Investigation of the Undrained Loading Effect and Chemical Effect on Shale Stability

Abstract

Abstract An investigation was performed to determine and compare pore pressure, stresses, and critical mud weights using poroelastic analyses with and without including the undrained loading effect. The undrained loading effect, which enables a fully-coupled poroelasticity analysis for wellbore stability, can be crucial to the stabilization of near-wellbore formations within the critical time even though it will diminish at long time. The critical time, or the duration, of the undrained loading effect is found to be inversely proportional to formation permeability and will vary considerably for formations with different permeabilities. It is also demonstrated in this paper that it is the dimensionless time, not the dimensional time, that controls the duration of the undrained loading effect. Chemical effects, caused by the imbalance between the water activity in the drilling mud and the shale water activity, are also found to affect shale stability by changing pore pressure and shale strength with exposure time. Including all effects (hydraulic diffusion, undrained loading, and chemical effects), one finds that the required mud weight can increase with time, decrease with time, or first decrease and then increase. Furthermore, the change in required mud weight with time is different at different radial positions. The chemical effect does not alter the undrained loading effect, but it may counterbalance the common destabilization of the undrained loading mode. Introduction Wellbore stability in shales has been studied for decades. Primarily, shale stability can be affected by poroelastic, chemical, and thermal effects. Poroelastic and chemical effects will be discussed in this paper. Shales can be regarded as a poroelastic material for which poroelastic induced pore pressure and stresses are important to hole stability. As an example of the fully-coupled poroelastic analysis, Detournay &Cheng1 decomposed the poroelastic behavior of a wellbore into three different loading modes. The full solution of pore pressure and stresses is obtained by the superposition of all three modes. Mode 1 loading represents the effect of wellbore pressure and mean stress. Mode 2 loading describes the homogeneous hydraulic diffusion, including both the hydraulic diffusion and thermal diffusion, if thermal diffusion is accounted for, the solution of which can be obtained by using the similar heat conduction approach. Mode 3 loading (or the undrained loading) describes the short-term behavior of a low-permeability rock when deviatoric stresses are applied to the wellbore. Under this circumstance, pore fluid can not escape from the low-permeability system within short time and pore pressure will build up inside the rock. The inclusion of mode 3 loading enables the full coupling of pore pressure and rock stresses. With fully-coupled poroelasticity, both the hydraulic diffusion and undrained loading contribute to the fully-coupled pore pressure and rock stresses for a low-permeability formation. Hydraulic diffusion dominates the pore pressure change and consequent stress redistribution for a partially-coupled analysis. Many wellbore stability analyses2–7 do not include the mode 3 loading effect. However, both the fluid diffusion and the undrained loading behavior will occur in low-permeability formations such as shales. The assumption of a short duration and/or a negligible undrained loading has to be made to neglect its effect. Therefore, it becomes important to investigate the duration of the undrained loading effect and its contribution to the pore pressure and stress alterations. An attempt is made in this paper to investigate the critical time of mode 3 loading and its effect on pore pressure and rock stresses for drilled formations with different permeabilities.