A permeability determination method for irregular rocks based on numerical Transient Pressure Pulse Decay (TPPD) testing

Abstract

Permeability is a key parameter characterizing the flow capacity within porous reservoirs. The Transient Pressure Pulse Decay (TPPD) method is a widely employed technique for permeability characterization; its reliance on the formula necessitates regularly shaped samples. However, samples often experience fragmentation during coring, making it difficult to obtain regular specimens. This study proposes an innovative method combining a modified TPPD experiment for irregular samples with a mathematical model describing the entire testing process, where permeability is determined through inverse problem optimization. Initially, the study conducts tests on regular samples. The results demonstrate strong agreement between the numerically simulated and experimentally measured pressure differential data, with fitting accuracy (R2) beyond 0.9776. Those confirm that the proposed model accurately captures the physical process of the TPPD experiment. The best inverted permeability of 0.148 mD (95 % CI: [0.142, 0.156] mD) closely matches the formula-derived value of 0.143 mD, further validating the model's accuracy. Subsequently, irregular specimens are prepared from the original regular samples and tested by the new method. The results show excellent fitting precision, with R2 consistently exceeding 0.9659, proving that the model can also precisely replicate the testing process for irregular samples. However, the permeability differs significantly from that of the original sample, which is attributed to imperfections in the preparation process. The new method has removed the limitation of laboratory permeability measurement on the shape of rocks, facilitating the research on the flow in unconventional reservoirs.