CO2/Brine/Rock Interactions in the Cedar Keys-Lawson Formation

Abstract

It has been reported that among the various geologic storage options, deep saline aquifers have the largest estimated capacity for CO₂ storage.  Obtaining knowledge of possible artificially geochemically induced changes to the permeability and porosity of host CO₂ storage deposits will enable us to gain insight on long-term reservoir behavior under CO₂ storage conditions. 

An experimental study of the interaction of CO₂/brine/rock on saline formations was conducted in a static system under CO₂ storage conditions.  Chemical interactions in the Cedar Keys-Lawson Formation carbonate during exposure to CO₂ and brine under sequestration conditions were studied.  Samples were exposed to the simulated in-situ reaction conditions for one and six months.  The samples were exposed to a model brine at 55 °C and CO₂ pressure of 23.8 MPa (3,500 psig). Computed tomography (CT), x-ray diffraction (XRD), scanning electron microscopy (SEM)-energy dispersive x-ray spectroscopy (EDS), brine composition, core porosity, and core permeability analyses were conducted prior to and after the exposure experiments.

Preliminary permeability measurements obtained from the core samples showed changes after they were exposed to CO₂-saturated brine for one and six months. This observation suggests that mineral dissolution and mineral precipitation could occur in the host deposit altering its characteristics for CO₂ storage over time. The 3D images of the pore space clearly illustrate the degree of dissolution that occurred during exposure.  It is noted that the dissolution that occurred during the six-month exposure could have enhanced the connectivity between voids. This may contribute the increase of permeability after the CO₂/brine exposure.  In addition, the primary minerals comprising the core are dolomite and gypsum. Both minerals could dissolve in the CO₂/brine environment resulting in the increase of permeability after the six-month exposure.