Geomechanical Factors Affecting Geological Storage of CO2 in Depleted Oil and Gas Reservoirs

Abstract

A key to the success of long-term storage of CO2 in depleted oil or gas reservoirs is the hydraulic integrity of both the geological formations that bound it, and the wellbores that penetrate it. This paper provides a review of the geomechanical factors affecting the hydraulic integrity of the bounding seals for a depleted oil or gas reservoir slated for use as a CO2 injection zone. Potential leakage mechanisms reviewed include fault reactivation, induced shear failure of the caprock, out-of-zone hydraulic fracturing, and poorly sealed casing cements in enlarged, unstable boreholes. Parameters controlling these mechanisms include the upper and lower bounds of pressure and temperature experienced by the reservoir, the orientation and mechanical properties of existing faults, rock mechanical properties, in situ stresses, and reservoir depth and shape. Approaches to mitigate the likelihood of geomechanics-related leakage include the identification of safe upper limits on injection pressures, preferred injection well locations, review of historical records for reservoir pressures, temperatures and stimulation treatments, drilling program design to mitigate rock yielding in new wells, and assessment of wellbore integrity indicators in existing wells. Introduction In order to achieve significant reductions in the atmospheric release of anthropogenic greenhouse gases, the implementation of technologies to capture carbon dioxide (CO2) and store it in geological formations will be necessary. Deep saline aquifers have the largest potential for CO2 sequestration in geological media in terms of volume, duration, and minimum or null environmental impact(1). The first commercial scheme for CO2 sequestration in an aquifer is already in place in the Norwegian sector of the North Sea, where 106 tonnes of CO2 are extracted annually from the Sleipner Gas Field and injected into the 250 m thick Utsira aquifer at a depth of 1,000 m below the sea bed(2). In light of the economic benefits of enhanced oil recovery (EOR) derived from CO2 injection in oil reservoirs(3), these types of reservoirs will be attractive CO2 injection targets and, most likely, CO2 storage in depleted oil and gas reservoirs (or in conjunction with EOR) will be implemented before CO2 storage in aquifers. An advantage of CO2 storage in depleted oil or gas fields is the fact that much of the infrastructure for fluid injection (e.g., wellbores, compressors, pipelines) is already in place. The Weyburn CO2 Monitoring and Storage Project in Saskatchewan, Canada(4) is an example of a large-scale application of EOR operations using anthropogenic CO2, in which the oil reservoir is being evaluated for subsequent use as a long-term storage zone. A key to the success of long-term storage in depleted oil and gas reservoirs is the hydraulic integrity of both the geological formations that bound it, and the wellbores that penetrate it. The initial integrity of this "bounding seal" system is governed by geological factors. A considerable amount of effort has been devoted to the development of procedures for assessing fault seal capacity in potential hydrocarbon reservoirs(5).

Coupled reservoir–geomechanical analysis of the potential for tensile and shear failure associated with CO2 injection in multilayered reservoir–caprock systems

“…In an analytical shear failure analysis, the horizontal poroelastic stressing may be estimated for the idealized case of a thin, laterally extensive reservoir (i.e., the so-called passive basin) according to [19]:…”

“…As mentioned in the introduction, analytical techniques are indeed very useful for first-order estimates of the maximum sustainable CO 2 injection pressure, and for identification of the most critically oriented faults in the system. A first-order estimate of poroelastic stressing may be based on previous field interpretations [21,22], or by the analytical and semi-analytical approaches described above [19,20]. A coupled reservoirgeomechanical numerical modeling, on the other hand, might be suitable once more sitespecific data on geometry and material properties are available.…”

A response surface methodology to address uncertainties in cap rock failure assessment for CO2 geological storage in deep aquifers

“…Several authors (Streit and Hillis, 2004;Hawkes et al, 2005;Rutqvist et al, 2007 andVidal-Gilbert et al, 2008) have shown that the CO 2 -injection operations lead to an increase in the pore pressure inside and around the host reservoir, which results in a general decrease of the effective stress (by convention, compressive stress is positive).…”

“…Streit and Hillis, 2004;Hawkes et al, 2005). The aquifer layer can be modelled by an idealized thin and laterally extensive reservoir.…”

“…Several authors (Streit and Hillis, 2004;Hawkes et al, 2005;Rutqvist et al, 2007 andVidal-Gilbert et al, 2008) have shown that CO 2 injection operations lead to reservoir fluid pressure increase and to mechanical stresses changes, which might potentially induce creation of new fractures or reactivation of pre-existing faults in the caprock layers. These mechanical discontinuities (fault or fracture) represent leakage pathways (e.g.…”