Pressure Data in 3-D Hydrofrac Stress Measurement

点击翻译按钮获取中文摘要

For in situ measurement of tectonic stress in the deep earth's crust, hydraulic fracturing technique is most promising. In the present project, physical background of the pressure data which are used in the technique to evaluate the tectonic stresses are clarified and several new methods for determining the pressure data from pressure-time records during hydraulic fracturing are proposed. Firstly, we introduced a new criterion for initiation of a crack from a wellbore surface subjected to a fluid pressure and succeeded to explain quantitatively the dependency of breakdown pressure on the wellbore diameter and also on the injection flow rate to show that the breakdown pressure can be useful pressure data bringing the information on the stress component acting parallel to the crack surface. Secondly, we analyzed, based on the fracture mechanics, closure process of the crack after shut in to find that the closure process consists of three stages : from cessation of crack growth until crack tip closure, from just after crack tip closure until complete crack closure, and from just after complete crack closure until the test is stopped. It is revealed that the inverse of the wellbore pressure decrease rate after shut-in is linear with respect to the fluid pressure in the first and the last stages. It is also shown that the far field minimum compressive stress can be determined on the basis of these characteristics. Finally, we also analyzed the reopening behavior of the crack during repressurization cycles, based on fracture mechanics and fluid dynamics. It is found that the so-called reopening pressure does not coincide the pressure at which the crack really opens. The so-called reopening pressure is highly dependent on the injection flow rate and approaches the magnitude of the compressive stress acting normally to the crack.