The consequences of using concentrated salt solutions for mitigating wellbore instability in shales

Wellbore instability in shale formation is the most troublesome and costly issue facing drilling engineers and operators. Early attempts to solve this problem recommended the application of osmosis where a chemical potential (water activity difference) is created across the wellbore in order to induce water flow out of shale. This required the addition of concentrated salts, especially when using non-aqueous drilling fluids, as a means to lower the chemical potential of the drilling fluid. While this practice may draw water out of shale, the benefit of such practice is compromised by the invasion of ions and their associated water owing to the ionic concentration difference created by the addition of salts to the drilling fluid. The invasion of ions and their associated water into the shale could change the pore fluid composition and mechanical properties of the shale, which could lead to strength reduction through cementing bonds weakening and cohesion degradation and thus matrix expansion and swelling, all of which result ultimately in shale failure.This paper analyzes prior experimental data that claims that shale generates high osmotic pressures by evaluating the experimentally measured membrane efficiency (63 data points) of shale when exposed to both aqueous and non-aqueous drilling fluids. In addition, a series of immersion tests and gravimetric measurements were performed on different shales, when exposed to concentrated aqueous solutions, to study the detrimental impact of ionic diffusion on shale stability. Moreover, biaxial stress tests were performed to investigate the impact of the invasion of water and ions on the compressive strength of shale as it interacts with concentrated aqueous solutions. Other important but ignored mechanisms that greatly impact wellbore instability in shales are also briefly addressed.Results show that while shales behaved as semi-permeable membranes, their membrane efficiencies were low (the average membrane efficiency reported was 0.034). In addition, it was shown that significant ionic invasion coupled with the flow of their associated water occurred when shales interacted with concentrated aqueous solutions. It was further observed that, shale samples failed after 48 h when they were immersed in concentrated aqueous solutions having water activities that were less than that of the shale samples. Also, results show that the compressive strength of shale was altered as a result of water and ion interaction with the shale matrix and pore fluid.