., preparation of oil shale beds for true in-situ processing, preparation of underground mineral deposits for solution mining, etc.With regard to oil and gas wells, well shooting is applied to introduce multiple fractures around a borehole rather than a single large fracture, as is created by hydraulic fracturing. Multiple fractures are desirable for eliminating skin damage, which may occur during drilling, and for connecting the wellbore to an existing fracture system, as might be present in a gas-bearing Devonian shale. Connecting a well to a fracture network often is not possible by hydraulic fracturing, since there is little or no control over fracture orientation and the hydrofracture often will run parallel to existing fractures.The term well shooting has many unfavorable connotations and conjures up many thoughts in the minds of petroleum engineers. Problems of wellbore damage, safety hazards, and unpredictable results have reduced the relative number of wells stimulated by high-strength explosives. The lack of predictability and the lack of definitive guidance in well shooting is pointed out in a statement from the Blasters' Handbook: Detailed instructions for shooting wells cannot be given since there are too many complicating factors. It is always advisable to have the work carried out by an experienced person, and even then there can be no assurance that the results will be satisfactory. There seems to have been few direct attempts to perform definitive studies of the physical phenomena involved with wellbore detonations to improve stimulation techniques. JPT P. 1305^