Abstract
The upper and lower limits and grading evaluation scheme of tight gas reservoirs (TGRs) at a microscopic pore structure level were explored by conducting high-pressure mercury intrusion porosimetry (HMIP), rate-controlled mercury intrusion porosimetry, nuclear magnetic resonance (NMR), and wettability experiments on tight rock samples from the Lower Cretaceous Shahezi (SHZ) Formation in the Xujiaweizi Rift, northern Songliao Basin. The results showed that the upper throat radius limit of the SHZ TGRs corresponds to 1.86 mu m, derived from the microscopic equilibrium state of buoyance and capillary forces. The above value is closely related to several geological factors, including a pore-to-throat radius ratio, the density difference between formation water and gas, formation dip angle, gas-water interfacial tension, and wettability. The SHZ Formation enters a tight reservoir stage when the burial depth is deeper than 3460 m. A new method integrating HMIP and NMR experiments was proposed to estimate the thickness of the adsorbed water film with a value of similar to 21.56 nm. The theoretical lower throat radius limit of the SHZ TGRs was determined as similar to 22 nm, considering the individual methane dimension. Natural gas can be scarcely injected into tight reservoirs below the theoretical lower limit, serving as theoretically invalid tight reservoirs in the study area. From the configuration relationship between pores and throats, two TGR types were identified in the SHZ Formation (types I and II). Type-I TGRs are char-acterized by relatively excellent seepage capacity, with the maximum connected throat radius and permeability of 0.4 mu m(-1).86 mu m and (0.015-0.27) x 10(-3) mu m(2), respectively, and their storage space mainly comprises abundant dissolution and residual intergranular pores. Furthermore, an adequate storage space (3.6% to more than 12%) and an appropriate burial depth (<5000 m) promote type-I TGRs to be the most favorable targets for current SHZ tight gas accumulation development in the Xujiaweizi Rift.
NSTL
Elsevier