Pore structure and multi-fractal analysis of tight sandstone using MIP, NMR and NMRC methods: A case study from the Kuqa depression, China

The pore structure and fractal characteristic of tight gas sandstone from Cretaceous Bashijiqike Formation in Kuqa depression were investigated, using thin section, scanning electronic microscope (SEM), mercury intrusion porosimetry (MIP), nuclear magnetic resonance (NMR) and NMR cryoporometry (NMRC) measurements. The pores of tight sandstone samples mainly contain residual primary intergranular pores, intergranular and intragranular dissolved pores, micropores related to clay aggregates and microfractures. The MIP, NMR and NMRC analyses show that the pore structure of tight sandstone is complex and heterogeneous at different pore scales, displaying as a multi-fractal feature. Fractal dimension Dg-s (with respect to small pore throats) from MIP and Dn-1 (long T-2 relaxation time, representing large pores and fractures or movable-fluid pores) are negatively correlated with porosity, permeability, median pore throat radius and skewness, and have inverse relationships with bound water saturation (S-bnd). Fractal dimension Dg-b ( > 3.0, with respect to larger pore throats) and Dn-s ( < 2.0, short T-2 relaxation time, representing small pores or bound-water pores) from NMR don't present clear relationships with most of pore structure parameters. Fractal dimension Dc-(1) and Dc-(2) from NMRC show more complex or inconspicuous relationships with pore structure parameters, in the pore diameter size < 500 nm. Hence, Dg-s from MIP and Dn-1 from NMR can be considered as effective parameters for pore structure and fluid properties characterization of tight sandstone. Fractal dimension Dg-b ( > 3.0) and Dn-s ( < 2.0) have no practical significance to characterize the pore structure of tight sandstone. The formation of residual primary intergranular pores and intergranular dissolution pores can reduce the fractal dimension which could be an important sign of effective reservoirs in the study area.