Drilling represents a significant portion of exploration and development costs in petroleum engineering. In many cases, borehole instability related problems may cause drilling to become extremely expensive. Traditional wisdom in drilling engineering often relies from experience and isothermal theories. In this paper we discuss cases where thermal effects are inevitable, and analyze their importance. Transient analytical solutions for temperature and pore pressure changes near a circular borehole or a spherical cavity subjected to a constant temperature and borehole pressure change are presented. The solutions couple conductive heat transfer with Darcy fluid flow in a poroelastic medium. They are applicable to low permeability porous media such as heavy oil reservoirs or shales, where conduction dominates the heat transfer process. Solutions are presented showing separately the effects of temperature and fluid flow on pore pressure and stress development. It will be shown that a substantial compressive stress may be induced near a borehole. Failure to incorporate such an effect may lead to serious error in estimating the borehole stability.
Borehole instability is mainly caused by the effective stress concentration near the borehole wall due to the far-field stress, and this concentration can be affected by diffusive fluid flow. Extensive studies have been conducted to investigate the coupled fluid-stress effect and one may refer to Rice and Cleary [1976], Detournay and Cheng [1988] for poroelasticity solutions. Another potential first-order factor is heat transport due to temperature differences. In petroleum engineering, the drilling fluid often has a temperature different from the formation, while in heavy oil exploitation, a borehole is often heated up to enhance production. Because of the thermal expansion and the differential expansitivities of the rock skeleton and the fluid contained inside the pores, thermal stresses maybe induced leading to borehole yield and failure. The quantification of the impact of thermally-induced stresses on borehole stability requires knowledge of the heat transport properties and the thermal expansion behavior. There have been extensive studies on borehole stability, but often their practical use has been limited due to the complicated nature of the problem. McLellan and Wang [1994] reviewed models for such problems. Maury [1989] suggested that cooled drilling mud may enhance stability in an abnormally warm formation. Several thermoelastic, thermo-poroelastic, and thermo- poroelastoplastic models have since been developed [Hojka and Dusseault, 1991; Wang and Dusseault, 1995]. In this paper, coupled heat- fluid-stress diffusion solutions are derived, and thermal effects on borehole stability are analyzed for a low-permeability formation such as shale.
A method of calculating tangential linear therm-poroelastic stresses around a borehole or a spherical cavity in an isotropic low permeability medium has been developed for the case of constant borehole or cavity temperature and pore pressure change. The solutions are valid for conductive heat transfer, and the effects of partial drainage are explicitly included through the Skempton coefficient approach. The solutions allow us to assess the stability of boreholes and cavities subjected to non-isothermal conditions and drilling guidance may be designed based on them.
