Time-Dependent Surface Mechanical Properties of Shale During Acid Fracturing: Microindentation Insights

Abstract

Abstract Acid fracturing is used to enhance oil and gas production in tight carbonate formations. Fractures are created, and flow paths are etched on the surfaces, allowing for sustained flow under fracture closure stress. The induced fractures aperture and stability depend on the time-dependent deformation of the contacting asperities. In this study, we investigate the effect of acid exposure on the time-dependent deformation of the rock surface using micro-indentation testing. Microindentation testing is used to investigate the change in the mechanical properties of cores extracted from the Eagle Ford surfaces following acid exposure for 1 hour and 24 hours. Specifically, time-dependent effects are captured by applying a constant load to a small area of the rock surface for a set period of time. Contact creep compliance rates are calculated from the measured deformations and contact areas. Our tests are conducted on both unaltered samples and samples exposed to varying acid strengths for different time intervals. We conduct multiple measurements on each core surface to minimize the effects of grain-scale heterogeneity. The results demonstrate that the contact creep modulus varies in a fluid- and time-dependent manner. Specifically, a one-hour soak in deionized (DI) water or weak acid (pH=4) led to an increase in the contact creep modulus, signifying reduced creep. Conversely, a sharp decrease was noted after a one-hour soak in a strong acid solution (pH=2). Additionally, it was observed that creep deformation varies with the logarithm of time. Unlike previous studies, the data indicate that the relationship between acid strength and soaking time on creep behavior is complex and merits additional scrutiny, particularly for extended soaking periods in strong acids. These observed trends have significant implications for the long-term behavior of fractures, influencing factors such as proppant embedment and the rate of fracture closure under varying fluid exposure conditions. This study reveals the critical role of coupled hydro-chemo-mechanical processes in controlling fracture closure during acid fracturing. The significant effect of creep on closure is highlighted, with novel experimental workflows employed using microindentation. Our study provides new insights into the behavior of shale under acid exposure and offers a framework for further research in optimizing acid fracturing.