Investigation on petrophysical properties of fractured tight gas sandstones: a case study of Jurassic Xujiahe Formation in Sichuan Basin, Southwest China

Integrated characterization methods from micro-scale to macro-scale were applied to thoroughly study petrophysical properties of the Upper Triassic Xujiahe fractured tight gas reservoirs. The lithology, pore types, pore structure, and porosity-permeability relationship were described based on experimental results of thin section analysis, computer tomography, and porosity and permeability measurements. And special attention was paid to characterize natural fractures and try to reveal the effects of fractures on gas storage and transportation. A dual porosity-dual permeability (DPDP) dynamic model of well L150 was established, and effects of natural fractures on natural gas productivity were studied. The lithology is mainly lithic arkose, feldspathic litharenite, and litharenite. The formation is highly tight due to compaction and cementation during the early and middle burial stages, and only some interparticle pores and grain dissolution pores remained. Fractures are classified into two types, which are structural fractures and interlayer fractures. The first type is caused by tectonic movement and is mainly developed in siltstone and fine sandstone, while the second type is developed between coarse sandstone beddings. Mercury injection capillary pressure experiment reveals that the pore size of the tight sandstone is 2–200 nm. Porosity of the samples is in the range of 2–6%, and permeability is 5 × 10 −3 –1 × 10 −1 mD. Yet permeability of some samples may be as large as 1000 mD because of the micro-fractures. Results of history matching and production predictions of the dynamic model indicate that natural fracture is important to natural gas production.