Leakage is one of the major concerns in a geological carbon sequestration project due to the adverse environmental consequences. The main leakage risk of CO2 through a thick, low permeable cap rock is identified to be along wells, especially in sedimentary basins that have a history of oil and gas exploration and production. To pursue a robust and cost effective real-time monitoring technology for CO2 leakage risk detection along the wellbore, a permanently downhole deployed coaxial cable casing imaging system is developed and tested for various deformation modes in laboratory in this paper.
The casing imager consists of a helically wrapped coaxial cable on the outside of the casing with coaxial cable strain sensors evenly distributed along the cable. A lab-scale prototype of the casing imager was deployed on both PVC sewer pipes and steel pipes for testing on four commonly observed casing deformation modes in the oil field, including axial compression, radial expansion, bending, and ovalization. The coaxial cable strain sensors were pre-stressed and then helically wrapped and attached onto the outer wall of the pipe at a pre-determined angle with high strength epoxy. Multiple LVDTs or strain gauges were used as independent measurement of the pipe actual deformation in comparison to the casing imager measured pipe deformation throughout all the tests.
The test results demonstrated the ability of the lab-scale casing imager prototype in real-time monitoring of casing axial compression, radial expansion, bending, and ovalization, which would prove great value in evaluating wellbore integrity and providing early warnings of leakage risk that will contaminate the ground water during CO2 injection. In addition, the low cost and high robustness of the distributed coaxial cable sensors will greatly lower the downhole monitoring cost and increase the system longevity.
Leakage is one of the major concerns in a geological carbon sequestration project due to its adverse environmental impact. The safety of drinking water would be threatened by the accumulated high concentration CO2 if it is leaked into a contained environment with possible consequences such as lowered pH and increased concentration of total dissolved solids (Bacon, 2013; Little & Jackson, 2010). Each CO2 sequestration project will have its unique leakage risk assessment, but the main leakage risk of CO2 through a thick, low permeable cap rock is commonly identified to be along existing wells or through faults and fractures (Cook, 2014; Edlemann et al., 2016; Metz et al., 2005; Moreno et al., 2005). Especially in sedimentary basins that have a history of oil and gas exploration and production, the existing wells provide possible pathways for leakage of waste fluids toward the shallow subsurface and the land surface (Bois et al., 2011, 2012; Nordbotten et al., 2004; Watson & Bachu, 2007). The cement sheath as one of the primary barriers to prevent wellbore leakage and failure, its integrity begins at the cementing operation and what happens there can greatly affect the long term integrity of the well (Weideman & Nygaard, 2014). Thus, it is of great importance to monitor the downhole activities during the cementing and CO2 injection process to provide early warnings of leakage risk.
