New Insights for Monitoring Shale Hydration Swelling Behaviors Using Distributed Fiber-Optics Sensing Technology

Summary The testing and evaluation of shale hydration behaviors have always been an essential problem of wellbore stability in drilling. As an excellent distributed measuring method of strain with a high spatial resolution in recent years, distributed fiber-optics sensing (DFOS) technology has been widely applied to wellbore integrity assessment. In this work, we propose a new experimental framework for monitoring the hydrational swelling strain of shale based on DFOS and optical frequency domain reflectometry (OFDR). Initially, we design a scheme of fiber wrapping helically on the shale surface to cover more sensing zones. Then, we carry out laboratory tests in an unconfined free-water soaking environment with a constant temperature to eliminate temperature influence. After several days of soaking and hydration, the hydrational strain of shale sensed by fiber optics can be monitored dynamically in real time with an enough acquisition frequency and a sufficient spatial resolution. We conduct a pretest using waterproof strain gauges and distributed fibers under the same condition, which demonstrates the great ability and sufficient accuracy of DFOS in hydrational strain monitoring. In addition, we establish a strain projection equation by tensor analysis, and for the first time define a convenient relationship of hydrational strain between circumferential and axial directions, called hydration self-similarity. The experimental results show that the hydrational strain growth varies with time and space, which indicates the native progressiveness over time and the significant heterogeneity in space. These typical hydration swelling behaviors of shale can be measured effectively by fiber optics. Such experimental framework and modeling analysis provide new insights into testing behaviors of shale hydration and understanding chemical fracture phenomena that may be further utilized for drilling mud optimization and wellbore stability enhancement.