Pore structure characteristics of an ultradeep carbonate gas reservoir and their effects on gas storage and percolation capacities in the Deng IV member, Gaoshiti-Moxi Area, Sichuan Basin, SW China

Evaluating reservoir storage and percolation capacities, which are closely associated with pore morphology and connectivity, is significant for the exploration and exploitation of oil and gas fields. However, ultradeep carbonate gas reservoirs have extremely high heterogeneity and complexity compared to other relatively shallow reservoirs. Therefore, investigations into all aspects of ultradeep carbonate face enormous challenges. To the best of our knowledge, no systematic study on the pore structure characteristics of ultradeep carbonate gas reservoirs has been conducted to date. In this paper, the pore structure characteristics and their effects on the gas storage and percolation capacities of the Deng IV Member gas reservoir were investigated using conventional petrophysical measurements, casting thin section analysis, scanning electron microscopy (SEM), high pressure mercury intrusion (HPMI) tests, and multiscale CT scanning analysis. The results show that the lithology of the Deng IV Member reservoir is dominated by algal clotted dolomite, algal stromatolite dolomite and algal doloarenite. The conventional petrophysical measurements indicate that the average porosity and permeability are 3.91% and 1.02 mD, respectively. However, the high porosity (8-4%) and high permeability (10-10,000 mD) intervals are developed locally. The casting thin sections, SEM and two-dimensional CT analyses identify six main types of reservoir spaces in the Deng IV Member: interparticle dissolution pores, intercrystal dissolution pores, fenestriform cavities, dissolution cavities, structural fractures and dissolution fractures. Furthermore, three types of throats, that is, necking, tubular and lamellar, were also detected. Four types of reservoirs, namely, matrix type, pore type, cavity type and fracture-cavity type, were classified according to the comprehensive analysis of HPMI data and curves. Three-dimensional pore throat network topologies were reconstructed for four types of representative core samples based on the multiscale CT scanning data and maximum-ball method. The distribution characteristics of pore throat radii, volumes and coordination numbers reveal the following: the reservoir space of fracture-cavity type is dominated by cavities and multiscale pore throats with the best pore-throat connectivity; the reservoir space of cavity type is primarily controlled by cavities and large-scale pore throats with moderate connectivity; the reservoir space of pore type comprises multiscale pore throats with moderate connectivity; and most areas of matrix type reservoir are occupied by rock matrix, and the pore-throat connectivity is the worst. In conclusion, the development of fractures and bedding cavities considerably improves the reservoir percolation capacity, and the development of cavities is an important supplement to the reservoir storage capacity. The reasonable combination of cavities and fractures is the basis for the efficient development of ultradeep carbonate gas reservoirs.