Dynamic stimulation techniques that 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 pre-existing fracture network. Previous studies have demonstrated that detrimental effects can occur with high-strength explosive techniques and that these effects can be avoided through the use of propellants.1,2  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 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,3 

  • Case B - a decoupled explosive with propellant booster,3 

  • Case C - a small-diameter propellant charge with pressurized water pad,4 

  • Case D - three successive shots of Case C, and

  • Case E - a full-diameter charge of a progressively-burning propellant.2 

While direct observation by mineback is highly beneficial, evaluation and analysis of these test results also depended heavily on other diagnostics. Thirty-six stress-meters 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 evaluations include TV log, caliper log, and 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.

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