Carbonates of the Wumishan Formation, Jixian System in the North China Plain: Mechanical properties under in-situ geothermal conditions

doi:10.13745/j.esf.sf.2024.7.11

Abstract

Abstract:

The uncertainties in the mechanical properties of carbonate rocks under high-temperature, high-pressure in-situ geothermal conditions lead to a lack of accurate data and mechanistic understanding for assessing wellbore stability. In this study, carbonates of the Wumishan Formation of the Jixian System are investigated, and suitable outcrops are identified along the stratum extension direction for sampling purposes. The mechanical properties of the reservoir rocks under the combined effect of high temperature and high pressure are studied, and the mechanism of the effect of temperature on the rock’s mechanical properties is analyzed. The increase in temperature led to a slower growth rate of the strength threshold of carbonate rocks with increasing confining pressure. When the confining pressure increased from 40 to 60 MPa, the peak strength increased by 15.9% at room temperature and only by 7.9% at 150 ℃. Above 110 ℃, the friction angle decreased while cohesiveness increased with increasing temperature. Within the temperature and confining pressure ranges in this study, the observed temperature effect was the result of the temperature-induced enhancement and weakening of rock mechanical properties. Within the studied temperature range, temperature had no effect on the chemical composition of minerals in carbonate rocks, but temperature variation could lead to uneven thermal expansion among different mineral particles, resulting in increased intercrystalline thermal stress and causing micro-fracture compaction between mineral particles. Results from this study provide accurate data and a research basis for wellbore stability analysis in the carbonate thermal reservoirs of the North China Plain.

Key words: North China Plain, carbonate, Wumishan Formation, Jixian System, rock mechanics, high temperature

CLC Number:

P642
P314

ZHOU Wei, MA Xiao, CHEN Wenyi, GAO Rui, WANG Yan, HU Dawei. Carbonates of the Wumishan Formation, Jixian System in the North China Plain: Mechanical properties under in-situ geothermal conditions[J]. Earth Science Frontiers, 2024, 31(6): 95-103.

Figures/Tables 12

Fig.1 Regional structural sketch map of Xian County. Adapted from [18].

Fig.2 Outcrop sampling site

Fig.3 Carbonate sample processing

Fig.4 Carbonate samples

Fig.5 Real-time, high temperature, true triaxial test system

Fig.6 Experimental setup for real time, high temperature test

Fig.7 Stress-strain curves of samples under different temperature conditions

Fig.8 Sample failure states

Fig.9 Method to obtain σci

Fig.10 Strength thresholds of samples

Fig.11 Effects of temperature on shear strength parameters

Fig.12 SEM image of carbonate sample after high temperature triaxial test

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