In-situ stress measurements in hot dry rock, Qinghai Gonghe Basin and simulation analysis of reservoir fracture modification

doi:10.13745/j.esf.sf.2024.7.14

Abstract

Abstract:

China has made some progress in recent years in the exploration and development of hot dry rock (HDR) resources and drilling technology, but is relatively lacking in basic research on high-temperature HDR fracturing technology. For enhanced geothermal systems, hydraulic fracturing is generally used to inject high-pressure fluid into the reservoir to expand and extend natural fractures, in order to increase the reservoir permeability and heat exchange area. Thus, reliable in-situ stress data are important for guiding the stimulation of reservoir reconstruction. Taking the Qinghai Gonghe HDR pilot project as an example, this paper first obtains the in-situ stress state by combining the anelastic strain recovery (ASR), diametrical core deformation analysis (DCDA), core cake, and image logging methods. Next, the influence of the in-situ stress state on the development of HDR reservoirs is discussed. Finally, to evaluate the effectiveness of reservoir fracturing under the current in-situ stress state, a three-dimensional fracture geological model of the hydraulic fracturing zone is established using the discrete fracture network (DFN) method. The sliding tendency (T_s) and dilation tendency (T_d) of each fracture are calculated through numerical simulation, and fracturing activities under different injection pressures are analyzed. According to the results, (1) the main type of in-situ stress state was the thrust faulting regime developed within the granite reservoir range (3500—4000 m) in the Gonghe HDR reservoir, with the horizontal compressive stress being the dominant stress. (2) The average direction of the maximum principal stress at 3500—4000 m depth was 39.35°±14.23°, predominantly oriented in the NE direction, consistent with the notion that the compressive stress was associated with the NE-trending compression movement on the northeastern rim of the Qinghai-Tibet Plateau. (3) The suitable fracturing pressure for the Gonghe HDR field area was 46—55 MPa, and most fractures tend to open after shear activation (high-dilation tendency), which was conducive to increasing the area of heat exchange and improving extraction efficiency. (4) Results of in-situ stress measurement and microseismic monitoring showed that natural fractures expanded horizontally and vertically under hydraulic fracturing, ultimately forming a horizontal-vertical fracture network. The research results provide a reference for future in-situ stress measurement and its application in the assessment of hot dry rock development in China.

Key words: Qinghai Gonghe, hot dry rock, in-situ stress measurement, hydraulic fracturing, reservoir evaluation

CLC Number:

P553
P314

XU Jiading, ZHANG Chongyuan, ZHANG Hao, BAI Jinpeng, ZHANG Shi’an, ZHANG Shengsheng, QIN Xianghui, SUN Dongsheng, HE Manchao, WU Manlu. In-situ stress measurements in hot dry rock, Qinghai Gonghe Basin and simulation analysis of reservoir fracture modification[J]. Earth Science Frontiers, 2024, 31(6): 130-144.

Figures/Tables 12

Fig.1 Geological map of the hot dry rock research area and location of the Gonghe HDR pilot project (yellow star) in Gonghe, Qinghai. Adapted from [22].

Fig.2 Well layout

Fig.3 Principle of ASR method

Fig.4 Pasting diagram of rock core strain gauge by ASR method

Fig.5 Principle of DCDA method. Adapted from [33].

Fig.6 Anelastic strain recovery curves of a rock core

Fig.7 Anelastic principal strain recovery curves of a rock core

Fig.8 Magnitude of in-situ stress in the Gonghe HDR field

Fig.9 Induced tensile fractures (left), borehole breakout (middle), and maximum horizontal principal stress orientation (right) in the HDR field area

Fig.10 Sliding tendency of fractures under different injection pressures

Fig.11 Sliding and dilation tendency of fractures under different injection pressures

Fig.12 Microseismic events in Gonghe

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