Modeling Framework Developed To Assess Integrity of Legacy Wells for CO2 Storage

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 222449, “Development of a Risk-Based Modeling Framework for Integrity Assessment of Legacy Wells in CO2-Storage Applications,” by Saeed Ghanbari, Morteza H. Sefat, SPE, and David Davies, Heriot-Watt University, et al. The paper has not been peer-reviewed. _ Demonstrating the integrity of plugged and abandoned legacy wells during CO2 storage projects is a crucial requirement for regulators, stakeholders, and operators. The corrosive nature of CO2 may affect the integrity of such wells, jeopardizing the long-term containment of the CO2. This study illustrates the new capabilities, tailored for CO2 storage applications, of a modeling framework that provides a quantitative, risk-based assessment of the long-term integrity of legacy plugged and abandoned wells. Three new modeling modules are added to this integrated framework to account for key concerns. Introduction The plugging and abandonment (P&A) framework translates all well-flow paths into a numerical, spatially discretized, grid-based, digital representation. Multiple grid cell types represent the various well components (e.g., intact and impaired cement, casing, tubing, reservoir formation, and fluid-filled annular space). Appropriate fluid-flow properties are allocated to each cell. The model solves the cell-to-cell flow from source to sink for the entire well P&A system over long periods (typically 3,000 years). Short- and mid-term transient, unsteady flows as well as long-term, steady-state flow rates are then calculated using a finite-difference flow simulator as the back engine. The modeling framework has been field-tested successfully in risk-based P&A designs and integrity assessments. These studies assumed no major chemical or physical interactions between the fluids and P&A components. This is a reasonable assumption for wells in a depleted hydrocarbon field with noncorrosive reservoir fluids, where major geomechanical interactions from pressure and temperature changes in the well are not expected. Extending the framework for carbon capture, use, and storage (CCUS) applications required new workflows to capture the chemical and physical reactions resulting from recharging the reservoir system with CO2. These reactions include the following: - Geochemical interaction of CO2 with water, cement, and near-wellbore formation - Geomechanical effects caused by pressure and temperature changes during the CCUS project - Casing corrosion in the presence of CO2 CO2/Cement Geochemistry Modeling Within the P&A Framework Portlandite [Ca(OH) 2] and calcium silicate hydrate (CSH) are the most important minerals in oilwell cement. Carbonic acid is formed when CO2 dissolves in water. It is a weak acid whose reaction with cement is well-documented in the literature. Cement defects typically have a much greater effective permeability than the cement matrix. Interaction of carbonated water with cement along a cement defect can have significant effect on flow properties, depending on the flow conditions and defect properties. Initially, the carbonated water dissolves a portion of the cement minerals from the face of the defect. The dissolved minerals are transported and, depending on flow and defect-size characteristics, may subsequently precipitate within the defect. Under favorable conditions, the defect size [hydraulic diameter (HD)] may decrease continuously until it is fully blocked. This behavior is called self-sealing. Alternatively, continuous dissolution may dominate if the residence time is insufficient for precipitation to occur. This behavior, called self-degradation, is characterized by the cement defect’s increased hydraulic diameter. Previously published work confirms the authors’ decision not to consider further the effect of carbonated water on the cement matrix because this interaction is relatively small. The vast majority of any leakage occurs through cement defects.