Geomechanical Monitoring at CO2 storage sites with Distributed Fiber Optic Strain Sensing: Insights from laboratory and Field experiments

With the growing need for CO2 storage, risk management is essential to secure the storage sites; these risks include fault reactivation, ground surface deformation/sea-bottom (uplift), well and caprock integrity, and CO2 leakage; managing these risks could be achieved by understanding the hydromechanical behaviors of rock induced by the reservoir pressure build-up caused by CO2 injection. However, this remains a crucial challenge because the rock's mechanical and hydraulic properties are poorly constrained. Moreover, the conventional monitoring methods usually consider CO2 plume migration only, which is not enough to understand the induced pressure front that occurs far beyond the real pressure plume. Although several techniques could image the geomechanical deformation and investigate the surface deformation well, these monitoring methods do not provide a complete image regarding the deformation migration from the subsurface to the surface due to the limited measurement points in addition to the cost issue.

 In this paper, we will introduce Rayleigh scattering-based Distributed Optical Fiber Strain Sensing (DFOSS) as an effective tool for subsurface and surface geomechanical monitoring to track the dynamic responses at each spatial location along the cable due to the deformation caused by injection; this technology could overcome other conventional methods' limitations including continual spatiotemporal measurements, cost-effective installation: vertically along the wellbore and horizontally into the ground surface, covered area and sea bottom. We will review several laboratory and field experiments from our previous studies. First, we will show the laboratory results from the first laboratory test to track the movement of the CO2 plume as it enters the clay-rich critical regions in the reservoir–caprock system using DFOSS and monitoring of the injected water in a sandstone sample using DFOSS in the second test. Both results demonstrated that DFOSS could provide high-resolution information on deformation and fluid activity. Next, we will show our subsurface monitoring field results, where we conducted several water injection tests in a shallow well. We monitored the injection process by installing DFOSS in a monitoring well. Our outcome confirmed that DFOSS could provide critical information for rocks' properties and fluid migrations by geomechanical monitoring, and it could be a real-time and permanent monitoring tool for wellbore, caprock integrity, and CO2 leakage. Finally, we will show the surface deformation monitoring results, where we installed the fiber cable into the surface horizontally in a shallow trench; the airbag inflation and deflation tests were conducted under the fiber cable to simulate uplift and subsidence caused by the fluid injection and production in the subsurface. The results suggested that DFOSS could locate any anomaly along the cable.

 Our results demonstrate that installation of DFOSS in fiber cables horizontally into the surface around the injection site and vertically in a well to incorporate well-based strain sensing with surface monitoring, allowing geomechanical monitoring (horizontally into the surface and vertically in the subsurface) in three dimensions via a cost-effective, real-time and permanent monitoring system.