Use of Fiber Optic Distributed Acoustic Sensing for Measuring Hydraulic Connectivity for Geothermal Applications

Advanced methods in reservoir characterization are required to effectively harness geothermal energy from fractured high-temperature, high-pressure crystalline rock. It is critically important to determine sufficient fluid connection between injection and production wells to enhance sweep efficiency and optimize the lifetime of the reservoir. Measuring hydraulic pressure changes in offset wells in response to periodic injection is an alternative indicator of hydraulic connectivity (Sun et al., 2015). However, conventional electric pressure sensors are incapable of operating under conditions associated with geothermal activity. Distributed Acoustic Sensing (DAS) has recently been demonstrated as a means to sense fracture strain response to pressure oscillations. DAS uses fiber optic cable which can be designed to withstand high temperatures and pressures typical of geothermal reservoirs (Paulsson et al., 2014). Because DAS measures instantaneous strain rate for the entire length of cable (Daley et al., 2015), hydraulic connectivity can be established at any depth within the 10 m gauge length. This tool would provide nearly continuous pressure monitoring along the length of the wellbore, providing knowledge into hydraulic pathways even where perforations do not exist. Real-time hydraulic monitoring can be performed without interrupting normal field operations. Also, measurements can be obtained throughout the lifetime of the well, ideally accounting for any changes in operations or reservoir configurations. DAS for hydraulic monitoring would offer a relatively inexpensive and convenient tool to increase knowledge of geothermal reservoir connectivity as it can operate using existing fiber optic wellbore installations that were previously deployed for Distributed Temperature Sensing (DTS) or DAS seismic or acoustic monitoring. Therefore, we propose the following project to further verify and demonstrate that fiber optic DAS technology can be useful for geothermal inter-well pressure sensing to determine reservoir connectivity.