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| A model for inclined borehole stability in bedding rocks based on Mogi-Coulomb criterion of rock matrix |
| DING Liqin1，WANG Zhiqiao1，LU Jianguo1，SUN Yuandi1，2 |
| (1. Faculty of Engineering and Technology，China University of Geosciences(Beijing)，Beijing 100083，China；2. China Institute of Geotechnical Investigation and Surveying Co.，Ltd.，Beijing 100007，China) |
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Abstract The problem of instability is more prominent when drilling inclined wells in bedding layers. A new wellbore stability model is established considering the weakening action of bedding plane on rocks on the basis of the traditional borehole stability model. The model takes into account the arbitrary nature of 3D in-situ stress with three Euler angles，and applies Mogi-Coulomb criterion to strengthen the effect of intermediate principal stress，as well as combines the single weak-plane strength theory to fulfill the goal of guided drilling. The method and formula for calculating the collapse and fracture pressure using this model are given. The experimental and calculated results show that the rock gains a minimum strength when the angle between the loading direction and bedding plane is 30°.. Mogi-Coulomb criterion is more effective on the estimation of rock matrix strength because of the consideration of the influence of intermediate principal stress. The collapse area is markedly enlarged under the effect of bedding plane，and the collapse position is changed. The drilling direction for obtaining the minimum collapse pressure is not simply perpendicular to the bedding plane in space. And the boreholes in the symmetry in terms of principle in-situ stress plane underground have the same fracture pressure. The safe mud density window calculated with the model above can provide theoretical basis for the safe drilling and trajectory design of inclined boreholes.
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2017, Vol. 36 
