Abstract
Severe wellbore stability issues were reported while drilling in laminated formation with weak planes such as beddings. To accurately determine the safe mud weight according to the changing environment is of primary importance for safety control of drilling. Considering both the elastic and strength anisotropy of bedding formation, a novel theoretical model is established and the stress and failure around wellbores are analyzed. The accuracy and applicability of the theoretical model is verified by in situ field data. For the purpose of fulfilling real-time prediction, the method flowchart of programming is also provided. The results show that the model built can be conveniently used to predict the stress distribution, failure area, and collapse and fracture pressure while drilling, and rather good predictions can be made compared to real field data. In addition, the inhomogeneity of in situ stress and elastic parameters affect the upper limit of the safe mud weight window (SMWW) greater than the lower limit. Negative SMWW may appear with the direction change of the wellbore or weak plane, especially when the azimuths of them change. As to the magnitude of SMWW, the anisotropic effects of Young’s modulus are greater than the Poisson’s ratio. The method established in this paper can greatly help with the precise prediction of wellbore stability as drilling proceeds in bedding formation.
Highlights
Bedding rocks such as shale are widely distributed in traditional energy deposition areas worldwide
To meet the most general situation, the model built and the method developed in this paper should fully consider the factors of the in situ stress, orientations of wellbore and weak planes, strength and elastic parameters of anisotropic rocks, etc
The Boulder wells were directionally drilled to targets in the gas producing formations. In this region the geology consists of a series of stacked thrust sheets, with multiple faulted imbricates offsetting the weaker parts of the formations
Summary
Bedding rocks such as shale are widely distributed in traditional energy deposition areas worldwide. This model was further developed by Cui et al [21] to incorporate the stability assessment of inclined wells Based on this theory, Ding et al [22] considered the effects of anisotropic permeations that occurred around wellbores due to the existence of beddings. The other type of model incorporates the effects of elastic anisotropy on the calculation of stresses around wellbores [36,37,38,39,40] In this model, the elastic parameters such as Young’s modulus and Poisson’s ratio are assumed to have different values with regard to direction. The model considering elastic anisotropy of rocks was not commonly applied when assessing wellbore stability in drilling practice as this model is rather complex and inconvenient to use Both the elastic and strength anisotropic effects of bedding rocks are significant according to the test results in the literature. The influences of anisotropy, in situ stress and wellbore trajectory on the prediction results are analyzed
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