Study on the Accuracy of Fracture Criteria in Predicting Fracture Characteristics of Granite with Different Occurrence Depths

Abstract

The fracture network of a deep geothermal reservoir forms the place for heat exchange between injected fluid and rock mass with high temperature. The fracture resistance ability of reservoir rocks will affect the formation of fracture-network structure, heat exchange and transmission characteristics, and reservoir mechanical stability. However, there are few reports on the fracture toughness and trajectory prediction of geothermal reservoirs with different depths. In this paper, the modified maximum tangential stress criterion (MMTS) is analyzed. The results show that the experimental data are significantly different from the theoretical estimate of MMTS under the influence of different occurrence depths. It is found that the fracture process zone (FPZ) seriously affects the accuracy of predicting fracture initiation angle and mixed-mode (I+II) fracture toughness by MMTS. The FPZ value, considering the influence of different occurrence depths, is modified, and the accuracy of MMTS in predicting the fracture mechanical characteristics of granite is improved. In addition, the mechanical test results show that the Brazilian splitting strength (σt) of granite fluctuates increase with the increase in temperature. With the increase in deviatoric stress, the Brazilian splitting strength and the Brazilian splitting modulus of rock show a trend of first increasing, then decreasing, and then increasing.

Shear Strength and Energy Evolution of Granite under Real-Time Temperature

“…(1) Installing the specimen into the pressure chamber after measuring its size; (2) fixing the normal stress to the pre-determined value (10 MPa, 20 MPa, 30 MPa, 40 MPa); (3) connecting the AE instrument and tapping the surface of specimen to ensure that the equipment is functioning properly; (4) keeping the normal stress constant and using the automatic heating mode with a heating rate of approximately 1 °C/min; (5) when the setting temperature points (25 °C, 100 °C, 200 °C, 300 °C, 400 °C) are reached, keeping each temperature point constant for 2 h to ensure that the specimens are uniformly heated inside; (6) keeping the temperature unchanged, the shear test is carried out on the sample. A constant axial displacement rate of 0.05 mm/min is applied to the granite specimen to apply the shear stress.…”

“…Therefore, the development of geothermal energy must fully study the mechanical properties of rock under real-time high temperature. Relevant scholars previously carried out mechanical experiments on heat-treated rock samples, exploring the influence of temperature on rock mechanical properties [3][4][5][6].…”

Energy Extraction and Processing Science

“…Usually, the existing hot dry rock geothermal engineering is numerically simulated to study the rock strata movement, rock mechanical behavior and to engineer earthquakes related to heat extraction efficiency and safe mining. During this process, attention is paid to the complex conditions of the deep environment, including high temperature, high stress, high permeability, and the coexistence of multiple states (solid, liquid, gas) [5]. In fact, it is the study on solid-fluid thermal coupling and its engineering response mechanism.…”

Application and Optimization of CCUS Technology in Shale Gas Production and Storage

“…geological conditions, high temperatures, high ground stress, fluid seepage pressure, and other complex conditions [15][16][17][18][19][20][21]. This makes innovations in applying CCUS technology during deep shale gas exploitation urgent.…”