Experimental results are presented to show how the compressive strengths and acoustic velocities of different types of shale change when they are exposed to water-based fluids. The acoustic velocity (or slowness) and compressive strength of a soft, high water activity, Pierre I shale increase after exposure to different ionic solutions, while for the lower water activity Arco shale, the converse is true. By combining these tes ts with a new gravimetric test that quantitatively determines the flux of water and ions into and out of the shale, it was shown that these different effects correlate well with the movement of ions and water into the shale. In every case, water adsorption weakened, while ion adsorption strengthened the shale. The influence of salt type and salt concentration on the strength and acoustic velocity (slowness) of two shales was also investigated. It was observed that the ionic content of the water-based fluid had a significant effect on the changes in shale properties. These changes in acoustic velocity and compressive strength are highly correlated. This suggests it may be feasible to use acoustic logging data to determine changes in the mechanical properties of shale. Finally, the impact of the reported changes in the mechanical properties of the shale on wellbore stability was demonstrated through the use of a 3-D wellbore stability analysis program.
Wellbore instability related problems in shale formation have plagued the petroleum industry for many years. The primary cause of such problems is the adverse interaction between the drilling fluid and shale formations. This interaction may cause pore pressure and mechanical property alterations around the wellbore, thereby leading to formation collapse.
To illustrate the effects of such alterations on wellbore stability, Mohr-Coulomb failure diagram is often used. Referring to Figure 1, an increase in pore pressure (or decrease in effective normal stress) will shift the Mohr circle to the left, bringing the circle closer to the safe/failure boundary. On the other hand, if the strength of the formation is decreased due to interaction between shale and drilling mud, the safe/failure line will shift downwards, bringing it closer to the Mohr circle. Once the Mohr circle touches the safe/failure envelope, wellbore instability is normally assumed to have occurred. From this observation, it is clear wellbore stability can be improved by minimizing pore pressure rise and formation weakening. Many studies have been conducted on preventing pore pressure build-up around wellbore caused by the shale-drilling fluids interaction [1-6]. In this work, we focus on the selection of drilling fluids that will maintain or even increase shale mechanical strength.