Quantitative characterization of 3D pore structures and percolation characteristics in bioturbated reservoir media based on X-ray micro-CT: a case study of the Neogene Sanya Formation in the Qiongdongnan Basin, Northern South China Sea

Abstract

Organisms alter the primary texture of sediments, leading to alteration in petrophysical properties mediated by a textural and mineralogical contrast between burrow-fill and sediment host, which affects reservoir properties and fluid flow characteristics. In this article, in order to understand the microscopic pore structure and flow state of fluids in microscopic pores of bioturbated reservoirs, Ophiomorpha-bearing bioturbated reservoirs in the Neogene Sanya Formation of the Qiongdongnan Basin in the northern South China Sea were selected to research. Micro-CT was applied to scan selected core plugs, and a 3D pore structure model was established. The geometric characteristics of the microscopic pore structure were quantitatively and visually characterized by a modified maximal ball algorithm, and connectivity analysis of the pore structure was carried out. Numerical simulations of the percolation characteristics of the analyzed bioturbated reservoir samples were performed using digital core software (Avizo) and multi-physics field simulation software (Comsol). The results show that: (1) 3D pore structures reveal that pore volume, pore area, pore equivalent radius, throat area, throat equivalent radius and throat length have a large distribution range; among them, the pore volume has the largest distribution range, which can vary by six orders of magnitude, indicating that the pore size distribution of the bioturbated reservoir is uneven and has strong heterogeneous characteristics; (2) the connected pore structure is very complex, and as the equivalent radius of connected pore increases, the coordination number also gradually increases, the better the connectivity. Numerical percolation simulation results also suggested that larger connected pore space plays a key role in the effective permeability of the reservoir. This study has important implications for analyzing the modification effect of bioturbation on oil and gas reservoirs, and enhancing production and recovery in the study area.

Keywords:

• Bioturbation
• oil and gas reservoirs
• micro-CT
• pore structure
• percolation characteristic

Acknowledgements

The authors are grateful to Dr. Xianglong Wang and Zilu Xu for technical advice on pore structure extraction, and Dr. Li Wei (Research Institute of Petroleum Exploration & Development of PetroChina Southwest Oil & Gas Field Company) provided help and guidance with measuring and collecting porosity and permeability data. The authors appreciate the guidance and help from Dr. Andrew La Croix and Dr. Camilo A. Polo. Many thanks to Dr. Murray K. Gingras and Dr. Greg Baniak for their warm work earnestly.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was carried out with the support of the National Natural Science Foundation of China (Grant Nos.: 41472104 and 42272178), Natural Science Foundation of Henan Province, China (Grant Nos. 202300410185 and 212300410349), the Program for Innovative Research Team (in Science and Technology) in Universities of Henan Province, China (Grant No. 21IRTSTHN007), the Program for Innovative Research Team (in Science and Technology) of Henan Polytechnic University (Grant Nos. T2022-5 and T2020-4) and the Fundamental Research Funds for the Universities of Henan Province (Grant No. NSFRF200340).