Estimation of the rock mechanics and in-situ stress parameters of carbonate reservoirs using array sonic logging: A case study of Shunbei No.4 block

Abstract

Objective Geomechanical analysis plays a crucial role in exploring and developing oil and gas reservoirs. However, the study of in-situ stress in the Yijianfang–Yingshan formations of the Middle to Lower Ordovician in the Shunbei oilfield has lagged behind. This paper compares and analyzes the rock mechanics and in-situ stress characteristics of different types of carbonate reservoirs in the Yijianfang-Yingshan formations of the Lower Paleozoic Ordovician in the Shunbei No. 4 block, and explores the control effect of micro-grain structures on the in-situ stress and rock mechanics parameters of carbonate reservoirs. It aims to provide fundamental geological data for the evaluating of ultra-deep carbonate reservoirs in the Shunbei area. Methods This study, based on the spring combination model, determined the rock mechanics and in-situ stress characteristics through rock mechanics experiments and array acoustic logging, and characterized the micro-pore structure of the limestone using casting thin sections and X-ray μ-computed tomography analysis. Results The results indicate that the Young's modulus of the Yijianfang–Yingshan formations ranges from 50 to 89 GPa, the compressive strength from 99 to 136 MPa, and the Poisson's ratio from 0.25 to 0.32. The maximum horizontal principal stress of the formations ranges from 200 to 225 MPa, while the minimum horizontal principal stress ranges from 125 to 160 MPa. Conclusion Significant differences in rock mechanics parameters and in-situ stress exist among different types of carbonate reservoirs in the Yijianfang–Yingshan formations. From Type I to Type III and non-reservoir carbonate rocks, the Young's modulus, compressive strength, and maximum horizontal principal stress increase dramatically, while the Poisson's ratio and minimum horizontal principal stress show little change. Compared to micritic limestone, sandy–bioclastic limestone has larger calcite particles and reduced particle cohesion, resulting in decreased compressive strength and Young's modulus. Some calcite particles appear sub-rounded or rounded with higher three-dimensional particle sphericity, also leading to weaker intergranular engagement and further reduction in compressive strength and Young's modulus. The presence of numerous pores and fractures in sandy-bioclastic limestone facilitates frictional sliding and potential failure along microfractures or particle interfaces, contributing to lower compressive strength and Young's modulus as well. In addition, micritic limestones in the Yijianfang–Yingshan formations are affected by hydrothermal activity, with some dolomite being replaced by quartz, which increases the compressive strength and Young's modulus. The macroscopic mechanical characteristics and rock mechanics parameters of the rock are constrained by the micro-grain structures. The low-value zones of the Young's modulus, compressive strength, and maximum horizontal principal stress in the Yijianfang–Yingshan formations are identified as the advantageous reservoir development zones in the Shunbei No. 4 block.

Keywords

• shunbei no. 4 block
• carbonate rock
• rock mechanic parameters
• in-situ stress