二氧化碳羽流地热系统中地层水回流和岩石-流体作用对采热能力的影响

摘要

Complicated fluid flow characteristics and rock-CO2 interaction can cause geothermal reservoir damage and reduce CO2 heat mining rate during geothermal exploitation via supercritical CO2 (SCCO2) injection. Based on thermodynamics and geochemistry principles, an integrated numerical model for heat mining via SCCO2 injection was established, and different processes including formation water evaporation, CO2 geochemical reaction with formation water and rocks, salt precipitation and dissolution, and the variation of formation porosity and permeability were considered and simulated. The model can be used to analyze the effects of CO2 flow and fluid-rock interaction on CO2 heat mining rate. The simulation results show that high formation temperature and continuous injection of SCCO2 can facilitate formation water evaporation and cause a reversed water saturation gradient, which lead to back flow of formation water toward the injection well when the capillary pressure is greater than the differential displacement pressure. The back flow of formation water can accelerate salt precipitation and geochemical reactions , which can reduce SCCO2 flow rate by~40% and greatly decrease heat mining rate. Sensitivity analysis shows that the salinity of formation water is the most significant factor on salt precipitation and heat mining rate, while temperature, pressure, porosity, permeability and injection-production pressure difference also have certain effects. Water injection with low salinity before SCCO2 injection can diminish the damage caused by salt precipitation and increase heat mining rate.%针对超临界CO2(SCCO2)开采地热过程中的渗流特征和可能产生的储层伤害，基于热力学原理和地化反应机理，建立了考虑地层水蒸发、CO2-地层水-岩石地化反应、盐沉淀/溶解和储层孔渗变化的SCCO2地热开采综合模型，分析了流体渗流特征和岩石-流体作用对采热速率的影响，并提出了提高SCCO2采热速率的相应方法。研究结果表明，干燥SCCO2持续注入会加剧地层水蒸发，造成注入井附近地层水饱和度梯度增加，使得气液毛管力大于驱替压差情况，导致地层水回流。地层水回流加剧了地化反应和盐析对注入井区域的伤害，使得SCCO2渗流速度和采热速率降低。敏感性分析表明，储层温度、压力、孔隙度、渗透率和注采压差均会影响SCCO2采热速率，其中地层水矿化度是影响储层盐析及采热速率降低的最重要因素。注SCCO2前预注低矿化度水可有效缓解盐析对储层的伤害。