Abstract
Currently, water fracturing under deep geothermal conditions remains poorly understood because the reservoir rocks are usually high-strength crystalline rocks characterized by high temperatures. The aim of this study is to investigate the effects of rock properties, injection rates, and temperatures on hydraulic fracturing behavior and the induced crack characteristics through experimental investigations. A series of hydraulic fracturing experiments was conducted on two Indosinian granite types to investigate the differences in hydraulic fracturing behavior caused by rock properties. Among others, six samples were tested under a room-temperature condition at different injection rates from 1 to 30 mL/min to clarify the effect of the injection rate and three samples were tested under a high-temperature condition (150 °C) to simulate specific geothermal environments. The results indicated that granites with different rock properties have different injection rate thresholds. When the injection rate is below the threshold, the injection pressure finally reached a constant value without fracturing. For rocks with the same properties, the temperature effect can lead to a high injection rate threshold due to the occurrence of thermally-induced cracks. The number of acoustic emission events recorded during the room-temperature experiments increased linearly with increasing injection rate, while high-temperature tests increased sharply. The investigation results imply that a complex hydraulically-induced crack network is expected to be achieved in geothermal reservoirs by a high injection rate or high temperature differences (between injected fluid and rock). Additionally, the characteristics of the hydraulically-induced cracks were investigated by cutting through the sample blocks and measuring the residual pressure. The results indicated that the induced crack aperture can maintain a fluid conductivity of 0.1–0.8 mm/s at a closure pressure of 12 MPa.
Highlights
Granite is a common geothermal resource and rock masses whose temperature reaches the standard (150 ◦ C) of hot dry rock (HDR) are almost all granitic
Water fracturing treatments have shown the most success in developing HDR systems in granites—e.g., Rosemanowes in the UK
Compared with the traditional hydrocarbon industry, HDR resources are characterized by deeper burial depth and higher temperature, making it difficult to conduct on-site hydraulic fracturing investigations in an enhanced geothermal system (EGS) environment
Summary
Granite is a common geothermal resource and rock masses whose temperature reaches the standard (150 ◦ C) of hot dry rock (HDR) are almost all granitic. The most effective means for developing HDR resources is the enhanced geothermal system (EGS) [1]. Energies 2019, 12, 2138 in low-permeability high-temperature rock strata by hydraulic fracturing. Water fracturing treatments have shown the most success in developing HDR systems in granites—e.g., Rosemanowes in the UK (1977–1991), Hijiori in Japan (1981–1986), Basel in Switzerland (2005–2006), and Soultz-sous Forêts in France (1987–present) [3,4,5]. Compared with the traditional hydrocarbon industry, HDR resources are characterized by deeper burial depth and higher temperature, making it difficult to conduct on-site hydraulic fracturing investigations in an EGS environment. Qiabuqiageothermal geothermal field is located in issurrounded surroundedbybythree three orogenic belts: the West
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