Chemoporoelastic solution of transversely isotropic saturated porous media.

Abstract

The solution is closed-form in the Laplace domain; and is then inverted to the time domain by using Stehfest's (1970) algorithm. This solution simulates the chemoporoelastic process triggered by drilling a borehole in saturated porous media, and subject to a step constant mass fraction change together with mud pressure acting on the borehole wall. Results indicate that osmosis alters the pore pressure and effective stresses, which are function of the swelling coefficient and the reflection coefficient of the formation as well as the solute mass fraction of the mud.

Instability of shales involves fully coupled chemo-mechanical processes, in which fundamental intermolecular surface forces are acting between the clay layers. These forces are function of the composition, type, amount, and micro-fabric of the clay content in the rock which are responsible for the global shale behavior. Thus, for wellbore stability problems one needs to include the mechanical properties of these materials interacting with the pore fluid.

The purpose of this dissertation is to provide civil, petroleum and geological engineers, a model of chemical poroelasticity for fluid saturated argillaceous rocks, appropriate for laboratory and field applications. The theory couples mechanical, hydraulic and chemical processes in fluid-saturated transversely isotropic porous media. The model of hydration swelling is derived from non-equilibrium thermodynamics; it is a hybrid of an extended version of the theory of poroelasticity and Onsager's (1931) transport phenomenology. In particular, it is an extension of Heidug and Wong's model (1996) to accommodate compressibility of the fluid as well as anisotropy. The rock constitutive equations expressing the total stresses, the pore volume fraction per referential volume are constructed from the internal energy of the wetted clay matrix in terms of the solid strains, the pore pressure, and the fluid component chemical potentials. The total stresses, and the variation of fluid content are linearly related to these variables through anisotropic material coefficients characterized via micro-homogeneity and micro-isotropy assumptions. The analysis incorporates four coupled transport processes of fluid, and solute in addition to mechanical deformation, and chemical swelling. The phenomenological equations relating the fluxes to their conjugate forces are constructed from the definition of the dissipation function and the generalized forces associated with it. Field equations for the linear chemical loading are derived and applied to vertical and inclined boreholes subjected to non-hydrostatic far-field loading.

Wellbore stability, mainly in shales, is one of the major problems encountered during the drilling of wells, with an estimated cost exceeding hundreds of millions of dollars each year in remedial works. Presently, the design of improved water-base muds for shale stability is of primary concern for the drilling industry. A key factor in selecting the appropriate drilling fluid is a better understanding of the swelling phenomenon in shales; it is associated with the chemical composition and characteristics of the material. However, until now, the experimental data did not totally and effectively explain the observations.