Studying Fluid Leakage Along a Cemented Wellbore: The Sustained Casing Pressure Analogue, the Influence of Geomechanics and Chemical Alteration on Leakage Pathway Conductivity, and Implications for CO2 Sequestration
Creators: name: Huerta, Nicolas J.
Publisher: name: The University of Texas at Austin
Year: 2009
Subjects: CO2
Geomechanics

description: Geologic storage of anthropogenic CO2 remains the technology most likely to allow society to maintain current and future levels of energy production from fossil fuels and also reduce CO2 emissions. A key technological challenge for geologic storage is defining the probability of CO2 leaks and estimating their impact. Leakage along faults and wells, whether they are pre-existing or created during operations, represent the most likely pathway by which CO2 will leave the storage reservoir. Understanding leakage along a well is an extremely complex problem due to the variability of well completions, pressure and temperature changes during the life of a well, poor quantity and fidelity of well data, and the unknown fate of a well when subject to the environments expected during a CO2 sequestration project. This thesis considers two key aspects of well leakage (1) how to use an analog to estimate a well’s leak path conductivity and (2) how leakage risk is affected by the coupling between pathway conductivity, cement alteration, and geomechanics in a well. To address the first topic, a model of sustained casing pressure described in the literature (Xu and Wojtanowicz, 2001) is used to match observed pressure build up history on a well annulus. The effective permeability of cement is treated as the matching parameter. Results from case studies showed effective permeability values orders of magnitude larger than what is expected for intact cement. This strongly suggests that gas flows along distinct defects in the cemented annulus. The characteristic length for gas flow along several discrete pathways matched values from pathways generated in laboratory studies of zonal isolation. Such leakage paths respond differently to mechanical stress than a permeable matrix would. Thus an important implication of the sustained casing pressure analogue is that the geometry of the leakage pathway should be modeled explicitly, rather than treated as an equivalent permeable medium if one expects changes to the stress state around a wellbore or to the mechanical properties along the pathway. To address the second idea we use core-scale experiments to quantify the coupling between flow and geomechanical stress along a cement/cement interface. The model flow channel is a fracture created by loading the sample according to the Brazilian method until it fails in tension. Using flow rate and pressure drop measurements we infer a fracture aperture. By cycling the confining stress we observe a reproducible relationship between confining stress and fracture aperture. This behavior is a function of the fracture geometry and mechanical properties of the cement. We then examine how chemical alteration of the cement along the pathway can alter the flow/stress relationship. The observed change implies a fracture can be self-sealing to leakage of CO2-saturated brine or of CO2, although the exact mechanism, material plugging or cement strength decrease, remains unverified.