CO₂ Storage Geomechanics for Performance and Risk Management

Controlling the trapping of CO₂ in the subsurface, i.e. storage containment, is of fundamental importance for a safe geological storage of carbon dioxide. During CO₂ injection, increasing fluid pressure, temperature variations, and chemical reactions between fluids and rocks inherently affect the state of stress inside the reservoir and in its surroundings. Besides, the mechanical properties of the rocks exposed to CO₂ may be altered. The impact of the resulting deformations on seal integrity must therefore be assessed in order to properly manage containment performance and leakage-incurred risks.

The analysis starts with the construction and the calibration of a Mechanical Earth Model of the site, through joint analysis of geologic, seismic, logging, drilling, and laboratory test data. Such a model consistently describes ambient stresses, fluid pressures, and poro-mechanical and strength properties of the formations. It is linked to a reservoir model to achieve initial equilibrium and also to further simulate the coupled transport, chemical and mechanical processes occurring during CO₂ injection operations and the subsequent re-equilibration. The predicted stress path allows the evaluation of the mechanical stability of both cap-rock and faults (which may bound the reservoir, penetrate the cap-rock or intercept wells). The stability of wells in formations experiencing strain is also investigated.

In addition, an accurate Mechanical Earth Model contributes to optimizing well construction and stimulation operations. Profiles of stresses and mechanical properties along a planned-well trajectory allow designing a drilling operation that will maximize subsequent hydraulic isolation of the well by optimizing the wellbore condition. Similar information along existing wells helps to control hydro-fracture propagation when injectivity enhancement is required.

The Mechanical Earth Model can be used to develop operating envelopes for well placement, hydraulic fracturing, and CO₂ injection that best ensure containment while achieving injectivity and capacity requirements.