Determination of shale matrix permeability through dynamic methane production experiments using variable pressure gradients

The accurate determination of shale matrix permeability is a crucial factor in simulating shale gas flow from matrix to fractures, evaluating reservoir potential and forecasting gas production. Current methods for measuring permeability with helium using small pressure gradients (SPG) may lead to erroneous results when applied to actual field production with significant and variable pressure gradients (VPG). This paper establishes a method that measures shale matrix permeability using real gas as close to the actual gas production conditions (PVT) as possible. The mathematical model accommodates desorption and uses both pseudo-pressure and normalized time to accommodate the effect of variations of pressure-dependent gas PVT propeties. Approximate analytical solutions allow the matrix permeability to be estimated by nonlinear fitting to match the approximate solution with the experimental data. Dynamic gas production tests are performed on powdered shale samples of 100~120 mesh with constant external pressure for each production stage with a designated pressure gradient. Both the normalized and actual time solutions are used to eatimate the shale matrix permeability. The results indicate that the normalized and actual time solutions are almost equivalent when external pressure is relatively high, and a deviation occurs between the two solutions at relatively low pressures. For the particular samples, the permeability is estimated of the order of magnitude of 10^-7md and decreases with a decrease in external pressure. Permeability errors are reduced by using the normalized time solution for parameter estimation at relatively low pressures. The normalized time transformation yields results obtained from the VPG method closer to actuality and may be directly used for gas production project design and production capacity prediction.