Influence of water on shale pore heterogeneity and the implications for shale gas-bearing property - A case study of marine Longmaxi Formation shale in northern Guizhou

Shale pore heterogeneity (as described by the fractal dimension) may influence the shale gas-bearing property (GBP). Previous studies on the fractal dimension and its influence on shale GBP were mainly focused on the dry shale samples. Our study investigates the pore structure characteristics under both the dry and moist conditions utilizing low-pressure N2/CO2 adsorption experiments, water-bearing property, and GBP on the eleven as-received (moist) marine Longmaxi shale sampled from the XK3 well in northern Guizhou. Moreover, the fractal dimensions D1 (P/P0: 0–0.5) and D2 (P/P0: 0.5–1.0) for the dry and moist shales were determined respectively, using low-pressure N2 adsorption data based on the Frenkel-Halsey-Hill (FHH) method. The results show that the ranges of D1 and D2 for the dry shales are 2.5759–2.7387 and 2.7681–2.8756, respectively, with averages of 2.6970 and 2.8470, while those for the moist shales are 2.0879–2.2215 and 2.6190–2.7252 respectively (averages 2.1591 and 2.6903). Shale water occurs mainly on nonmicropores (>2 nm) surfaces, and part of water fills the micropores (<2 nm), which reduces the nonmicropores BET specific surface area (SBET), micropore volume (Vmic), and pore size distribution (PSD) and increases the average pore size (APS), thus reducing the D1 and D2 (19.94 and 5.50% on average, respectively). The shales have high gas contents with an average of 2.59 m3/t, which is dominated by adsorbed gas (75.21% on average). Compared with D2, D1 is more suitable for indicating the pore heterogeneity (especially nonmicropores surface heterogeneity) and shows stronger positive linear relationships with the shale GBP, implying that a larger pore surface heterogeneity is favorable for gas adsorption. Water mainly occurs in the clay minerals and reduces the D1 of the clay pores, thus the TOC content controls the D1 and shale GBP.