Climate Change:

Confronting Global Warming: The Geological Sequestration of CO₂ 

LA-UR-06-3870
Author: Bill Carey, EES-6
June 2006

The continued use of fossil fuels threatens to heat up the climate as CO₂ emissions may lead to global warming. Our research team is investigating one solution to this problem: Sequestration of CO₂ in geologic reservoirs.  With this approach, CO₂ is captured or separated at point sources, such as power plants, and injected underground into depleted oil reservoirs or saline (non-potable) aquifers. The viability of sequestration depends both on the economics of CO₂ capture and injection as well as the permanence of CO₂ storage.

There are five primary mechanisms of permanent CO₂ storage: 1) Stratigraphic trapping of the buoyant CO₂ plume by impermeable caprocks (such as shale, salt, etc.); 2) Capillary trapping of CO₂ in microscopic pores;  3) Dissolution of the CO₂ into the brine; 4) Reaction of dissolved CO₂ with minerals in the reservoir (such as calcite and clay minerals) to produce bicarbonate ion; and 5) Precipitation of carbonate minerals such as calcite and dawsonite. Some of these processes may take decades to hundreds of years to occur. In the short term, demonstrating the integrity of the  primary seals to upward migration of CO₂ is critical to CO₂ reservoir performance.

The wellbore seal is probably the most vulnerable component of a CO₂ reservoir because the Portland cement that provides a barrier to fluid migration is reactive in CO₂-bearing fluids (see figure to right). The concern extends from the wells used to inject the CO₂ to any older wells that may penetrate the CO₂ reservoir.

To address the wellbore integrity issue, we have collected samples from a wellbore with 30-years of CO₂ exposure,  conducted experimental studies, and performed numerical modeling to develop a predictive basis for wellbore performance. The samples were obtained through the generosity of  Kinder Morgan CO₂ Company who re-entered a well in the SACROC Unit, which is the oldest CO₂-enhanced oil recovery project in the United States.

The sample at the right shows a polished slab of cement collected 10 feet above the reservoir-caprock contact in Well 49-6. It is about 5 cm in length such that the casing is on the left and the shale caprock is on the right. The (gray) cement adjacent to the casing is relatively unaltered but does have calcite-filled fractures. The orange, altered cement adjacent to the shale is strongly carbonated to an assemblage of calcite-aragonite-vaterite with residual amorphous alumina and silica. The interface between the orange and gray cements is relatively dense and siliceous.

Our experimental efforts to understand cement-interface reactivity include flow-through studies at elevated temperature and pressure. In this image, a sample of cement with a central fracture was saturated with brine and then CO₂ was injected from bottom towards the top. The CO₂ penetrated along the fracture and produced an orange alteration with central regions of gray, relatively unaltered cement, quite similar to the observations at SACROC.

The experimental results and SACROC observations have been used to construct a numerical model for observations are being used to construct a numerical model for cement behavior in a CO₂-saturated wellbore environment. We have used the reactive transport code FLOTRAN  to calculate the effect of 30 years of diffusion-based attack of CO₂-saturated brine on cement. The observations at SACROC were used to calibrate porosity-tortuosity-reaction rate parameters for wellbore cement. We have successfully reproduced a number of the SACROC features including the 0.5 cm zone of intense carbonation and the distribution and alteration of primary cement minerals (see Figure below).

The figure above shows the results of a simulation of brine saturated with 180 bar CO₂ pressure diffusing from a high-porosity shale at > 0.05 m (representing a porous wallcake residue of drilling mud) into a cement with at 30% initial porosity. After 30 years, approximately 0.005 m of cement has been completely altered and replaced by an assemblage of calcite and cement residue (amorphous silica, secondary calcium-silicate-hydrate,  gibbsite, representing amorphous aluminum hydroxide, and gypsum). This predicted alteration assemblage closely resembles the observed orange-altered cement in the polished slab shown above. The shale mineralogy is essentially unaltered with a narrow deposit of amorphous silica and gypsum at the interface. The cement interior is largely unaltered, although the diffusion of chloride transforms the monosulfate to Friedel's salt and changes in pH and sulfate result in the formation of ettringite.

The observations at SACROC suggest that despite its inherent reactivity Portland cement can survive for at least decades in a CO₂-rich environment. Additional observations at other wellbores and in different environments are necessary before drawing more definitive conclusions on wellbore performance in CO₂ reservoirs. However, the SACROC samples do show that the most important potential leakage pathways are not due to CO₂-induced alteration of the cement matrix but rather due to potential migration of CO₂ along the cement-caprock or cement-casing interface. Our current work is focused on evaluating the factors that control whether CO₂ migration widens these interfaces due to cement dissolution or actually closes these interfaces due to carbonate precipitation.

The SACROC samples provide a snapshot in time. Our experimental and modeling programs are designed to  understand the implications of the observations. In particular, we eventually hope to be able to estimate the amount of CO₂ migration implied by the orange alteration zone and to predict whether the interface is stable, widening, or closing with time.
This work was funded by DOE-NETL (contract #05FE01, "CO₂-Water-Rock Interactions and the Integrity of Hydrodynamic Seals) and LANL LDRD-DR 20040042DR, "Science of Geological Carbon Sequestration").  For more information on CO₂ Sequestration and Wellbore Integrity, please contact Bill Carey. For more information on Los Alamos' programs in carbon-neutral energy technolgies see the Energy Security home page.

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