Storing carbon dioxide in depleted petroleum reservoirs is a viable strategy for carbon mitigation, but ensuring that the sequestered CO2 remains in the formation is vital to the success of such projects. There is great concern for the development of leakage pathways through annuli between the well cement and the formation or the casing. Predicting the behavior of such potential leakage pathways is critical. Numerical simulations conducted using a reactive transport module match well with experimental studies , but also show the necessity of quantifying the transport and mechanical properties of the leached solid cementitious solids - predominantly silica gel - produced by carbonic acid corrosion of well cement. Bench-top experiments have been performed with the following goals in mind: 1) to investigate the parameter space of relevant corrosion boundary conditions, e.g. pH, CO2 concentration, and calcium ion concentration, 2) to produce samples that can be used to quantify the transport and mechanical properties of acid corroded Class H well cement, and 3) to validate and improve the accuracy of numerical simulations of the reaction of well cement with carbonic acid. Class H cement samples were uniaxially corroded via exposure to a brine of constant composition. Constant composition is ensured by constant renewal of the brine at a rate larger than cement reaction rate. H+, Ca2+ and CO2 total aqueous concentration in the NaCl brine are controlled independently by adding known amounts of NaCl, HCl, CaCl2 and NaHCO3 and by controlling CO2 partial pressure. Microscopic (30X) time-lapse videos were taken of each sample so that corrosion front movements could be accurately measured. These experiments have yielded corrosion front measurements that clearly show that corrosion front advancement is diffusion controlled (i.e., linear as a function of the square root of time). The uniaxial corrosion of these samples has not only allowed for detailed measurements of the corrosion front, but also affords the opportunity to measure the mechanical properties of the corroded samples as a function of depth. The one-dimensional corrosion also allows for measuring the diffusion coefficient of the outer layer of silica gel by low field Nuclear Magnetic Resonance (NMR). Measuring the kinetics under various boundary conditions has validated the modeling results reported by Huet et al. . The measurements of mechanical and transport properties can now be used to improve the predictive power of these simulations by providing much needed information on the exterior layer of corroded Class H well cement. Additionally, these experiments offer experimental validation that the corrosion kinetics are enhanced by the presence of CO 2 and open the door to better understanding of the mechanism of, and boundary conditions that might lead to, "pore-plugging" by the corrosion products, which in turn leads to a drastic retardation of the corrosion reaction.
All Science Journal Classification (ASJC) codes
- Acid leaching of Portland cement
- Carbonic acid
- Class H well cement
- Wellbore integrity