Poroelastic modeling of cutting bottom-hole rock – Part I: Stress state of bottom-hole rock

Abstract

This is the first part of a series of two papers modeling cutting bottom-hole rock during drilling. This paper studies the stress state of a bottom-hole rock that a downhole bit is drilling. A bottom-hole rock underground is subjected to far field in-situ stresses. During drilling process, the rock materials above the bottom-hole rock are gradually removed and replaced with drilling fluid, thus altering the stress state of the bottom-hole rock.In this paper, a model is developed based on the theory of Linear Poroelasticity to calculate the evolution of stresses and pore pressure in a bottom-hole rock during drilling process. The bottom-hole rock under consideration can be selected at any depth underground. To solve the model, superposition principle and finite difference method are employed. The model is also extended for inclined wellbores in arbitrary directions.A series of case studies with various drilling parameters in different formations are presented. The results show that during drilling bottom-hole rock will expand, leading to the pore pressure decrease if formation has a low permeability. The pore pressure decrease in a bottom-hole rock will increase differential pressure, eventually increasing difficulties in drilling. The results show that low permeability formation drilled with a greater rate of penetration will lead to a lower pore pressure in a bottom-hole rock. Moreover, during air drilling in hard formation, the pore pressure decreases significantly in bottom-hole rock.