Fragmentation law of carbonate rocks under different confining pressure in Gaoyuzhuang Formation, Gaoyang geothermal field, Xiong'an New Area

The paper is the result of geothermal geological survey engineering.

Objective D34 well of Gaoyang geothermal field in Xiong'an New Area is the highest temperature geothermal well in North China Basin drilled by Chinese Academy of Geological Sciences and China Geological Survey in 2020. The wellhead temperature is 123.4℃, and the water output is 94.5 m³/h. Obviously, such high temperature corresponds with a relatively low flow. In order to provide important theoretical support for fracturing stimulation, this study intends to clarify the fragmentation law of reservoir carbonate rock under different confining pressures.

Methods Based on previous studies, this paper takes the deep Gaoyuzhuang Formation reservoir carbonate rocks drilled from D34 well as the research object, and carries out conventional triaxial compression and tensile experiments.

Results The experimental results show that: (1) The tensile strength characteristics of the reservoir carbonate rock in the Gaoyuzhuang Formation are similar with that of compressive strength, and shows obvious compression failure features. With the increase of the surrounding rock pressure, the compressive strength shows a typical secondary increase. (2) The compressive strength and the distribution of the fractures show differences with the depth. Among them, the uniformly developed multiple fractures and single main fractures are the main forms of expansion. Under the confining pressure of 20 MPa and 40 MPa, the yield strength changes synergistically with the Young's Modulus and Poisson's Ratio, respectively. (3) The expansion methods of tensile fractures are different with different depth. The shallower tensile fractures mainly expand with small amplitude and multi-stage expansion, while the deeper rocks show a uniform expansion trend.

Conclusions Combined with the result of conventional triaxial compression and tensile experiments, it is concluded that the possibility of forming a complex fracturing network in the shallow part of the reservoir is higher than that in the deep part, but the overall brittleness index is less different, so there is no need to adopt different construction plans for different depths of the reservoir.