Multidisciplinary Workflow for CO2 Injection in Depleted Gas Reservoirs

Summary The growing interest in Carbon Capture and Sequestration (CCS) projects challenges the subsurface studies to overcome the classical approach, focused on field development and hydrocarbon exploitation. The need of a completely new workflow is embedded in the change of perspective that CO2 storage projects, where reservoir is located downstream the whole process, bring with them. ENI implemented a dedicated workflow where the classical geomodelling approach has been modified and adapted to satisfy these new needs. This workflow is suitable for being applied to CCS projects in depleted reservoirs. Depleted reservoirs are particularly appealing for storage purposes due to the extensive field knowledge obtained during their production phase, the possible presence of facilities already in place and, most important, the safe containment they can guarantee, proven by the natural hydrocarbon accumulation process. 3D static and dynamic model construction has been considered to be the core of the workflow for properly determining the effective storage capacity of the reservoir. Classical history-matching process has been reshaped to better focus on parameters like injectivity, re-pressurization and CO2 plume evolution. Furthermore, a correct description of the fluids in the reservoir at the very end of its production, ideally corresponding to the injection start-up, has been considered a fundamental output of the history-match process. Uncertainty analysis has, also, been run in this new perspective, to provide key inputs and information to other disciplines (i.e. drilling and completion, monitoring, environmental impact evaluation). Specialistic studies, considered mandatory, assumed a key role in the workflow, providing crucial information about CO2 containment during the operations and beyond, and marking the project success. They include geochemical studies used for detecting both near-wellbore issues but also for assessing medium to long-term containment. Geomechanical studies may provide information about reservoir behavior and cap-rock integrity during injection phase and they may include several studies like e.g. fault stability assessment, cap-rock integrity evaluation and thermal induced fracture (TIF) modelling, that can be applied to the specific reservoir situation. The paper will focus on each step of the new workflow when applied, being it particularly comprehensive and effective.