摘要：

CO₂压裂具有降低起裂压力、增强热应力致裂并促进复杂裂缝网络形成等优势，被认为是干热岩储层无水压裂改造中最具发展潜力的技术之一。现有CO₂压裂多采用高纯CO₂作为工质，但其捕集、提纯、液化和输运成本较高，增加了储层改造投入，制约技术规模化应用。为降低压裂工质成本，本文提出利用炼厂、电厂等工业排放源经简单纯化后的含N₂非纯CO₂直接压裂干热岩的新思路。基于非纯CO₂流体物性，建立非纯CO₂压裂干热岩热—流—固—损伤耦合裂缝扩展数值模型，研究不同CO₂/N₂配比、地应力差和注入速率条件下裂缝起裂与扩展规律。研究表明：①CO₂/N₂配比是影响主裂缝—天然弱面相互作用方式的关键因素，高CO₂比例条件下流体传压和换热能力较强，主裂缝更易穿透天然弱面并保持连续扩展；②随N₂比例升高，混合流体可压缩性增强，压力传递效率和裂缝尖端有效驱动力降低，裂缝更易沿天然弱面偏转或被捕获，损伤面积整体减小；③提高注入速率可增强裂缝尖端流体补给，部分弥补高N₂比例条件下传压能力不足的问题，促进裂缝扩展；④地应力差主要控制裂缝扩展方向和损伤展开尺度，地应力差增大时裂缝趋于定向扩展，侧向分支和弱面扩展受到抑制。研究结果可为工业源非纯CO₂压裂干热岩工质利用和参数优化提供依据。

关键词: 非纯CO₂, 干热岩压裂, 无水压裂, 裂缝扩展, 热—流—固—损伤耦合

Abstract:

CO₂ fracturing has advantages in reducing fracture initiation pressure, enhancing thermally induced cracking, and promoting the formation of complex fracture networks, and is regarded as one of the most promising waterless stimulation technologies for hot dry rock reservoirs. Existing CO₂ fracturing commonly uses high-purity CO₂ as the working fluid; however, its capture, purification, liquefaction, and transportation costs are high, which increases reservoir stimulation investment and restricts large-scale application. To reduce fracturing-fluid costs, this study proposes a new concept of directly using N₂-bearing impure CO₂, obtained from simply purified industrial emissions from refineries and power plants, for hot dry rock fracturing. Based on the fluid properties of impure CO₂, a thermo-hydro-mechanical-damage coupled numerical model for fracture propagation in hot dry rock under impure CO₂ fracturing was established to investigate fracture initiation and propagation under different CO₂/N₂ ratios, in-situ stress differences, and injection rates. The results show that: ①the CO₂/N₂ ratio is a key factor affecting the interaction between the main fracture and natural weak planes. Under high CO₂-ratio conditions, the fluid has stronger pressure transmission and heat transfer capacities, and the main fracture is more likely to penetrate natural weak planes and maintain continuous propagation; ②as the N₂ ratio increases, fluid compressibility increases, pressure transmission efficiency and fracture-tip effective driving force decrease, and fractures tend to deflect along or be captured by natural weak planes, resulting in an overall reduction in damage area; ③increasing the injection rate can enhance fluid supply at the fracture tip, partly compensate for insufficient pressure transmission under high N₂-ratio conditions, and promote fracture propagation; ④the in-situ stress difference mainly controls fracture propagation direction and damage extension scale. As the in-situ stress difference increases, fractures tend to propagate directionally, while lateral branching and weak-plane extension are inhibited. These results provide a basis for the utilization of industrial-source impure CO₂ and parameter optimization in hot dry rock fracturing.

Key words: impure CO₂, hot dry rock fracturing, waterless fracturing, fracture propagation, thermo-hydro-mechanical-damage coupling