ABSTRACT:

The development of a scientifically robust and practical physicochemical theory for describing shale deformation while considering chemical and poroelastic processes is described herein. Ion transfer in the mud/shale system is coupled to formation stresses and pore pressure. The field equations are derived within the framework of a Biot-like isotropic poroelastic theory and allow determination of the mechanical response of shales to drilling-induced chemical, stress, and pore pressure disequilibria. These field equations are solved analytically to yield the solute mass fraction, pore pressure, and the stress distributions around a borehole using a generalized plane strain approach. The impact of ion transfer on pore pressure/stress fields around a borehole in shale has been studied by considering a vertical well. It has been determined that ion transfer causes the chemical-osmosis to become time dependent. As time increases, the transfer of ions leads to osmotic pressure dissipation and re-establishment of a pore pressure regime characteristic of hydraulic flow. Furthermore, it has been found that while the wellbore may be supported by a mud pressure of significant magnitude, the rock can experiences a tensile effective radial stress due to the invasion of solute into the formation. The distribution of tangential effective stress is also affected by ion transfer; it is reduced at the borehole wall, however, this reduction is initially overcome by the larger increase resulting from the reduction of pore pressure near the wellbore. The transfer of ions in the mud/shale system has a significant impact on stress and pore pressure distributions around a borehole. Hence, its contribution to borehole failure is significant and should be considered in the process of optimizing the mud properties.

INTRODUCTION

Oil-based muds can remedy some shale instability problems and enhance hole stability. However, oil-based muds are toxic and poorly biodegradable. Water-based muds may be used a,*, an alternative, but they have shown poor shale. drilling performance (van Oort et al. 1996), became when in contact with a drilling fluid, shale absorbs water into its clay matrix between aggregates, particles, and basal planes of crystals. The sorption of water is accompanied by a change in the interlayer spacings between clay surfaces which causes the rock to swell. In addition to the chemical composition of the drilling fluid, shale swelling and reactivity are influenced by the chemical composition, amount, and distribution of water within the shale itself. The influx of mud filtrate into the formation leads to excessive stress, pore pressure, and ionic exchange between drilling fluids and shale. Therefore, it is a major contributing factor to shale deterioration and instability. Additional causes of borehole instability include thermal stresses and the anisotropy of the formation as well as the in-situ stress state. The chemo-mechanical processes causing shale deterioration and borehole instability while drilling have been studied by a number of investigators (Mody and Hale, 1993; Ghassemi et al. 1998).

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