An enhanced geothermal system using carbon dioxide (CO) for both reservoir creation and thermal energy extraction has attracted attention; however, studies on the CO fracturing of volcanic rocks under geothermal conditions are lacking. This study aimed to elucidate CO fracturing characteristics and processes in geothermal volcanic rocks via integrated lab-scale fracturing experiments and numerical simulations of basalt and andesite at 250°C and a confining pressure of 30 MPa. Moreover, it proposed and demonstrated an efficient CO-based fracturing method based on the obtained insights. Fracturing experiments on basalt and andesite with relatively low initial porosity or permeability implied that CO fracturing can be initiated at a lower pressure and produce a more complex fracture pattern than water fracturing because of the ease of fluid permeation (e.g., fluid pressure propagation) in rocks owing to the lower viscosity of CO. The experiments also revealed that the ease of CO permeation can produce thinner fractures than those produced by water fracturing, which may severely inhibit the fracturing of andesite rocks with high initial porosity/permeability. Fracturing simulations of rocks with different initial porosity/permeability provided results consistent with the experimental observations. Moreover, the simulations clarified that fluid pressure propagation during CO permeation within an unfractured part of the rock and between induced fractures and the surrounding rock matrix could reduce the fracture initiation pressure by effective stress reduction, increase the complexity of the fracture pattern by large stimulated zone development, and inhibit fracture opening and propagation by decreasing the pressure difference between the fractures and matrix. Based on these findings, we proposed a combined CO-water fracturing system that addresses the drawbacks while maintaining the advantages of CO fracturing.

中文翻译：

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地热条件下火山岩CO2压裂：特征与过程

使用二氧化碳 (CO) 来创造储层和提取热能的增强型地热系统引起了人们的关注；然而，关于地热条件下火山岩CO2压裂的研究还很缺乏。本研究旨在通过综合实验室规模的压裂实验和玄武岩和安山岩在250°C和30 MPa围压下的数值模拟，阐明地热火山岩中CO的压裂特征和过程。此外，根据所获得的见解，提出并演示了一种有效的二氧化碳压裂方法。对初始孔隙度或渗透率相对较低的玄武岩和安山岩进行的压裂实验表明，由于岩石中流体渗透（例如流体压力传播）容易，CO压裂可以在较低压力下启动，并产生比水压裂更复杂的裂缝模式。由于CO的粘度较低，实验还表明，CO渗透的容易性可以产生比水压裂产生的裂缝更细的裂缝，这可能会严重抑制初始孔隙度/渗透率较高的安山岩的压裂。具有不同初始孔隙度/渗透率的岩石的破裂模拟提供了与实验观察一致的结果。此外，模拟还表明，CO 渗透过程中流体压力在岩石未破裂部分内以及诱导裂缝与围岩基质之间的传播可以通过有效应力降低来降低裂缝起裂压力，通过大刺激区增加裂缝模式的复杂性并通过降低裂缝与基质之间的压力差来抑制裂缝张开和扩展。基于这些发现，我们提出了一种组合式二氧化碳-水压裂系统，该系统可以解决二氧化碳压裂的缺点，同时保持二氧化碳压裂的优点。