Wellbore instability in shales is the most challenging and costly issue in drilling operations. Wellbore instability in shales can be attributed to many factors some of which have been well studied and documented. However, the physicochemical and mechanical properties alterations in shales which eventually leads to wellbore failure have been largely ignored. Water and ions movement in and out of shales plays a major role in the alteration of the physicochemical and mechanical properties of shales thus leading to wellbore instability problems and possible hole collapse. Water and ions can move in and out of shales by many mechanisms including, but not limited to, diffusion osmosis, chemical osmosis convective flow and capillary suction.
This work presents experimental data analyzing the impact of chemical osmosis, and diffusion osmosis on water and ion movement when shale interacts with drilling fluids. The adopted experimental work minimized the effect of convective flow and capillary suction. Results show that water movement is not only controlled by chemical osmosis (water activity) as previously thought, but is also influenced by diffusion osmosis. This insight provides information and guidelines to optimize drilling fluids to effectively control and mitigate wellbore instability when drilling through troublesome shale.
Furthermore, pressure transmission tests were used to experimentally measure capillary entry pressures of various non-wetting fluids (oil-based mud, crude oil and Nitrogen gas) through shales. These capillary entry pressures are needed for the estimation of shales seal capacity (h).