An integrated method of measuring gas permeability and diffusion coefficient simultaneously via pressure decay tests in shale

In shale gas plays, both the permeability and diffusion coefficient are indispensible parameters to reservoir assessment and production forecast. Unlike previous works which measured the two parameters individually, this paper first proposed an integrated method of measuring the gas permeability and diffusion coefficient simultaneously with a single pressure decay curve. The experiments were conducted through one-chamber pressure decay tests (PDT) with shale core plugs under stressful conditions, simplifying the apparatus and time saving. According to the gas transport mechanisms, the whole gas flow in shale plugs was divided into three stages: viscous and slip flow stage (stage I), non-surface diffusion stage (stage II), and surface diffusion stage (stage III). Then using the linear permeability and bidisperse diffusion models, the permeability was calculated to be on the order of 10− 5 mD, and the effective diffusion coefficients (D/r2) of stages II and III were about 10− 5 s− 1 and 10− 7 s− 1, respectively, which got well agreements with the existed methods. Given that the multiple gas transport stages in shale are actually interacted and coupled with each other, the integrated measuring values in this paper may be more representative.