Three-Way Coupled Modeling Evaluates CO2 Sequestration in Depleted Reservoir
Authors: Carpenter Chris 
Published: February 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 206156, “Importance of Three-Way Coupled Modeling for Carbon-Dioxide Sequestration in a Depleted Reservoir,” by Prasanna Chidambaram, SPE, Pankaj K. Tiwari, SPE, and Parimal A. Patil, SPE, Petronas, et al. The paper has not been peer reviewed.

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Three major depleted gas reservoirs in the Central Luconia field offshore Sarawak, Malaysia, are being evaluated for future carbon-dioxide (CO2) storage. A three-way coupled modeling approach that integrates dynamic, geochemistry, and geomechanics models is used to obtain the cumulative effect of all three changes. This integrated model provides a more-accurate estimate of CO2 storage capacity, caprock-integrity evaluation, CO2-plume migration path, and volume of CO2 stored through different mechanisms.

Background
The CO2 storage sites being evaluated are depleted gas reservoirs that have been in production for a few decades. At the end of their producing life, they have the potential to be converted into CO2 storage sites. The Central Luconia sedimentary basin is in a seismic-free zone with limited faults and consists of shale interbedded with high-sand-content sediment, making it ideal for CO2 storage. These reservoirs provide the required geological characteristics and volume needed to ensure long-term CO2 storage in a safe and economical way.
The depleted gas reservoirs have an in-place volume of approximately 1.5–3 Tscf. Their thickness ranges from 100 to 150 m, with a porosity of 15–32% and permeability of 10–1600 md. These fields are believed to be supported by a regional aquifer several thousand feet thick. Large seafloor subsidence has been observed in these reservoirs.

Storage-Capacity Estimation
Storage capacity of a depleted hydrocarbon reservoir is affected by several factors including voidage created; aquifer influx and efflux during the production and CO2-injection phases, respectively; maximum injection pressure; rock compressibility; and geochemical effects. Depending on which of these factors are prominent in the storage reservoir, CO2-storage capacity may be estimated using a simple material-balance model or may require a more-complex approach to capture these effects.
Use of the Three-Way Coupled Model Injected CO2 is anticipated to react with reservoir rock, leading to either dissolution of reservoir rock or precipitation of solids that are products of the geochemical reactions, causing a net change in porosity and permeability.
With regard to geomechanical effects, uplift is anticipated to occur during CO2 injection. The degree to which subsidence is reversed depends on whether compaction of the reservoir is fully elastic or if plastic deformation has occurred. During production, reduction in porosity and permeability has occurred because of subsidence.
Conventional or stand-alone reservoir simulation does not capture geochemical and geomechanical effects. Hence, it is critical to use an integrated model that captures the effects of dynamic, geochemical, and geomechanical changes caused by CO2 injection in order to evaluate suitability of the reservoir for long-term CO2 storage. A three-way coupled modeling approach that integrates dynamic, geochemistry, and geomechanics models provides a more-accurate estimate of CO2 storage capacity, along with estimation of subsidence.
In three-way coupled modeling, the dynamic model is at the center, which passes input parameters to the geochemical and geomechanical models. Once it receives updated porosity and permeability values back from the geochemical and geomechanical models, the dynamic model incorporates these changes before proceeding to the next simulation timestep.