Unconventional reservoirs suffer from an ultra-low permeability, causing their drainage area to be limited to tens of feet. An efficient technique that can be used in combination with conventional hydraulic fracturing to increase the drainage area is pulse power plasma. In this study, we used an experimental approach to study the effect of pulsed power plasma discharge on the permeability change around wellbore under tri-axial confining stress conditions.
We designed and used equipment, allowing for the generation of the shock wave in a true tri-axial cell to perform the analysis in this study. The equipment has a capacity for rock samples of 14 in on each edge with 1.5 in diameter well in the center. Using the equipment, the stored electrical energy in capacitors is instantaneously released into a fusible link creating a thermite reaction, which creates a shock wave in the wellbore and is transmitted to the rock afterward. The shock wave affects the permeability of samples, even in situations where the generated stress loading is below rock strength.
Several types of material, such as limestone, sandstone, and concrete are tested in this study. Samples were investigated before and after the electrohydraulic discharge to find the extent and magnitude of the induced fractures (permeability enhancement) and their relationship with the released energy. Also, the effect of repetitive low-magnitude shock waves for creating micro-cracks in rock is studied. It is observed that even under sub-critical loading conditions, micro-cracks are generated in the rock samples that might be a result of the main shockwave or reflection of the stress wave from the boundary. These fractures were less controlled by the stress orientation as compared with hydraulic fractures. However, they contributed to the permeability enhancement around the wellbore that can be up to orders in magnitude. Finally, the optimum discharge energy for the maximum permeability enhancement is suggested.
In this study, for the first time, we tested the rock under confining stresses and imaged them using computer tomography (CT) scanning. Also, change of permeability around the wellbore using pulse power plasma is a novel use of pulse fracturing technology that can effectively be mixed with hydraulic fracturing treatment in unconventional reservoirs and maximize the stimulated reservoir volume (SRV). The results of this paper can help to maximize the EUR from the reservoir.