Microscopic pore structures and controlling factors of the Micro- and Nano- Scale tight sandstone reservoirs in the Western Sichuan Depression, China

Q. Wang, D. Chen, B. Zhang, L. Li
China University of Petroleum,
China

Keywords: tight sandstone gas, microscopic pore structures, micro- and nano- scale reservoirs

Summary:

Western Sichuan Depression (WSD) has a natural gas resource of about 31646.98×108 m3 and 251 commercial gas wells among which the largest single well productivity can be as high as 151×104 m3/d (Well X851), indicating huge exploration potential. However, complex geological condition, complicated gas-water relation, large amounts of nano-scale reservoir and the lack of study in microcosmic field restrict the exploration and development of tight sandstone gas. In this study, we choose the samples from the Upper Triassic to Jurassic to analyze their pore throat morphology, radius, distributions and relationship with the use of cast thin section, field emission scanning electron microscopy, micro CT scan, constant-rate mercury injection and high-pressure mercury injection in quality and quantity. The result shows WSD has a high densification degree and strong heterogeneity with low to medium porosity (5.16%~16.75%, with an average of 8.13%), low permeability (3 μm) are rare which are common in conventional reservoirs. Cementation is the main cause of this distribution because of the cements including calcite (8.1%), siliceous cement (4.6% in average), kaolinite (4.2%) and chlorite (3.9%) can block throats to make throat radius decrease (decreased by 76.8% in average) and resulting in lower permeability(0.455×10-3 μm2 in average) compare to conventional reservoirs in WSD (43.831×10-3 μm2). The pore throat relationship is complex for the difference in ratio of pore-throat radius (99.5~377.0 in average) mainly from 0 to 200 (33.1%) and from 200 to 400 (49.2%) which correlate to different water-gas relationship. This different ratio is attribute to dissolution that can enrich local intergranular pores with their pore radius but without throat radius because of the narrow space for pore fluids to pass through. Microscopic pore structure and influence factors on pore development can provide an important basis for data support to further explain gas filling process in tight gas reservoirs, gas distribution variability and gas-water relationship in WSD, which is of great significance for the study of reservoir heterogeneity and lays a foundation for the further development of tight gas reservoirs.