コアを用いた地下の応力状態の評価手法

―最適な水圧破砕のデザインを目指して―

抄録

Knowledge of in-situ stress state in oil and gas reservoirs is necessary and important for the prediction of orientations of hydraulic fracturing and for the design of optimum parameters of the hydraulic fracturing aimed at enhances of hydrocarbon energy resource recovery. Although many types of stress measurement techniques have been suggested and developed up to today, there is not a perfect and reliable method for obtaining the full stress state information (the three-dimensional stress tensor) in a large depth well. For better understanding to the present situation of stress measurements and their insufficiency, we reviewed and summarized the existing stress measurement methods which can be applied into a deep wellbore. In principle, measuring of the in-situ stress under in-situ condition is better; for example, applications of the hydraulic fracturing, leak-off test or extended leak-off test, as well as analyses of wellbore failure stress indicators including borehole breakouts and drilling induced tensile fractures are desirable. In some cases, the hydraulic fracturing and/or leak-off tests are impossible and the wellbore failure stress indicators are not available. Even such stress data are available, they may be insufficient, for example, lack of the maximum horizontal stress magnitude. Therefore, the core-based stress measurements are useful as the complementary approach. We described the details of anelastic strain recovery (ASR) method and diametrical core deformation analysis (DCDA) method which are widely and frequently applied in the recent scientific and engineering deep drilling projects as the core-based stress measurement methods. We concluded that these core-based methods are applicable and helpful in deep drillings related to the hydrocarbon energy resource recovery projects.