Characterization of the pore system in an over-mature marine shale reservoir: A case study of a successful shale gas well in Southern Sichuan Basin, China

Abstract

During the past two years the shale gas exploration in Southern Sichuan basin received some exciting achievements. Data of a new appraisal well showed that the gas producrtions of vertical well and horizontal well are ~1.5 × 10⁴ m³/day/well (with maximum ~3.5 × 10⁴ m³/day/well) and ~12.5 × 10⁴ m³/day/well (with maximum ~40 × 10⁴ m³/day/well), respectively, indicating a good gas potential in this area. Eight core samples from the reservoir were investigated by using a carbon sulfur analyzer, microphotometry, x-ray diffractometry, field-emission scanning electron microscopy (FE-SEM), mercury injection porosimetry (MIP), and low-pressure nitrogen adsorption to obtain a better understanding of the reservoir characteristics of the Upper Ordovician-Lower Silurian organic-rich shale. Results show that the total organic carbon (TOC) content ranges from 0.5% to 5.9%, whereas the equivalent vitrinite reflectance (VRr) is between 2.8% and 3.0%. Pores in the studied samples were observed in three modes of occurrence, namely, interparticle pores, intraparticle pores, and intraparticle organic pores. The total porosity (P) ranges from 1.6% to 5.3%, and MIP data sets suggest that pores with throats larger than 20 nm contribute little to the pore volume. Low-pressure N₂ adsorption isotherms indicate that the total specific surface area (S_BET) ranges from 9.6 m²/g to 18.9 m²/g, and the pore volume (V) ranges from 0.011 cm³/g to 0.020 cm³/g. The plot of dV/dW versus W shows that the fine mesopores (pore size_(BJH) < 4 nm) mainly contribute to the pore volume. The P, S_BET, and V show a good positive correlation with TOC and a weak positive correlation with the total clay mineral content, thus indicating that the nanopores are mainly generated by the decomposition of organic matter. The reservoir characteristics of the Upper Ordovician-Lower Silurian organic-rich shale are comparable with commercial shale gas plays in North America. The sample gas contents with TOC >2% are more than 3.0 m³/ton. The observation can be a good reference for the future exploration and evaluation of reservoir in this area.

Keywords

Pore system / SEM imaging / Mercury injection porosimetry / Low-pressure nitrogen adsorption / Wufeng-Longmaxi gas shale

Acknowledgments

The Authors are grateful to Lei Xie, Xiaowei Yang, Bing Shu and Yanni Ma, for their help in sampling and field work. This study was Supported by the National Natural Science foundation of China (Grant No. 41302123), the Doctoral Program of Higher Education (Specialized Research Fund) of China (Grant No. 20125121130001), and the Science foundation of Education Department of Sichuan Province (Grant No. 13ZB0190).

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