Full-range pore throat radius distribution and fractal characteristics of different types of reservoirs in deep carbonate gas reservoirs:A case study of Deng-4 Member in Gaoshiti-Moxi Area,Sichuan Basin

ZOU Rui; WANG Lu; LUO Ruilan; ZHANG Lin; DENG Hui; MENG Zhan; ZOU Run; ZHANG Yifan

doi:10.13247/j.cnki.jcumt.20240059

Abstract：The typical characteristics of deep carbonate gas reservoirs with diverse reservoir media and complex pore throat structure in deep carbonate reservoirs make it difficult to reveal their full-range pore throat distribution and fractal characteristics.Therefore,the high-pressure mercury injection and constant-rate mercury injection experiments were jointly applied to quantitatively characterize the full-range pore throat size distribution of various reservoir types in deep carbonate gas reservoirs in Sichuan Basin.Subsequently,the complexity and mi-cro-heterogeneity of pore throat structures at different scales were revealed through fractal the-ory.Finally,the relationship between fractal dimension and storage and percolation capacities was revealed,and the controlling factors of fractal dimension were analyzed.The results show that the full-range pore throat radius of four types of reservoirs in deep carbonate reservoirs is in the range of 3.6 nm and 500 μm,and all distribution curves have multi-peak characteristics.Deep carbonate gas reservoirs mainly develop nano-scale pore throats,but the differences in micro-scale pore throats are obvious.The average values of total fractal dimension(DT)of fracture-cavity type,cavity type,fracture type and pore type reservoirs are 2.438,2.509,2.762 and 2.804,respectively,reflecting the differences in micro-heterogeneity among different types of reservoirs.All four types of reservoirs have triple fractal characteristics of small pore—fine throat section(r＜0.027 μm),moderate pore—coarse throat section(0.027μm≤r≤3.431 μm),and large pore—fracture section(r＞3.431 μm).Moreover,the differ-ences between different types of reservoirs are mainly reflected in the small pore—fine throat section,resulting in the micro-heterogeneity of the reservoir being mainly affected by the pro-portion of pore throat volume and the value of fractal dimension in this section.The fractal di-mension of nano-scale pore throats is lower than that of micro-scale pore throats,indicating that the heterogeneity and complexity of micro-scale pore throats are stronger than those of nano-scale pore throats under the effects of fractures and cavities.The correlation between DT and porosity,displacement pressure,skewness,maximum mercury saturation,and sorting co-efficient is good,and the correlation between the fractal dimensions of nano-scale pore throats and small pore—fine throat sectionis relatively strong with these structural parameters,indica-ting that the small pore—fine throats at the nano-scale has a greater impact on the overall pore throat connectivity and heterogeneity of deep carbonate gas reservoirs.