Recognitions on the flow mechanism of shale during the plug pulse decay measurement

Abstract When using the plug pulse decay method to measure shale permeability, a comprehensive analysis of various factors influencing the measurement and analysis precision is conducive to improving the accuracy of the test result. A great number of nano-scale pores are developed in shale, so the lower the test pressure is, the more easily the non-Darcy flow tends to emerge. In order to ensure the gas is in the form of Darcy flow in the process of measurement, we put forward the lower pressure limit of Darcy flow in pores according to the definition of Knudsen number. Then, the gas desorption-flow coupling model for the gas flow process during the plug pulse decay measurement was established by considering the effects of gas on the adsorption in organic pores. After the partial differential equation set was solved and derived, the corresponding permeability calculation method was proposed. Finally, after the plug pulse decay measurement, the supporting nitrogen isothermal adsorption test and methane isothermal adsorption test were conducted on shale samples. And the following research results were obtained. First, during the plug pulse decay measurement, the flow of gas in rock samples is one-dimensional linear, so when establishing the flow equation, the gas pseudopressure can be replaced with the gas pressure to simplify the calculation. Second, the adsorption of nitrogen in shale is much less than that of methane in shale, so the influence of nitrogen desorption on the flow is negligible when the pressure difference (i.e.