Impact of Microscopic Pore Structures on Shale Stress Sensitivity
Rui Zhang, Zhengfu Ning, Feng Yang, and Xinwei Liao
China University of Petroleum, Beijing, China
The investigation of stress sensitivity is of primary importance during the life cycle of a shale gas reservoir. This work focuses on how the microscopic pore structure characteristics affecting the stress-dependent permeability of shale.
A pulse-decay permeability stress sensitivity measurements were performed on two shale samples in niutitang formation of lower Cambrian, southern Chongqing, China. The results demonstrated that the empirical correlation was obtained using the exponential function, which presented good curve fitting in two stress sensitivity samples.
The derivation of capillarity theory shows that a smaller capillary radius would be retained with increasingly effective stress in a higher stress-dependent permeability sample. Due to their easy-compressed shape, the slot nanopores in shale reservoirs lead to severely stress sensitivity. Combined with the petrographic information from SEM analyses, it appears that pore-throat size and shape are one of the controlling factors on stress sensitivity.
Nitrogen adsorption experiment results present the difference of pore-throat size frequency distribution in two samples. The sample with higher micropore frequency distribution but lower macropore frequency distribution is more likely to lose its gas flow channels. It is probably because that micropore close first with effective stress increased (Fig 1).In other words, the more easy closed micropores existed in all pore scales, the less flow channels left for macropores. The reduction of flow channels for macropores is another main reason of severely stress sensitivity.
AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013