Effects of hydration on physical properties and rock microstructure of shale oil reservoir in the Triassic Chang 7 Member of Southern Ordos Basin

ABSTRACT

Shale oil is an unconventional oil resource with significant reserves and considerable prospects. There is a substantial shale oil reservoir in the Triassic Chang 7 Member of the Southern Ordos Basin. However, hydration would change the physical properties and rock microstructure during the drilling process. Drilling stuck, lost circulation, collapse, and other accidents easily occur, which restricts the efficient and safe development of shale oil. In order to study the effect of hydration on physical properties and microstructure of shale oil reservoir, mineral composition test, hydration test, and rock microstructure test were carried out on the downhole cores. The results show that there is a large number of laminated thin sandstone in the upper shale of the reservoir and the weak cementation between shale and sandstone provides a channel for hydration. The lower part of the reservoir is a tuff with a small amount of irregular structural fractures. Compared with the weak hydration reaction of shale, the tuff expanded, deformed, and softened after encountering water, and its water sensitivity is more obvious than that of shale section. The porosity and permeability of the reservoir are very low, and the heterogeneity is strong. The rock pore size is concentrated in the range of 1–10 nm and 50–1000 nm, in which the intergranular and intragranular pore content is high. Hydration would lead to mineral shedding and fracture extension, and the number and size of pores and fractures would increase. Porosity and permeability also increased with the change of rock structure, which was obvious in the early hydration stage of the downhole cores.

KEYWORDS:

• Hydration
• mineral composition
• rock microstructure
• shale oil
• Triassic Chang 7 Member
• tuff

Acknowledgments

The authors would like to express their gratitude to the editors and anonymous reviewers for their constructive comments on the draft paper.

Additional information

Funding

The financial support from the National Science and Technology Program during the 13 Five-year Plan Period (Grant No. 2017ZX05049), the National Natural Science Foundation of China (Grant No. 51774246 and 51474185), and the National Key Basic Research and Development Program, (973 Program) China (Grant No.2013CB228003) is appreciated.

Notes on contributors

Pengfei Zhao

Pengfei Zhao is a doctoral student majoring in oil and gas well engineering of Southwest Petroleum University. His interests include unconventional petroleum geology, unconventional oil development, and mechanics related to petroleum engineering. He is the corresponding author.

Xiangyu Fan

Xiangyu Fan is a professor of oil and gas well engineering in the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation at Southwest Petroleum University. He received his Ph.D. from Southwest Petroleum University. His interests include tectonic stressfields, geological engineering, and rock mechanics related to petroleum engineering.

Qiangui Zhang

Qiangui Zhang is an associate professor of oil and gas well engineering at Southwest Petroleum University. He received his Ph.D. from Chongqing University. His research focuses on rock mechanics and geotechnical engineering stability, wellbore stability related rock mechanics, unconventional gas development, and engineering geomechanics of rock mass.

Mingming Zhang

Mingming Zhang is a doctoral student majoring in oil and gas well engineering of Southwest Petroleum University. His interests include wellbore stability related rock mechanics and tectonic stressfields.

Bowei Yao

Bowei Yao is a doctoral student majoring in oil and gas well engineering of Southwest Petroleum University. His interests include logging of oil and gas well engineering and natural gas hydrate development.