A method for calculating the stress-strain state of an inclined borehole in a layered transversally isotropic rock formation with an arbitrary orientation of the bedding planes is presented. Based on this method, software was developed to calculate the shear failure gradient in anisotropic rocks. The software was tested by comparing the results with numerical finite element calculations.
Examples of stability calculation of inclined and horizontal wells are given. It is shown that the anisotropy of elastic and strength properties significantly affects the stress state and stability of the inclined borehole. The effect of the inclination and azimuth of the well on its stability was studied. It is established that at inclination up to 20-30° the shear failure gradient is determined by the strength of the matrix. When the inclination is increased, the rock strength is determined by the shear bedding layers and a higher mud density is required to ensure the borehole stability. It is also shown that the optimum drilling path in an anisotropic rock formation may be the azimuth of the maximum horizontal stress.
An example of calculating the shear failure gradient in the interval of clay rocks in one of the oil fields in Western Siberia is given. The obtained results confirm that neglecting the anisotropy of the elastic and strength properties of rocks leads to an underestimation of mud density and drilling problems. Laboratory measurements of the mechanical properties of rocks are required for a reliable prediction of the borehole stability.
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