Cement/caprock fracture healing experiments to assess the integrity of CO2injection wells

It has been speculated that fractures along wellbore cement/caprock interfaces may provide a path for release of carbon from both long-term sequestration-sites and CO2-based enhanced oil recovery operations. The goal of this study is to evaluate the potential for fracture growth and healing in the wellbore environment, and its impact on wellbore permeability. A series of flow-through experiments was conducted, in which sample cores containing a planar fracture between impermeable caprock (compacted quartz, from 13,927’ depth in Kern County) and cement (Portland G cured by ATSM standards) were reacted with brine containing variable amounts of carbonic acid (pCO2 between 0 and 3 MPa). The initial fracture geometry was controlled by grinding the caprock and cement pieces flat, and then bead blasting topography into the cement surfaces. Runs lasted 4-8 days with cores and brine maintained at constant temperature (60 °C). Constant confining pressure (24.8 MPa) was applied to cores, while brine was flowed with constant rates (0.05-0.10 mL/min) and pore pressure (12.4 MPa). Geomechanical and geochemical responses of the fractures were monitored by in situ measurements of differential pressure, and by periodically sampling output brine to analyze compositional changes. In every experiment the total permeability of samples cores decreased substantially. For runs using brine with pCO2 = 3 MPa, sample permeability continually decreased by over a factor of 200. Sample permeability also decreased by a factor of 50 having stabilized after ~3 days in a run using brine without CO2 (pCO2 = 0 MPa). These reductions in permeability appear to be the result of chemically-induced changes to the mechanical properties of the cement surface. Prior to reaction, the cement-caprock samples had high strength and elastic response to changes in stress during loading. After the experiments, the samples were weaker, and showed inelastic response to changes in stress during unloading. All cement surfaces exposed to CO2-rich brine were heavily reacted, as evidenced by coatings of rust-colored amorphous material. X-ray micro-tomography images revealed a series of reaction zones consistent with the results of related experiments by other researchers [e.g. Kutchko et al. 2007]. The mechanical properties of the individual reaction zones were evaluated by nano-indentation. Sampling during runs indicated that brine with pCO2 = 3 MPa became substantially enriched in Ca, Si, and Al, whereas composition of output brine with pCO2 = 0 MPa had little change over the run duration. The enrichment of Al in the brine with pCO2 = 3 MPa indicates that both Al -bearing minerals and amorphous calcium-silicate-hydrate (CSH) dissolved from the cement. Geochemical reaction pathways were further characterized in the reacted zones with the cement by scanning electron microscope, x-ray diffraction, and solid state NMR spectroscopy. These results suggest that the evolution of fractures in our experiments are determined by 3 competing factors: 1) swelling of CSH through hydration from the brine, 2) dissolution of cement into brine containing CO2, and 3) mechanical weakening of cement by chemical reaction with CO2. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-Eng-48 and Contract DE-AC52-07NA27344.