Analysis Model of Borehole Stability in Bedding Formations

Abstract:

Bedding formations are often encountered in drilling operations. On the basis of the stress tensor coordinate transforming relation and the borehole stress concentration equation, both the weak-plane shear slip criterion and rock mass Mogi-Coulomb criterions are introduced to build an analysis model for borehole stability. Researches on the reasonable drilling fluid density and safe drilling direction in typical reservoirs show that the existence of bedding plane significantly aggravates the rock damage and changes the damage position on the borehole wall; the collapse pressure of horizontal well continually varies along with the changes of drilling direction, thus the minimum collapse pressure is obtained in a particular drilling direction; in the inclined borehole, the minimum collapse pressure direction acquired is not vertical to the bedding plane experimentally, while the symmetrical inclined boreholes about principal planes have the same fracture pressure in space; the optimum drilling direction is the one with both larger safe density window of drilling fluid and smaller collapse pressure, and the drilling trajectory curve should be controlled strictly in the azimuth where the density window of drilling fluid changes greatly.