Horizontal Bedding Shale in-Situ Stress Calculation Inverted from Adjacent Beds

Due to rock friability and the strong difference of rock mechanical characteristics between parallel and vertical bedding directions of the horizontal bedding shale, existing in-situ stress calculation methods could not meet the accuracy requirements and it is difficult to perform laboratory in-situ tests on the cores. Therefore, shale in-situ stress evaluation methods were studied in this paper so as to provide the effective guidance for well drilling and fracturing design. The shale elasticity parameters in different bedding directions were obtained by means of uniaxial compression tests, and the transversely isotropic characteristics were presented obviously. Based on the constitutive relation of transversely isotropic materials, the model for horizontal bedding shale in-situ stress inverted from its adjacent sand-shale in-situ stress was established, after it was assumed that there was no relative displacement between the formations in the process of the deposition and later tectonic movement. An analysis was conducted on sensitivity factors and influence laws. It is shown that the shale in-situ stress was controlled by its own elastic parameters and the elastic modulus, Poisson’s ratio and in-situ stress value of its adjacent sand-shale beds. The two horizontal in-situ stresses calculated with this method were higher than those obtained with the Terzaghi and Newberry models, and they were between the upper and lower limits of Huang Rongzun model.The research results in this paper provided a new method for evaluating the in-situ stress of horizontal bedding shale.