Reusing O&G Depleted Reservoirs for CO₂ Storage: Pros and Cons

Two main types of reservoirs are considered for geological storage of CO₂: deep saline formations and depleted oil and gas reservoirs. The former offer very large potential capacity and a more even distribution, at the expense of high uncertainty due to the very poor characterization of their properties, including their sealing capacity; the latter offer smaller overall capacity, but with a reduced risk due to better reservoir knowledge. Gas reservoirs have also provided a proven seal to pressurized gas.

However, reusing depleted O&G reservoirs presents challenges that must be considered in the evaluation of performance factors and the risks associated

Depletion can cause pore collapse in the reservoir – with an associated loss of capacity and injectivity – weaken caprock and bounding faults, or even well completions, leading to possible containment losses due to mechanical failure. O&G reservoirs are also intersected by many wells and it is likely that stricter regulatory requirements on well integrity and the quality of zonal isolation will force operators to recomplete or work over wells which will be exposed to CO₂, with an obvious impact on cost.

Low reservoir pressure may also mean that injection of CO₂ in a dense phase would result in reservoir fracturing and very strong thermal effects that may lead to injectivity problems. In the reservoir, chemical and physical differences in behavior between CO₂ and methane may adversely affect geological containment and injectivity.

Economics and financing present another set of constraints: use of injected CO₂ for EOR/EGR vs. storage may lead to difficulties in obtaining emission credits. Infrastructure decommissioning and conflict with resource exploitation may reduce the attractiveness of depleted reservoirs.

The benefits and challenges of depleted O&G reservoirs will be analyzed with respect to all performance factors (capacity, injectivity, containment) and the major drivers of costs and risks will be identified. Fixed costs and risks, associated to uncertain capacity for deep saline formations or uncertain containment for depleted reservoirs, will then be used to compare the two candidates for CO₂ geological storage and the paper will propose criteria to aid in the selection of possible storage sites.