Optimizing Permanent CO 2Sequestration in Brine Aquifers

Abstract

Geological sequestration of CO 2in brine-saturated formations has been proposed as a possible method to reduce emissions of this greenhouse gas to the atmosphere. To optimize the effectiveness of this method, the largest possible volume of CO 2should be sequestered over geological time. Sequestration over geological time can be thought of as permanent for the purposes of relieving climate-changing increases in atmospheric CO 2concentration. The least risky way to achieve permanent sequestration is to store the CO 2as a residual phase within a brine aquifer. Geological conditions that impact the volume of CO 2stored as a residual phase include petrophysics, burial effects, temperature and pressure gradients, and CO 2pressure-volume-temperature character. Analyzing and integrating all of these parameters result in an optimal CO 2sequestration depth for a given geological subprovince.

The integrated sequestration optimization model was constructed using petrophysical, geological, and CO 2characteristics. Sequestering CO 2as a residual nonwetting phase is one way to ensure its residency in rock over geological time. Thus, residual saturation and porosity were pivotal modeling characteristics. Sediment burial depth affects porosity, temperature, and pressure; thus depth is a key input variable that integrates the other parameters. Finally, CO 2density as a function of temperature and pressure was accounted for, resulting in a model that combines all the salient properties that affect the amount of CO 2that can reside within buried rock.

A model for predicting residual nonwetting-phase saturation and a sequestration optimization curve (SOC) was developed. Results indicate that a sandstone porosity of 0.23 is optimal for CO 2sequestration. The SOC for the Frio Formation, upper Texas Gulf Coast, indicates that the largest volume of CO 2could be trapped as a residual phase at about 3048–3657 m (10,000–12,000 ft). The SOC of depth versus CO 2residual-phase bulk volume is a concave-down parabolic shape with a broad maximum indicating the optimal sequestration depth. Additionally, greater depth decreases the risk of surface leakage and increases the pressure differential between hydrostatic and lithostatic so that higher injection pressures and, thus, higher injection rates can be obtained.