This paper describes the development of a borehole stability model capable of considering certain chemo-mechanical processes that are operative iv. drilling fluid-shale interactions. Specifically, phenomena related to osmotic and poroelastic processes are included. A generalized plane strain solution is utilized which fully couples the chemical potential and deformation of shale. Drilling fluid parameters such as mud weight and salt concentration can be optimized to alleviate borehole instability when using oil-based or water-based muds. The user can choose the Drucker-Prager failure criterion or the Mohr-Coulomb criterion when conducting stability analyses. The model was used to investigate the impact of drilling fluid chemistry on the stability of an inclined wellbore in shale. The results demonstrate that osmosis significantly impacts fluid flux into the formation whereby stability can be achieved by increasing the salinity of the drilling mud. However, the contribution of osmosis to hole stability is overestimated by the ion exclusion model. Preliminary results of a boundary element model with ion transfer indicate that the diffusion of ions progressively decreases and eventually eliminates osmosis by reducing the chemical potential gradient between the mud and the formation.
When holes are drilled for petroleum production, more than 90% of the drilled length involveshales. Prior to drilling, a shale formation is in a state of hydraulic, thermal, chemical, and stress equilibrium. Drilling action disturbs this equilibrium and introduces gradients of stress, pore pressure, temperature and chemical potentied in the rock material surrounding the hole. To reestablish equilibrium, shale deforms, swells and may det?. riorate causing borehole instability. In the past, the petroleum industry has used oil-based muds to prevent wellbore instability. However, due to their toxicity, oil-based muds are environmentally hazardous and can lead to very large remedial costs. Water-based muds provide an attractive alternative, but have shown poor shale-drilling performance (van Oort et al. 1996). The extent to which a shale is disturbed due to excavating a well can be assessed by a suitable analysis of the forces imposed on the rock by the disequilibria resulting from the drilling action. The chemomechanical processes causing shale deterioration and borehole instability while drilling have been studied by a number of investigators. Although significant progress has been made (Mody and Hale, 1993; Onaisi et ed. 1993; van Oort et al. 1996; Sharma et al. 1998), an adequate tool for analyzing shale deformatioh while drilling is not presently available. The development of such a tool requires consideration of those. parameters which drive transport processes in a borehole stability model. In this paper the macroscopic processes of osmosis, and ion transfer are described and their potential contribution to shale instability are delineated. This is followed by the presentation of a borehole stability model which considers the effects of hydraulic and chemical potentieds. Results of the application of the model are discussed and future improvements are outlined.
