Radial Permeability Measurement for Shale Using Variable Pressure Gradients

Shale gas is becoming an important addition to worldwide energy supply with permeability a critical controlling factor for gas production. Helium permeability determined using small pressure gradient (SPG)methods may lead to erroneous results when applied to actual field production with variable pressure gradients (VPG). In this paper, a VPG method using real gas (rather than He) is established to render permeability measurements more representative of reservoir conditions and hence response. Dynamic methane production experiments are performed to measure permeability using the annular space in shale cores. Boundary pressure is maintained constant within each production stage with a designated pressure gradient and the gas production with time is measured. A mathematical model explicitly accommodating gas desorption uses pseudo-pressure and normalized time to accommodate the effects of variations in pressure-dependent viscosity and compressibility. General and approximate solutions to the model are obtained and discussed. These provide a convenient approach to estimate radial permeability in the core by nonlinear fitting to match the approximate solution with the recorded gas production data. Results indicate that the radial permeability of the shale determined with methane is of the order of of 10-6∼10-5md and decreases with an increase in average pore pressure. This is contrary to the observed change in permeability estimated with helium. Permeability errors obtained from the SPG method using helium are several times greater than those obtained from the VPG method using methane. Bedding geometry has a significant influence on shale permeability. The superiority of the VPG method is confirmed by comparing permeability test results obtained from both VPG and SPG methods. The VPG method has two advantages: The first is that reservoir gas can be used in the VPG method instead of helium, better incorporating potential desorption impacts in permeability evltuion. The second is that realistic pressure dependent impacts can be accurately accommodated, making this method more applicable to gas production conditions in the reservoir. Although several assumptions are used, the results obtained from the VPG method are much closer to reality and may be directly used for actual gas production evaluation and prediction.