Paper No. 6
Presentation Time: 9:20 AM
EXPERIMENTAL EVALUATION OF WELLBORE INTEGRITY ALONG THE CEMENT-ROCK BOUNDARY
NEWELL, Dennis, Department of Geology, Utah State University, Logan, UT 84321, CAREY, J. William, Earth and Environmental Science Division, Los Alamos National Laboratory, Los Alamos, NM 87545 and CROW, Walter, BP Alternative Energy, dennis.newell@usu.edu
Leakage of CO
2 and brine from geologic storage reservoirs along wellbores is recognized as a major risk factor for the success of geologic carbon sequestration. Our previous work has shown that the most important leakage pathways occur along the interfaces between well casing and cement and between the cement and rock both in the CO
2 reservoir and in the caprock seal. Leakage along the casing interface could result in casing corrosion and chemical reaction with the cement. The cement-rock interface is likely to include pulverized rock from drilling and some disaggregated cement. Leakage along this interface may result in chemical reaction will all these materials along with changes in permeability of the interface. To evaluate the chemical and permeability changes along the cement-rock interface, we conducted core-flood experiments at reservoir conditions. Composite core samples were constructed of cement and rock with a 2.5 mm thick interface of a crushed (~125 micron) mix of rock and cement. Rocks from the caprock seal and from the reservoir were tested. Cores were wrapped in copper foil to ensure a seal to CO
2 leakage from the experiment. Cores were flooded with brine and supercritical CO
2 at ~100 bar and ~60°C.
During a sandstone-cement experiment, permeability decreased from >50 mD during brine injection to ~15 mD during co-injection of CO2. CO2 co-injection also resulted in a pH drop from ~11 to 4. Major element analysis of brine samples supports the carbonation observed in the cement component of the core sample. Optical microscopy and SEM-EDX analysis show a carbonation front extending 3 mm into the cement half of the core parallel to the cement-sandstone contact. Carbonation is also evident along fractures in the cement. The mineral ettringite is present in un-reacted cement, but is absent in the carbonated cement. Within the interface between the cement and sandstone, the cement phases appear leached and carbonate is minor to absent. Progressive flow and reaction along the interface may have removed previously formed carbonate due to the persistent low pH environment.
