• Energy Research

Abstract A modular reactive transport model, Dynaflow ™ , is used to simulate the reactivity of cement in CO2-saturated water of intermediate salinity (0.5 M). Methodology for coupling transport and geochemical modules is derived and its assumptions are discussed. The modules are coupled in a sequential iterative approach to accurately model: (1) mineral dissolution/precipitation (2) aqueous phase speciation and (3) porosity-dependent transport properties. Simulation results reproduce qualitatively the dissolution of cement hydrates (CH, C-S-H, AFm, AFt) and intermediate products (CaCO3) that have been observed experimentally. However, when using a standard power law to relate effective transport properties to porosity, modeling and experimental results do not coincide; here, agreement between simulations and observations is obtained by modifying the functional dependence of effective diffusivity on mineralogy. Furthermore, for this particular system for which concentration gradients are the only driving force, the assumption of neglecting the mass balance of water or density changes might show its limits. Therefore, future work should investigate the likely need to account for reaction-driven advection.

AbstractExperimental results of Portland cement reactivity in CO2 rich fluids have so far seemed inconsistent, providing different values of reaction rates for apparently similar experimental conditions. Coupled transport-reaction models allow reconciling experiment al evidence within a consistent framework. Experimental and numerical results suggest two classes o f controlling mechanism. For Class G/H cements, either reactivity is Ca or CO2 diffusion-limited, depending on the boundary conditions, or reactivity stops because of pore clogging by calcium carbonate formation.

AbstractStoring 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 CO2 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.

AbstractCarbon dioxide (CO2) geological storage relies on safe, long-term injection of large quantities of CO2 in underground porous rocks. Wells, whether they are the conduit of the pumped fluid or are exposed to CO2 in the storage reservoir (observation and old wells) are man-made disturbances to the geological storage complex, and are thus viewed by some as a possible risk factor to the containment of the injected CO2.Wells are composite structures, with an inner steel pipe separated from the borehole rock wall by a thin cement sheath (∼2 cm) that prevents vertical fluid migration. Both carbon steel and cement react in the presence of CO2, although evidence from production of CO2-rich fluids in the oil and gas industry and from lab experiments suggests that competent, defect-free cement offers an effective barrier to CO2 migration and leaks.However, reactivity of cement and steel may result in CO2 migration pathways degrading over time, thus in the leakage risk increasing during the life of the storage project. The issue then becomes how to best integrate preventive verification of zonal isolation/well integrity in the storage site monitoring plan. An analysis of the order of magnitude of possible CO2 leaks, and of their path to potable aquifers or the atmosphere, is also necessary to optimize the assurance (mitigation) monitoring of the storage site.Evidence gathered during the MovECBM project indicates that migration of small quantities of CO2 happened during injection in a coal seam in Southwest Poland. The evidence, gathered from casing and cement logging as well as soil gas monitoring over a 3-year period, was coupled with laboratory testing and extensive modeling of the chemo-mechanical behavior of cement and steel to determine if CO2 migration might have been responsible of the observed behavior.The three lines of evidence were: The detection of very small CO2 fluxes, coupled with less controversial helium concentration in soil; the occurrence of a thin pathway at the interface between cement and casing; and the change in mechanical properties of cement, suggestive of partial carbonation.Whereas the observations suggest that limited CO2 migration might have happened in the well, they are by no means proof that the migration did happen. Nonetheless, the integration of measurement and modeling yields important lessons for wellbore monitoring.First, it puts a probable ceiling on the order of magnitude of expected leaks from reasonably well-cemented wells at around 100 metric tons per year (less than 0.05% of the injected mass in a well like Sleipner or In Salah). It also suggests that cement may be a very effective leak detector: Exposure to CO2 modifies its mechanical properties, which in turn can be detected using cement evaluation logs. Finally, coupling with dispersion modeling suggests the precision and accuracy required from soil gas and atmospheric monitoring, as well as the placement of sampling points; it also suggest that hysteresis, due to the accumulation in CO2 in surface aquifers and to the time required for it to be transported to the survey points, may delay initial detection; the same hysteresis may at the same time prolong the occurrence of CO2 shows long after the leak has stopped.

AbstractThe risk minimization of carbon dioxide (CO2) storage relies to some extent on well integrity assurance. While chemical reactions between the constitutive materials of the wellbore–such as cement–and stored CO2 do not jeopardize the efficiency of a defect-free cement sheath, the reactive flow through an existing pathway in a cemented annulus may alter its initial properties, and thus change the associated risk. On one hand, the cement will react with the CO2 rich fluid, and be leached away by the leaking flow. On another hand, under specific conditions, the released minerals can re-precipitate downstream and clog the pathway.The evolution of a CO2 leak through a pre-existing leak path in the cement sheath is examined theoretically and numerically. A numerical model of the flow has been built, that takes into account the particular physics and geometry of the problem. The governing equations are investigated in order to identify the driving mechanisms and define the dimensionless groups that rule the process, leading to a reduction in the dimension of the parameters space. We use this analysis to predict the domain of stability of the defect by extracting a general criterion corresponding to the clogging conditions.

Abstract To evaluate the risk of corrosion of cement by geosequestered CO2, samples are being retrieved from wells placed in natural CO2 deposits [e.g., Crow et al., 2009]. If the cement passing through the cap rock is carbonated, it may indicate that annular gaps or cracks have allowed carbonic acid to come into contact with the cement. However, it must be recognized that the pore water in the cap rock has become saturated with CO2 over geological time. After the well is placed, the CO2 will diffuse toward the cement and react with it. A simple analysis of the diffusion kinetics demonstrates that carbonation depths of millimeters to centimeters can be expected from this reaction within the lifetime of a well, in the absence of any cracks or gaps. Therefore, the occurrence of carbonation in cement sealing natural CO2 deposits must be interpreted with caution.