A Chemo-poroelastic Solution for Stress and Pore Pressure Distribution around a Wellbore in Transversely Isotropic Shale

Abstract

Abstract It is generally accepted that shale deterioration and borehole instability are significantly influenced by excessive pore pressure, ion exchange between drilling fluids and shale, and the anisotropy of the in-situ stress state as well as the formation. This paper describes a model for estimating the influence of elastic and chemo-mechanical anisotropy on the distribution of stress and pore pressure around a well in transversely isotropic shale. The model is based on a practical theory that couples ion diffusion, chemical osmosis, and hydraulic flow to stresses and pore pressure. The field equations of the model are derived within the framework of a continuum chemoporoelastic theory that linearly relates total stresses and variation of fluid content to the strains, pore pressure, and solute mass fraction through anisotropic material coefficients. The field equations are solved analytically for the problem of a wellbore in transversely isotropic shale to yield the solute mass fraction, pore pressure, and the stress distributions around the borehole. The solution has been applied to a typical field situation and the results indicate that the osmotic pressure tends to stabilize the borehole. However, osmotic pressure dissipates with time due to ion diffusion. Furthermore, it is found that the anisotropy in chemo-mechanical material coefficients strongly affects the effective stresses around the borehole and enhances the potential for borehole failure. Introduction When drilling for oil and gas, a significant portion of the drilled sections contain shales that are the major source (90%) of wellbore instability problems. These problems often depart from the classical mechanical failure mechanism exhibiting time-dependent mud support requirements and loss of strength associated with physico-chemical processes. Prior to drilling, a shale formation may be considered in a state of mechanical, hydraulic, chemical, and thermal equilibrium. Drilling disturbs this virgin state and imposes hydraulic, chemical potential, and thermal gradients. The ensuing influx of mud filtrate into the formation weakens the rock and leads to excessive stress, pore pressure, and ionic exchange between drilling fluids and shale. As a result, rock deforms and may faile leading to borehole collapse.