A Toolkit for Carbon Capture and Storage in Offshore Depleted Gas Field
Authors: Tewari Raj Deo , Tan Chee Phuat , Sedaralit M ohd Faizal 
Published: March 2022

Carbon dioxide (CO2) capture, utilization, and storage is the best option for mitigating atmospheric emissions of CO2 and thereby controlling the greenhouse gas concentrations in the atmosphere. Despite the benefits, there have been a limited number of projects solely for CO2 sequestration being implemented. The industry is well-versed in gas injection in reservoir formation for pressure maintenance and improving oil recovery. However, there are striking differences between the injection of CO2 into depleted hydrocarbon reservoirs and the engineered storage of CO2. The differences and challenges are compounded when the storage site is karstified carbonate in offshore and bulk storage volume.

It is paramount to know upfront that CO2 can be stored at a potential storage site and demonstrate that the site can meet required storage performance safety criteria. Comprehensive screening for site selection has been carried out for suitable CO2 storage sites in offshore Sarawak, Malaysia using geographical, geological, geophysical, geomechanical and reservoir engineering data and techniques for evaluating storage volume, container architecture, pressure, and temperature conditions. The site-specific input data are integrated into static and dynamic models for characterization and generating performance scenarios of the site. In addition, the geochemical interaction of CO2 with reservoir rock has been studied to understand possible changes that may occur during/after injection and their impact on injection processes/mechanisms. Novel 3-way coupled modelling of dynamic-geochemistry-geomechanics processes were carried out to study long-term dynamic behaviour and fate of CO2 in the formation.

The 3-way coupled modelling helped to understand the likely state of injectant in future and the storage mechanism, i.e., structural, solubility, residual, and mineralized trapping. It also provided realistic storage capacity estimation, incorporating reservoir compaction and porosity/permeability changes. The study indicates deficient localized plastic shear strain in overburden flank fault whilst all the other flaws remained stable. The potential threat of leakage is minimal as target injection pressure is set at initial reservoir pressure, which is much lower than caprock breaching pressure during injection. Furthermore, it was found that the geochemical reaction impact is shallow and localized at the top of the reservoir, making the storage safe in the long term. The integrity of existing wells was evaluated for potential leakage and planned for a proper mitigation plan. Comprehensive measurement, monitoring, and verification (MMV) were also designed using state-of-art tools and dynamic simulation results. The understanding gaps are closed with additional technical work to improve technologies application and decrease the uncertainties.

A comprehensive study for offshore CO2 storage projects identifying critical impacting elements is crucial for estimation, injection, containment, and monitoring CO2 plume. The information and workflow may be adopted to evaluate other CO2 projects in both carbonate and clastic reservoirs for long-term problem-free storage of greenhouse gas worldwide.