58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

The Arbuckle Dolomite deep saline reservoir is being evaluated as a carbon capture, utilization and storage (CCUS) site.  Each potential CCUS site has a unique set of geological, microbiological, and geochemical characteristics that must be evaluated prior to the injection of supercritical CO₂ (CO_2(SC)).  Understanding how CO_2(SC) exposure affects Arbuckle aquifer and seal materials will provide data for improved injection and well-monitoring plans, and may provide insight into future CCUS projects.

The purpose of this research is to characterize near-injection well weathering reactions of the Arbuckle dolomite (aquifer material) and Lower Mississippian seal (considered an alternate primary seal) materials upon exposure to CO_2(SC).  These studies will provide information to help assess the impact of CO_2(SC) exposure on the injectivity and storage capacity of the Arbuckle, as well as predict seal integrity for the system. 

Core plugs from the Arbuckle dolomite reservoir and the Lower Mississippian dolomitic siltstone were reacted with synthetic brine (modeled on Arbuckle pore water geochemistry) and 100% pCO_2(SC) under injection zone conditions (2500 psi and 50 °C) in stationary steel autoclave reactors for >30 days.  A set of identical experiments was pressurized with N₂, and served as the control. The core plugs were powdered and sieved to a size fraction of 63-125 μm to insure reaction rates within the time period of the experiment. 

Brine geochemistry from experimental reactors display a decrease in pH of ~ 2 pH units compared to N₂ controls.  The brine concentrations of Mg²⁺, Ca²⁺, and HCO₃^- increased for both the Arbuckle and Lower Mississippian experiments after exposure to CO_2(SC).  Geochemical modeling of these changes suggest that CO_2(SC) injection led to undersaturated conditions and dolomite dissolution.  Scanning electron microscopy of reacted Arbuckle Dolomite confirms dissolution features.  An increase in the total sulfur concentration of the brine in the CO_2(SC)-reacted Lower Mississippian seal materials combined with observations via SEM suggest pyrite dissolution.  Secondary clay precipitation can also be observed via SEM in the Lower Mississippian solids exposed to CO_2(SC).

Future work includes a series of biotic experiments that will test the microbiological effects of CO_2(SC) exposure on Arbuckle and Lower Mississippian materials.  Bulk mineralogical changes for all current and future experiments will be determined via X-ray diffraction (XRD).

Results to date suggest that CO_2(SC) injection will lead to dissolution of some minerals in the Arbuckle Dolomite and Lower Mississippian materials.  Dissolution of aquifer material will only enhance porosity and storage capacity.  More information is needed to assess the integrity of the Lower Mississippian seal materials, but initial data suggests that secondary mineral formation (clay minerals and possibly anhydrite) will offset, at least in part, any porosity enhancement due to dissolution reactions.