2. Research Status
Geostress is a natural stress that is present in the formation and not disturbed by engineering. There are several ways of geostress research, including: rock mechanics experiment, well point stress measurement and logging interpretation method. The method of rock mechanics test obtained reservoir rock mechanics parameters (including the Poisson's ratio, elastic modulus, density, etc.) mainly through mechanical tests with core samples; The method of well pointstress measurement uses many measurement method to research geostress, which include the imaging logging, dip logging, cross multi pole array acoustic logging, drilling borehole caving methodanddrilling induced crack method. These methods are based on acoustic emission and hydraulic fracturing techniques at key wells point.
Currently mature geostress researches is mainly for sand shale section, including:
(1)Borehole breakouts theory: It’s characteristic is thatthe minimum horizontal principal stress of sandstone layer is substantially equal to the maximum horizontal principal stress,and the difference between the minimum and maximum horizontal principal stress is large where the shale expand.
(2)Construct stress coefficient method：It’s characteristic is that horizontal minimum principal stress of sandstone layer is obviouslydifferent of horizontal maximum principal stress. Because the resultof the entire region is based on single wells, so the minimum horizontal principal stress of shale layer is muchdifferent of horizontal maximum principal stress too.
Neither of these two method is perfect. Borehole breakout theory gives the exactly opposite result with the fact. The result of construct stress coefficient method is also wrong, because of the mistake of the concept of stress. So there is also a big error of the calculation of the fracture pressure and collapse pressure.
(3)We calculate geostress by using simulation orthogonal dipole stress difference. It’s characteristic is thatthe minimum and maximum horizontal principal stress is obviouslydifferenton sandstone layer, and equal to each other on shale layer.
In many simulation experiments and experiences in the past, there were so many parameters that were too complex and difficult to obtain, or measured inaccurately because the constraint of lithology formation conditions. Our method using simulation orthogonal dipole stress difference overcomes theabove method’s weakness, and improves the computational accuracy of formation stress parameters. This method will be the trends of stress computing technology.