Abstract
Gas transport in shale matrix is complex due to multiple mechanisms and is difficult to be investigated by macroscopic experiment. For Gas Research Institute (GRI) method, which is the most accepted one for gas transport investigation in shale matrix, the apparatus was modified by adding an automatic gas supplement and pressurization (AGSP) system, and a numerical model considering the variation of real gas property and the Klinkenberg effect was established for data interpretation. Then, the intrinsic permeability and Klinkenberg coefficient were effectively obtained by maintaining high expanding speed of gas in apparatus and eliminating the negative effect of low filling degree of sample. By analysis, the ideal gas transports faster than real gas due to the viscosity difference at low pressure and the deviation factor difference at high pressure. For Wufeng-Longmaxi shale matrix, the positive influence of Klinkenberg effect on gas transport would attenuate with increasing pressure and is more powerful than bulk shale sample with fractures. Therefore, the gas transport in real shale matrix could be well known, which is meaningful to production forecast and evaluation in oil and gas fields.
Highlights
The cognition of gas transport in shale matrix has direct connection to the production of shale gas [1,2,3], which is significant to the extraction of shale gas reservoir
By experimental evaluation of AGSP system, the gas expands from reference chamber (RC) to sample chamber (SC) much faster than the original apparatus, and the negative effect of low filling degree on gas expansion would be eliminated
(2) Considering the real gas effect and Klinkenberg effect, an explicit numerical model was established with variable gas properties and apparent permeability
Summary
The cognition of gas transport in shale matrix has direct connection to the production of shale gas [1,2,3], which is significant to the extraction of shale gas reservoir. For steady-state experiment, the permeability of shale core plug was tested at constant pore pressure and overburden stress conditions [10]. For non-steady-state experiment, a pressure pulse was applied to the shale pore plug [7, 11,12,13], or crushed shale rock [11, 14,15,16,17], and the permeability of shale could be obtained by analyzing the variable pressure data. The non-steady-state experiment with crushed particles, called Gas Research Institute (GRI) method, was the most recognized one in studying the gas transport process in shale matrix [13, 15, 21, 22], due to the fact that the particle was small enough to reduce the existence of microcracks greatly
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