A Study of Wellbore Stability in Shales including Poroelastic, Chemical, and Thermal Effects

by
Guizhong Chen, Ph.D.

University of Texas at Austin, 2001
Supervisors: Martin E. Chenevert and Jon E. Olson

Shale is always a troublesome rock during oil and gas drilling operations. Shale (in)stability has been of great concern in the oil industry for decades. It has also been a costly problem and has perplexed the industry for many years. A better understanding of the wellbore stability mechanism in shales is imperative. The object of this study is to develop a comprehensive model to deal with borehole instability problems in shales.

Wellbore stability problems are caused by changes in near wellbore pore pressure and rock stresses. The excess of rock effective stresses over the rock strength can cause collapse (shear) or breakdown (tensile) failure of the drilled formation. This imbalance between the rock stress and rock strength always happens when the in-situ rock is drilled out and is replaced by the drilling fluid. Pore pressure alterations due to osmotic effects are a function of the water activity in the drilling fluid and the membrane efficiency of the shale.

In this work, thermo-mechanical stresses coupled with the osmotic contributions are used to compute conditions under which the wellbore becomes unstable. The osmotic contribution is added to the hydraulic potential to form the net driving force of the fluid flow.

Changes in pore pressure have been observed in shale experiments. An alteration of the shale strength was also observed when shales are exposed to different drilling fluids. It is necessary to consider shale strength alterations when inspecting the wellbore stability status and determining critical mud weights.

Thermal diffusion inside the drilled formation induces additional pore pressure and rock stress changes and consequently affects shale stability. Thermal effects are important because thermal diffusion into shale formations occurs more quickly than hydraulic diffusion and thereby dominates during early times. Rock temperature and pore pressure can be partially decoupled for shale formations. The partially decoupled problem can be solved analytically under appropriate inner and boundary conditions. The analytical solutions are consistent with the finite-difference solution to the coupled problem. The decoupled temperature and pore pressure variables are programmed to calculate rock stresses and wellbore failure status.

User-friendly input and output interfaces are developed in order to implement this model in the field. This model can also be applied to other petroleum rocks like sandstones.

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