Factors controlling the compressive strength and acoustic properties of shales when interacting with water-based fluids

This paper investigates the impact of water-based fluids on the compressive strengths and acoustic velocities of different types of shales. 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. A new gravimetric test that quantitatively determines the flux of water and ions into and out of the shale is utilized to show that changes in compressive strengths and acoustic velocities of shales correlate well with the movement of ions and water into the shale. The results showed that water adsorption weakened, while ion adsorption strengthened the shale. In addition, this study investigated the influence of salt type and salt concentration on the strength and acoustic velocity (slowness) of shales. 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 were found to be highly correlated. This suggests that it may be feasible to use acoustic logging data to determine changes in the mechanical properties of shale. Finally, a 3-D wellbore stability analysis program was used to demonstrate the impact of the reported changes of the mechanical properties of shale on wellbore stability. (C) 2007 Elsevier Ltd. All rights reserved.