This paper describes a computer model which simulates the movement of fluid between the wellbore and shale formation due to their chemical potential difference, and the associated time-dependent pore pressure change as the drilling fluid interacts with the shale. The coupled process of stress-strain, water flow, solute flow and volume change incorporates the effects of reflection coefficient of the shale, chemical properties of the salt and drilling fluid properties. The model was used to simulate two laboratory experiments designed for the study of the chemical potential mechanism and the simulation results are in qualitative agreement with the results of similar experiments by other researchers. A parametric study was carried out to demonstrate the effects of salt concentration of the drilling fluid and reflection coefficient of the shale on the variation of pore pressure in the formation.
Wellbore instability, experienced mainly in shale sections, is one of the principal causes of drilling delays and in some cases, even suspension of wells prior to reaching the target. These instabilities may be induced by either in-situ stresses that are high relative to the strength of the formations or physicochemical interactions of the drilling fluid with the shales (Tan and Willoughby, 1993, 1994, Mody and Hale 1993 and Gazaniol et al., 1994). The instability induced by drilling fluid-shale interactions can be managed more efficiently through a better understanding and development of a capacity to model the pore pressure change due to chemical potential difference between the drilling fluid and the shale, and subsequent wellbore failure due to the interactions. Coupled chemical potential mechanism has been studied as a method of stabilising shales drilled with water-based muds (van Oort et al., 1994, 1995, Mody and Hale, 1993 and Wong and Heidug, 1994). This paper describes a computer model which simulates the movement of fluid between the wellbore and shale formation due to their chemical potential difference, and the associated time-dependent pore pressure change as the drilling fluid interacts with the shale. The model was used to simulate two laboratory experiments designed for the study of the chemical potential mechanism. The effects of salt concentration of the drilling fluid and reflection coefficient of the shale on the variation of pore pressure in the formation were demonstrated through a parametric study.
The conceptual model is shown schematically in Figure 1. The physical processes have been considered as two initially independent processes. The basic processes are the undrained load- deformation-failure and the fluid flow processes, and their interaction is achieved by the coupling processes or mechanisms. One coupling process models the pore water pressure change as a result of the stress or strain change and the other, the change of strain (often called volume change) or stress as a result of the change of pore water pressure. The model can follow time, stress, strain or displacement paths using an iterative-incremental procedure. Each path through the model can then also be repeated iteratively until convergence is reached before proceeding to the next increment.
