Pulsed Power Plasma Stimulation Technique - Experimental Study on Single Pulse Test for Fractures Initiation

This paper reports an experimental study of the application of Pulsed Power Plasma Discharge (P³D) to wellbore laboratory models. The ultimate goal is to investigate the potential of intense electrical discharge as a cost-effective wellbore stimulation technique that does not require the massive injection of water or the use of chemical additives. Considered complimentary to hydraulic fracturing, the technique envisions the stimulation of the wellbore by creating self-propped short fractures and by changing the near wellbore permeability. This study is a laboratory scale investigation of the technique. In these experiments, a plasma pulse is created within a concrete reservoir model by rapid electrical discharge. The plasma arc generated by the discharge creates a pressure shock wave and electromagnetic fields. The generated high-pressure shock waves were shown to create multiple fractures emanating from the discharge. The generated EM field was measured and modeled in a simulation and shown to have potential in reservoir monitoring applications. Two groups of cylindrical test samples were built under different conditions: water-to-cement ratio, wellbore size, and tubing perforations. Their purpose was to test the effect of the high-pressure shockwave over a range mimicking common rock properties and current completion methods. Experiments using 2kJ of stored energy achieved peak pressures of up 200 MPa (29,000 psi). These shockwave pressures were generated by typical plasma durations of 6–16.5 microseconds. Use of appropriate ionizable material in the plasma discharge creates thermite ionic recombination reactions that enhance the produced mechanical energy by up to two orders of magnitude. The high peak pressures generated in these experiments indicate the thermite reaction made a significant contribution. However, no separate pressure pulse due to a thermite component was detected suggesting that electrical discharge and ionic recombination occurred almost simultaneously. In all cases, significant multiple micro-fractures were created and propagated. Small-bore vertical models with casing perforations produced the most significant effects. The electromagnetic fields generated by an intense current arc of short length and duration were simulated in a finite element model. In all these experiments, the relevant electromagnetic fields were measured at some distance from the plasma arc and compared with the finite element model. The transient electromagnetic fields generated by the arc current operate on microsecond time scales. In both laboratory and near zone field applications where transient magnetic effects dominate, the techniques to measure dynamic magnetic fields are limited. Therefore flat induction search coils based on Faraday's law were used to measure the time derivative of the magnetic fields generated. In order to compare with the experimentally measured data from the search coils the time derivatives of the simulated fields were calculated. Good agreement was obtained between simulation and experiment. Matching the simulation with experiment at laboratory scale provides support for extension of the model to reservoir scale.