Classifying Multiscale Pores and Investigating Their Relationship with Porosity and Permeability in Tight Sandstone Gas Reservoirs

Since tight sandstone usually contains pores of multiscales and various types, it is important to classify pores in scale and investigate their distinct contributions to porosity and permeability for better understanding of the storage and percolation mechanism of tight gas sandstone reservoirs. In this study, rate-controlled porosimetry (RCP) was performed to probe the pore connectivity and fractal structures and classify pores' size, while low temperature N-2 adsorption and nuclear magnetic resonance (NMR) were conducted to determine the specific surface area (SSA) and the relative content for different scales of pores, respectively. Based on the differences in pore connectivity and the contributions to storage and percolation, pores in tight sandstone are divided into nanopores (mainly < 0.5 mu m), micropores (mainly 0.5-1.5 mu m), and mesopores (mainly > 1.5 mu m). Nanopores consist of the clay-associated pores and intraparticle dissolution pores, contributing to both percolation and storage, especially to the SSA; micropores comprise the narrow slits between grains and the quartz intercrystalline pores, mostly dominating the permeability, while mesopores, dominated by the interparticle-related pores, must be connected with micropores/nanopores and therefore mostly contribute to storage. The weak correlation between porosity and permeability is mainly attributed to the combination of diagenesis and compaction, because they damage the correlation of micropores content with porosity. For tight sandstones with the weak correlation between petrophysical properties, permeability established by producible porosity, Coates model, and Pittman method are better than that by the SDR (Schlumberger Doll Research) model. Tight sandstone reservoirs with different content of micropores and nanopores show a distinct gas storage and percolation mechanism; with decreasing microporosity, the contribution of nanopores becomes predominant, the adsorbed gas content becomes greater, and the decreasing rate in production with pressure decay becomes slow.