Thermoelastically-induced fracture permeability enhancement associated with cold fluid injection

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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.