Abstract Efforts to maintain and enhance reservoir permeability in geothermal systems can contribute to sourcing more sustainable energy, and hence to lowering CO2 emissions. The evolution of permeability in geothermal reservoirs is strongly affected by interactions between the host rock and the fluids flowing through the rock's permeable pathways. Precipitation of secondary mineral phases, the products of fluid-rock interactions, within the fracture network can significantly reduce the permeability of the overall system, whereas mineral dissolution can enhance reservoir permeability. The coupling between these two competing processes dictates the long-term productivity and lifetime of geothermal reservoirs. In this study, we simulate the conditions within a geothermal system from induced fracturing to the final precipitation stage. We performed batch and flow-through experiments on cores of the Carnmenellis granite, a target unit for geothermal energy recovery in Cornwall (UK), to understand the role of mineral dissolution and precipitation in controlling the permeability evolution of the system. The physico-chemical properties of the cores were monitored after each reaction-phase using ICP-OES, SEM, hydrostatic permeability measurements, and X-ray Computed Tomography. Results show that permeability evolution is strongly dependent on fluid chemistry. Undersaturated alkaline fluids dissolve the most abundant mineral phases in granite (quartz and feldspars), creating cavities along the main fractures and generating pressure-independent permeability in the core. Conversely, supersaturated alkaline fluids, resulting from extended periods of fluid-rock interactions, promote the precipitation of clay minerals, and decrease the permeability of the system. These results suggest that chemical dissolution during geothermal operations could generate permeable pathways that are less sensitive to effective stress and will remain open at higher pressures. Similarly, maintaining the circulation of undersaturated fluids through these granitic reservoirs can prevent the precipitation of pore-clogging mineral phases and preserve reservoir permeability in granite-hosted geothermal systems.

中文翻译：

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流体化学对花岗岩渗透率演化的作用：在自然和人为系统中的应用

摘要 在地热系统中维持和提高储层渗透率的努力有助于获得更多可持续能源，从而降低二氧化碳排放。地热储层渗透率的演变受到母岩与流经岩石渗透路径的流体之间相互作用的强烈影响。次生矿物相的沉淀，流体-岩石相互作用的产物，在裂缝网络内可以显着降低整个系统的渗透率，而矿物溶解可以提高储层渗透率。这两个相互竞争的过程之间的耦合决定了地热储层的长期生产力和寿命。在这项研究中，我们模拟了从诱导压裂到最终降水阶段的地热系统内的条件。我们对 Carnmenellis 花岗岩（英国康沃尔的地热能回收目标单元）的岩心进行了分批和流通实验，以了解矿物溶解和沉淀在控制系统渗透率演变方面的作用。在每个反应阶段之后，使用 ICP-OES、SEM、静水渗透率测量和 X 射线计算机断层扫描监测岩心的物理化学性质。结果表明渗透率演化强烈依赖于流体化学。不饱和碱性流体溶解花岗岩（石英和长石）中最丰富的矿物相，沿主要裂缝产生空腔，并在岩心中产生与压力无关的渗透率。相反，由于长时间的流体-岩石相互作用而产生的过饱和碱性流体，促进粘土矿物的沉淀，降低系统的渗透性。这些结果表明，地热作业过程中的化学溶解可能会产生对有效应力不太敏感的渗透通道，并且在更高的压力下将保持开放。类似地，保持不饱和流体在这些花岗岩储层中的循环可以防止堵塞孔隙的矿物相的沉淀，并在以花岗岩为主体的地热系统中保持储层渗透率。