Formation of Superaitical Geothermal Resevoirs by Means of Supercritical Water-Induced Cracking

该研究项目确定了利用超临界水与造岩矿物之间的相互作用在温度和压力条件超过水临界点的深层岩体中创建人工地下地热库的可行性。本研究取得的主要成果如下：1.通过超声波波速测量，证明了在高温试验中施加围压是有效的，以减轻由于岩石矿物热膨胀失配而引起的热应力引起的微破裂。2.利用厚壁圆柱体试件对几种类型的花岗岩进行了渗透性测量。岩样包括从一个实际地热田的深层高温岩体中提取的岩芯。结果表明，当水的条件超过临界po…时，测得的渗透率显著跳跃。对于水，无论研究中使用的岩石类型如何，INT都更高。渗透率的显著增加是由于超临界水与成岩矿物之间的相互作用导致了颗粒尺度的微破裂。取自深层岩体的岩样渗透率增加的幅度小于取自露头的岩样。3.为了考察应力对上述超临界水致开裂现象的影响，进行了三轴压缩试验。结果表明，当温度高于临界温度时，最终断裂面附近的微裂纹密度明显大于远场的微裂纹密度。微裂纹密度的增加表明超临界水在应力作用下加速了裂纹的萌生。这种加速开裂被称为超临界水强化应力腐蚀开裂。实验观测结果表明，超临界水强化应力腐蚀裂解技术在水力地热藏扩容中具有潜在的应用前景。4.对超临界岩体进行了长期热采数值模拟分析，综合考虑了围岩中的流体流动特性(即循环水)和热传递特性。考虑了超临界流体温度、压力以及流体密度、粘度等因素对超临界流体的影响，结果表明，结合亚临界温度条件，从超临界油藏中提取热能存在一个最佳注入量。较少

The research project determined the feasibility of utilizing the interactions between supercritical water and rock-forming minerals in the creation of an artificial subsurface geothermal reservoir in a deep-seated rock mass, in which temperature and pressure conditions exceed the critical point for water. The main results obtained in this study are summarized below.1.Application of confining pressures in high temperature testing was shown tot be effective in order to mitigate the thermal stress induced micro-cracking due to thermal expansion mismatch of the rock-forming minerals by conducting ultrasonic wave velocity measurements.2.Permeability measurements were made for several types of granitic rocks using thick-walled cylindrical specimens. The rock specimens included cores taken from the deep-seated high temperature rock mass at an actual geothermal field site. It has been shown that the permeability measured jumped significantly when the condition of water exceeded the critical po … More int for water regardless of the rock types used in this study. The significant increase in permeability was shown to be due to grain-scale microcracking induced from the interaction between the supercritical water and rock-forming minerals. The extent of the permeability increase was less for the rock specimens taken from the deep-seated rock masses than that for the rock types obtained from outcrops.3.Tri-axial compression tests were conducted in order to examine the effect of stress on the above-mentioned supercritical water induced cracking phenomenon. It has been demonstrated that the microcrack density at the vicinity of the final fracture plane was signifcantly larger than that at the far-field when the temperature was greater than the critical temperature. The increase in the microcrack density points out that the supercritical water induced cracking was accelerated by the effect of stress. The accelerated cracking is newty named as supercritical water enhanced stress corrosion cracking. The experimental observation may suggest the potential use of the supercritical water enhanced stress corrosion cracking the expansion of hydraulically-induced geothermal reservoirs.4.We undertook long-term thermal extraction numerical modeling analysis, for a supercritical rock mass, which incorporated fluid flow characteristics (i. e. circulating water) and thermal transfer in the surrounding rock mass. We considered the effects of temperacure and pressure in the supercritical fluid regime, as well as fluid density, viscosity etc. Based on the numerical results, it has been shown that there is an optimum injection rate to extract the heat energy from the supercritical reservoir combined with subcritical temperacure rock mass. Less