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Abstract

DOI: 10.1306/09051817138

Wettability and connectivity of overmature shales in the Fuling gas field, Sichuan Basin (China)

Rui Yang,1 Qinhong Hu,2 Sheng He,3 Fang Hao,4 Xusheng Guo,5 Jizheng Yi,6 and Mengdi Sun7

1Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan, China; [email protected]; [email protected]
2School of Geosciences, China University of Petroleum (East China), Qingdao, Shandong, China; Department of Earth and Environmental Sciences, University of Texas at Arlington, Arlington, Texas; [email protected]
3Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan, China; [email protected]
4School of Geosciences, China University of Petroleum (East China), Qingdao, Shandong, China
5Exploration Company, Sinopec, Chengdu, China
6Jianghan Oilfield Branch Company, Sinopec, Wuhan, China
7Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan, China

ABSTRACT

The Fuling gas field in Sichuan Basin, China, has produced greater than 1.5 × 1010 m3 (0.53 tcf) of natural gas from overmature Upper Ordovician Wufeng and lower Silurian Longmaxi shales. To systemically investigate the characteristics of wettability and connectivity and to understand the underlying causes of production behavior, we study five samples of Wufeng and Longmaxi shales with different total organic carbon contents and mineral compositions. Complementary approaches include mercury intrusion capillary pressure (MICP), contact angle measurement, spontaneous imbibition and saturated diffusion, and tracer (both nonsorbing and sorbing) migration mapped via laser ablation inductively coupled plasma mass spectrometry. According to measured contact angles and imbibition tests conducted on aqueous (deionized water and brine) and oleic (n-decane) phases, Wufeng and Longmaxi shales are strongly oil wet and moderately strong water wet. The lower boundary of estimated permeability obtained from n-decane imbibition can reach 137 nd, which is higher than the geometric mean permeability derived from the MICP method (5.5–68.8 nd). Effective diffusion coefficients of the Wufeng and Longmaxi shales are in the range of 10−13 m2/s (1.1 × 10−12 ft2/s). Tests of imbibition and saturated diffusion using tracer-containing brine show that concentrations of nanometer-sized tracers decrease rapidly (a factor of >10) over a migration distance of a few millimeters from the sample edge, suggesting the presence of poorly edge-connected water-wet pores. Sparsely connected hydrophilic pores, mixed wettability, and highly restricted pathways collectively contribute to the limited migration of nano-sized tracers, which probably results in the production behavior of initial steep decline and low overall recovery in the Fuling gas field.

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