Depleted oil and gas reservoirs provide ideal sites for the long term storage of CO2 that is one aspect of the carbon capture and storage (CCS) concept. CCS is a technique that potentially could contribute to the reduction of the anthropogenic greenhouse gases emissions to meet the 1.5-2 ℃ target set in Paris Agreement. The high number of drilled wells in oil and gas reservoirs can put the storage aspect of CCS at risk. Any defects in these type of wells can be potentially a source of CO2-leakage. These damaged wells could compromise the time and expense invested in a CCS project. The integrity of wells existed in a CO2 storage field should be examined prior to the implementation of any CCS project. A CCS project should be reconsidered or the wells in a CO2 storage site will need re-completion if CO2 leakage is predicted through them. This study investigates the integrity of both abandoned and injection wells within CO2 storage formations. Once CO2 is injected in target storage formations below the Earth it starts moving upwards due to the buoyancy force which mostly occurs close to the injection points. Then, the CO2 plume continues distributing beneath the caprock that traps its further upward trajectory. It would be possible for a CO2 plume to encounter abandoned wells. The area of the concern in this thesis, however, is the rock-cement-casing assemblage for the numerous well drilled the impermeable layers into the reservoirs below. The CO2 plume will freely move within porous layers but it is the caprock and impermeable layers which block its permeation to the upper porous layers. Therefore, it would be reasonable to consider any well trajectory across the impermeable layers, such as caprocks as the riskiest area which needs to be focused. In this thesis, it was assumed that CO2 is completely dissolved in brine as a ubiquitous phase present in CO2 storage sites. Indeed, this case is the worst condition which could be assumed within a CO2 storage site. Carbon dioxide should be dissolved in an aqueous phase to affect the rock otherwise the reaction between CO2 gas phase and solid phase seems impossible. This assumption reduces the dependency of the geochemical reactions on the stress state which is resulted from the overburden layers. Therefore, an iterative approach has been applied in which the initial and the boundary conditions are imported into the CrunchFlow code which is developed by Steefel et al. (2015) to calculate the geochemical alterations within the rock-cement-casing assemblage. The author developed a code in MATLAB (2019) for calculating the geomechanical alterations within the rock-cement-casing assemblage using the output data from the CrunchFlow code. The output data from the geomechanical section were subsequently inputted into the CrunchFlow code again. This iterative approach continues until the rock-cement-casing assemblage fails. The radial cracking and the radial compaction are proposed in this study as two processes that may occur in abandoned wells. An indefinite lifespan is observed for an abandoned well impacted by the radial compaction, although it reduces to less than a century while the radial cracking is active. However, the lifespan of the cement matrix increases with phenomenon reduces the porosity at the rock cement interface by one order of magnitude, and closes the gap between the cement and the casing. Therefore, the rock-cement interface becomes less porous than in its initial intact state, and would not account for a leakage pathway. In the case of abandoned wells and under the assumptions used in this study, the lifespan is again found to be indefinite due to governance of the compaction process over the rock-cement interface. Based on the results obtained from this study and considering the applied assumptions, in both abandoned and injection wells the lifespan of the cement sheath would be more than a thousand years due to the activation of the compaction process, the formation of a calcite precipitation zone, and the extremely low permeability of the cement matrix itself. The results of experiments reported in the reviewed literature show that the diffusion of CO2-bearing fluids into the cement matrix results in the formation of a calcite precipitation zone. This zone is close to the cement-brine interface. The strength of the cement matrix increases in this area due to a reduction in the porosity. In addition, this zone by itself reduces the diffusion of CO2-bearing fluids into the inner parts of the cement matrix. This process is also widely observed in the simulations in this study which helps to prevent the high degradation of the inner parts of the cement sheath. It should be noted, that the health and vigour of the rock-cement-casing initially assumed in calculations, and the in-situ horizontal stress is not negligible. With considering these conditions and based on the results obtained from this thesis, it is realised that the CO2 leakage through either injection or abandoned wells would be implausible. The results of this work would give a clear perspective on the safety of CCS projects which are going to be implemented in fields containing drilled wells regarding the tendency of CO2 leakage through those wells.
|Date of Award||Jul 2021|
|Supervisor||Seyed Shariatipour (Supervisor) & Essie Ganjian (Supervisor)|
Reactive transport modelling and geomechanical investigation of wellbore cements in CO2 storage sites
Bagheri, M. (Author). Jul 2021
Student thesis: Doctoral Thesis › Doctor of Philosophy