超临界CO2压裂裂缝温度场模型

超临界CO₂压裂中,裂缝流体物性变化、相态规律以及裂缝的几何尺寸、导流能力等参数均与裂缝温度场密切相关。针对超临界CO₂无造壁性、滤失能力强的特点,考虑滤失过程中的节流效应,推导了滤失过程中的岩石温度场解析模型及裂缝壁面上的热流函数表达式;以此为基础,考虑裂缝内超临界CO₂压裂液的相态、物性变化,以比焓为研究对象,建立了超临界CO₂压裂裂缝温度场模型。通过实例分析,计算结果表明:随着滤失时间的增加,裂缝壁面上的热流速率逐渐减小,对应位置处的裂缝流体温度逐渐降低;滤失系数越大,裂缝壁面上的热流速率越小,裂缝内流体和周围岩石温度变化越慢。高滤失系数条件下,由于节流效应,滤失流体存在明显的"冷却"过程,会对裂缝温度场产生很大的影响。压裂过程中,裂缝内流体会存在相态的转变,由超临界态转化为液态与超临界态并存,同时近井地带存在生成水合物的风险。

Fissure temperature field model of supercritical CO2 fracturing

During supercritical CO₂ fracturing, the physical property change and phase pattern of fissure fluid as well as geometrical dimensions, conductivity and other parameters of fissures are closely related to fissure temperature field. Aiming at the fact that supercritical CO₂ fracturing is featured by no wall building property and high filtration capacity, an analytical model of rock temperature field in the filtration process and heat flux function expression on fissure wall are derived in consideration of throttling effect in the filtration process. On this basis, in combination with phase and physical property changes of supercritical CO₂ fracturing fluid, the fissure temperature field model of supercritical CO₂ fracturing is established by taking specific enthalpy as the research object. The case analysis and calculation results prove that the heat flow rate on fissure wall is gradually reduced with the filtration time increasing, and fissure fluid temperature at the corresponding position is declined as well. The greater the filtration coefficient is, the smaller the heat flow rate on fissure wall will be, and the slower the temperature change of fissure fluid and surrounding rocks will be. Under the premise of high filtration coefficient, a significant cooling process exists in filtration fluid due to throttling effect, leading to great influences on fissure temperature field. During fracturing, the fissure fluid shows liquid and supercritical phase, instead of single supercritical phase. Meanwhile, there is a risk of hydrate formation near wellbores.