Thermoelastically-induced fracture permeability enhancement associated with cold fluid injection

当冷流体流入岩石内部的裂缝时，岩石会冷却下来，冷却导致裂缝周围的岩石热收缩。因此，裂缝可能会打开，并具有更强的渗透性。本文采用二维有限元程序分析了低温流体流动时远压应力对裂缝开度的影响。结果表明，远程压应力会抑制裂缝的开度，但即使在这种情况下，当注入流体温度小于临界值T_c时，裂缝的开度和渗透率也会出现较大的增加。这个值是作为远压应力、破裂压力和岩石弹性特性的函数给出的。为了验证理论预测，我们使用圆柱形试样进行了实验室实验。试件轴向存在一条人工断口，实验如下：(1)对试样施加围应力；(2)将试样温度升高到一定水平；(3)以恒压向裂缝注入冷水，并测量注入速率；(4)根据测量的注入速率估算裂缝渗透率。实验结果表明，随着注入温度的降低，裂缝渗透率逐渐增加，当注入温度低于理论预测的临界值T_c时，裂缝渗透率的增加速率发生了显著变化。

When cold fluid flows into fractures within a rock, the rock cools down and the cooling causes thermal contraction of the rock around the fractures. As a result, the fractures are likely to open and to be more permeable. We analyze the effect of remote compressive stresses on fracture opening associated with cold fluid flow by using a 2D FEM code. The results show that the fracture opening will be suppressed by remote compressive stresses, but even in such a condition, the considerable increase in fracture opening and permeability will appear when the temperature of injected fluid is smaller than a critical value T_c. This value is given as a function of remote compressive stresses, fracture pressure, and elastic properties of rock. In order to verify the theoretical prediction, we carried out laboratory experiments using a cylindrical specimen. There exists an artificial fracture passing through the specimen in the axial direction, and the experiments were proceeded as follows ; (i)apply a confining stress to the specimen, (ii)elevate the specimen's temperature to a certain level, (iii)inject cold water into the fracture by a constant pressure, and measure the injection rate, (iv) estimate the fracture permeability from the measured injection rate. From the results of experiments, we confirmed that the fracture permeability increases gradually with decreasing the injection temperature, and the increasing rate changes dramatically when the injection temperature becomes lower than the critical value T_c as theoretically predicted.