In Situ Evaluation Of Several Tailored-Pulse Well-Shooting Concepts

Abstract

Abstract Dynamic stimulation techniques that produce multiple fracturing in a wellbore produce multiple fracturing in a wellbore are being investigated for enhanced gas recovery. Multiple fracturing appears to be especially promising for stimulating naturally-fractured reservoirs, such as the Devonian shales, since this may be the most effective technique for connecting a wellbore to a preexisting fracture network. Previous studies have demonstrated that detrimental effects can occur with high-strength explosive techniques and that these effects be avoided through the use of propellants. The use of propellants and other so-called tailored-pulse techniques depend on a controlled pressure-time behavior to minimize wellbore damage and maximize fracture growth by gas penetration. This paper describes a series of five full-scale tests performed to evaluate various multi-frac concepts. The tests were conducted at the Nevada Test Site in cased, horizontal boreholes drilled in ash-fall tuff from a tunnel under 430 m of overburden. This site provides both realistic in situ conditions provides both realistic in situ conditions for the tests and access to the stimulated regions by mineback which permits direct observation of results. The five tailored-pulse concepts tested involve: Case A - a decoupled explosive, Case B - a decoupled explosive with propellant booster, Case C - a small-diameter propellantcharge with pressurized waterpad, Case D - three successive shots of Case C, and Case E - a full-diameter charge of aprogressively-burning propellant. While direct observation by mineback is highly beneficial, evaluation and analysis of these test results also depended heavily on other diagnostics. Thirty-six stressmeters and accelerometers were fielded in the surrounding rock to record the dynamic disturbances, and each borehole contained transducers to measure the actual cavity pressures. Pre-test and post-test pressures. Pre-test and post-test evaluations include TV log, caliper log, and permeability measurements. Permeability, permeability measurements. Permeability, which evaluates the effectiveness of the created fracture network to transmit fluids, was determined by analysis of constant-pressure, water-injection tests and the subsequent pressure decline after shut-in. Results show a large increase in formation permeability for Case E, modest increases for Cases B, C, and D and a decrease for Case A that appears due to the formation of a stress cage. A comparison of Case E results with previous tests suggests a multiple fracture criterion based on pressure rate with little effect of peak pressures. Introduction 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 this in days when a water well was sometimes rejuvenated by shooting a rifle down the well. Well shooting as discussed herein refers to any rapid release of energy from a chemical reaction in a wellbore for the purpose of stimulating production, presumably purpose of stimulating production, presumably by fracturing the reservoir rock. This includes explosives (solid, liquid, and gas) and propellants that deflagrate rather than explode. 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. P. 105