This paper presents a linear cherno-poroelastic model for analyzing borehole stability in transversely isotropic shales. The theory couples mechanical, hydraulic and chemical processes in fluid-saturated porous media. In particular, the model couples stresses and pore pressure to hydraulic conduction, chemical osmosis, and solute diffusion. Field equations are obtained by requiring the rock constitutive and transport models to satisfy the momentum, fluid mass, and solute mass balance equations. The field equations are solved analytically for the problem of vertical and inclined boreholes in shales to yield the distributions of the solute mass fraction, pore pressure, and the stresses around the borehole. The analytical solution is used to demonstrate the impact of the degree of anisotropy, chemical osmosis, solute diffusion and reflection, swelling, and chemical properties of shales on the pore pressure and the stress fields around a borehole drilled in anisotropic shale.
Oil and gas wells represent the fundamental infrastructure in hydrocarbon exploitation. Drilling the well bore is the first and, usually, the most expensive step in oil and gas production activities. Therefore, designing a stable and safe well has become a critical issue in the industry, especially due to the recent and ever-increasing complexity of geological settings requiring continuous improvement in drilling technology.
Well bore stability, mainly in shales, is one of the most challenging problems encountered during drilling. These problems often depart [Tom the classical mechanical failure mechanisms exhibiting time-dependent mud support changes and loss of strength according to physico-chemical processes. Presently, the design of improved water-base muds for shale stability is of primary concern for the drilling industry. As a results, a key factor in selecting the appropriate drilling fluid is a better understanding of the swelling phenomena and transport processes in such formations; being associated with the chemical composition and characteristics of the material.
Prior to drilling, a shale formation is in a state of mechanical, hydraulic, thermal, and chemical equilibrium. Drilling disturbs this natural equilibrium, resulting in the modification of local stresses accompanied by deformation of the borehole, as well as the enforcement of hydraulic, temperature, and chemical potential gradients (Hale et al., 1993; Mody & Hale, 1993; van Oort et al., 1996; and Dick et al., 1996).
This paper presents an extension of poroelasticity for fully coupling mechanical, hydraulic, and chemical processes in transversely isotropic porous media saturated by a compressible liquid consisting of a solvent and one or more solute.
Many different theories, (Low & Anderson, 1958; Chenevert et al., 1970, 1993; Fritz & Marine, 1983; Mody & Hale, 1993; Pashley & Israclachvili, 1984) have been presented to explain the swelling phenomenon of shales (capillary suction, osmosis pressure, and hydraulic pore pressure imbalance). However, until now, the experimental data have not totally and effectively been explained or even understood. Moreover, the extension of the classic poroelastic theory of Biot (1941) for coupling isothermal processes (mechanical, hydraulic, and chemical), that causing shale deterioration and borehole instability while drilling, have been studied by a number of researchers.