Occurrence and influence of residual gas released by crush methods on pore structure in Longmaxi shale in Yangtze Plate, Southern China

Abstract

The composition of gas released under vacuum by crushing from the gas shale of Longmaxi Formation in Upper Yangtze Plate, Southern China was systematically investigated in this study. The effect of residual gas release on pore structures was checked using low-pressure nitrogen adsorption techniques. The influence of particle size on the determination of pore structure characteristics was considered. Using the Frenkel-Halsey-Hill method from low-pressure nitrogen adsorption data, the fractal dimensions were identified at relative pressures of 0−0.5 and 0.5−1 as D1 and D2, respectively, and the evolution of fractal features related to gas release was also discussed. The results showed that a variety component of residual gas was released from all shale samples, containing hydrocarbon gas of CH4 (29.58% −92.53%), C2H6 (0.97% −2.89%), C3H8 (0.01% −0.65%), and also some non-hydrocarbon gas such as CO2 (3.54% − 67.09%) and N2 (1.88%−8.07%). The total yield of residual gas was in a range from 6.1 μL/g to 17.0 μL/g related to rock weight. The geochemical and mineralogical analysis suggested that the residual gas yield was positively correlated with quartz (R2=0.5480) content. The residual gas released shale sample has a higher surface area of 17.20−25.03 m2/g and the nitrogen adsorption capacity in a range of 27.32−40.86 ml/g that is relatively higher than the original samples (with 9.22−16.30 m2/g and 10.84−17.55 ml/g). Clearer hysteresis loop was observed for the original shale sample in nitrogen adsorption-desorption isotherms than residual gas released sample. Pore structure analysis showed that the proportions of micro-, meso- and macropores were changed as micropores decreased while meso- and macropores increased. The fractal dimensions D1 were in range from 2.5466 to 2.6117 and D2 from 2.6998 to 2.7119 for the residual gas released shale, which is smaller than the original shale. This factor may indicate that the pore in residual gas released shale was more homogeneous than the original shale. The results indicated that both residual gas and their pore space have few contributions to shale gas production and effective reservoir evaluation. The larger fragments samples of granular rather than powdery smaller than 60 mesh fraction of shale seem to be better for performing effective pore structure analysis to the Longmaxi shale.