Abstract
Background: Since the fragile anisotropy of shale, it is difficult to carry out laboratory experiments of geostress by shale cores. The existing geostress calculation model that is based on the homogeneous hypothesis also cannot meet the accuracy requirement. Therefore, it is necessary to establish the new geostress calculation model and test methods which are suitable for shale and provide the effective guidance for drilling and fracturing. Methods: Firstly, the triaxial stress experiments were carried out. It showed that the mechanical parameters had strong difference between parallel and vertical bedding direction. The characteristics of transversely isotropic were shown obviously. Then, the geostress calculation model which considers the mechanical parameters of anisotropy in different direction was established by the constitutive relation of transversely isotropic materials. Finally, it was assumed that there is no relative displacement between formations in the process of deposition and the late tectonic movement; the prediction method for the shale geostress was established by the adjacent homogeneous formation. The sensitivity factors and influence laws were analyzed for the horizontal bedding shale geostress. Results: The results showed that the shale geostress was controlled by the elastic parameters of its own and the adjacent beds’. Conclusion: The research can provide the theoretical basis and easy way for calculating the shale geosterss.
Highlights
The geosterss calculation model is able to reflect geosterss' physical nature and actual regularity
Since the fragile anisotropy of shale, it is difficult to carry out laboratory experiments of geostress by shale cores
It was assumed that there is no relative displacement between formations in the process of deposition and the late tectonic movement; the prediction method for the shale geostress was established by the adjacent homogeneous formation
Summary
The geosterss calculation model is able to reflect geosterss' physical nature and actual regularity. A lot of scholars have established many geosterss calculation models, which include, Kinnick model [1], Mattews and Kelly model [2], Terzaghi model and Anderson model [3], Newberry model and Huang Rongzun model [4]. These models have considered the composition characteristics of geosterss in different aspects, the rocks were mostly treated as a homogeneous isotropic linear elastic material [5 - 7]. It is necessary to establish the new geostress calculation model and test methods which are suitable for shale and provide the effective guidance for drilling and fracturing
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