Characterization of full pore size distribution and its significance to macroscopic physical parameters in tight glutenites

Characterization of microscopic pore structure plays an important role in understanding transport and accumulation mechanism of tight gas accumulation. Optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen gas adsorption (N2GA), mercury intrusion porosimetry (MIP) and nuclear magnetic resonance (NMR) experiments were performed on tight glutenite samples from the Lower Cretaceous Shahezi Formation in the northern Songliao Basin of China in order to gain insight into the microscopic pore structure (e.g., pore types and full pore size distribution), and to discuss the control of full pore structure properties on macroscopic physical parameters. The results show that the tight glutenite samples in this study contain four types of pores: inter-granular pores, dissolution pores, inter-crystalline pores and even some pores associated to micro-fractures. The inter-crystalline pores and inter-granular/dissolution pores can be determined by the surface fractal dimensions and mercury intrusion features because of different pore connectivity. The combination of N2GA and MIP were applied as complementary techniques and revealed that the studied samples have a broad pore size distribution ranging from 2 nm to 200 μm (mainly from 10 nm to 2 μm). Nano-scaled pores become the crucial storage space in tight glutenite samples. A total porosity derived from full pore size distribution curves is between 1.18% and 5.64%, which is closer helium porosity than that originated from N2GA and MIP techniques, respectively. Tight glutenite samples with a greater proportion of small pores (diameter<50 nm) mainly corresponding to inter-crystalline pores always have worse macroscopic physical parameters. However, tight glutenite samples with a greater proportion of large pores (diameter>200 nm) mainly corresponding to inter-granular/dissolution pores always correspond to better macroscopic physical parameters. The development of different pore types controls the macroscopic physical parameters. A series of empirical equations developed from multiple regression analysis between permeability, porosity and pore radius were established in tight glutenite samples. Pore radius corresponding to 15% cumulative pore volume from the combination of N2GA and MIP techniques is the best permeability estimator for tight glutenite samples.