• Energy Research

Abstract Fractures along interfaces between host rock and wellbore cement have long been identified as potential CO2 leakage pathways from subsurface CO2 storage sites. As a consequence, cement alteration due to exposure to CO2 has been studied extensively to assess wellbore integrity. Previous studies have focused on the changes to either chemical or mechanical properties of cement upon exposure to CO2-enriched brine, but not on the effects of loading conditions. This paper aims to correct this deficit by considering the combined effects of the fracture pathway and changing effective stress on chemical and mechanical degradation at conditions relevant to geologic carbon storage. Flow-through experiments on fractured cores composed of cement and tight sandstone caprock halves were conducted to study the alteration of cement due to exposure to CO2-enriched brine at 3, 7, 9, and 12 MPa effective stress. We characterized relevant reactions via solution chemistry; fracture permeability via changes to differential pressure; mechanical changes via micro-hardness testing, and pore structure changes via x-ray tomography. This study showed that the nature and the rates of the chemical reactions between cement and CO2 were not affected by the effective stress. The differences in the permeability responses of the fractures were attributed to interactions among the geometry of the flow path, the porosity increase of the reacted cement, and the mechanical deformation of reacted asperities. The suite of observed chemical reactions contributed to change in cement mechanical properties. Compared to the unreacted cement, the average hardness of the amorphous silica and depleted layers was decreased while the hardness of the calcite layer was increased. Tomographic imaging showed that preferential flow paths formed in some of the core-flood experiments, which had a significant impact on the permeability response of the fractured samples. We interpreted the observed permeability responses in terms of competition between dissolution of cement phases (leading to enhanced permeability) and mechanical deformation of reacted regions (leading to reduced permeability).

Abstract Beneficial pore space and permeability enhancements are likely to occur as CO2-charged fluids partially dissolve carbonate minerals in carbonate reservoir formations used for geologic CO2 storage. The ability to forecast the extent and impact of changes in porosity and permeability will aid geologic CO2 storage operations and lower uncertainty in estimates of long-term storage capacity. Our work is directed toward developing calibrated reactive transport models that more accurately capture the chemical impacts of CO2-fluid-rock interactions and their effects on porosity and permeability by matching pressure, fluid chemistry, and dissolution features that developed as a result of reaction with CO2-acidified brines at representative reservoir conditions. We present new results from experiments conducted on seven core samples from the Arbuckle Dolostone (near Wellington, Kansas, USA, recovered as part of the South-Central Kansas CO2 Demonstration). Cores were obtained from both target reservoir and lower-permeability baffle zones, and together these samples span over 3–4 orders of magnitude of permeability according to downhole measurements. Core samples were nondestructively imaged by X-ray computed tomography and the resulting characterization data were mapped onto a continuum domain to further develop a reactive transport model for a range of mineral and physical heterogeneity. We combine these new results with those from previous experimental studies (Smith et al., 2013; Hao et al., 2013) to more fully constrain the governing equations used in reactive transport models to better estimate the transition of enhanced oil recovery operations to long-term geology CO2 storage. Calcite and dolomite kinetic rate constants (mol m−2 s−1) derived by fitting the results from core-flood experiments range from kcalcite,25C = 10−6.8 to 10−4.6, and kdolomite,25C = 10−7.5 to 10−5.3. The power law-based porosity-permeability relationship is sensitive to the overall pore space heterogeneity of each core. Stable dissolution fronts observed in the more homogeneous dolostones could be accurately simulated using an exponential value of n = 3. Unstable dissolution fronts consisting of preferential flowpaths could be simulated using an exponential value of n = 3 for heterogeneous dolostones, and larger values (n = 6–8) for heterogeneous limestones.

AbstractA reactive transport modeling approach was used to assess key geochemical reactions between wellbore cement, formation mineralogy, and injected supercritical CO2 at the Krechba natural gas field at In Salah, Algeria. Characterization of these reactions is important for understanding changes in porosity and permeability and the propagation or sealing of fractures in the formation or wellbore cement. Experiments involving the reaction of CO2 with reservoir mineral assemblages and wellbore cement under in situ conditions, combined with geochemical modeling, were used to identify candidate reactive mineral phases and reaction rates specifically applicable to CO2 injection at In Salah. These findings informed a reactive transport model which considered advective transport of CO2 along the wellbore-formation interface and diffusive transport of CO2 and brine constituents into juxtaposed wellbore cement. Model results indicate shallow carbonation of the cement along the interface, leading to changes in cement porosity. Diffusive transport of cations such as Ca, Fe, and Al between the cement and formation materials results in mineralogical changes in the formation material immediately adjacent to the cement, including localized dissolution of calcite and precipitation of siderite, magnesite, gibbsite, smectite, and amorphous silica. In contrast to the cement, the modeled porosity changes in the formation material appear to be minor. Taken together, these results suggest (1) significant retardation of the rate of advance of CO2 along the interface, and (2) relatively minor impacts to permeability.

Deep wells provide a possible pathway for CO2 and brine leakage from geologic storage reservoirs to shallow groundwater resources and the atmosphere. The integrity of wellbore cement in these environments is of particular concern, because it is not known if changes in cement properties resulting from reaction with CO2-rich brines will lead to enhanced leakage over the life cycle of the storage reservoir. Assessment of wellbore leakage will ultimately be answered through models that capture both the chemical and physical processes and the uncertainty of key parameters within the wellbore environment. Towards this end, we use the results for 13 core–flood experiments conducted at variable partial pressures of CO2, flow rate, durations, and cement–caprock apertures to constrain a wellbore model that couples chemical processes important to assessing the long-term integrity of wellbore cements in geologic carbon storage environments. X-ray computed microtomography collected prior-to and following the experiments was employed to spatially resolve the interface and the extent of the reaction zones, and time dependent solution chemistry was used to track the chemical alteration over the course of the experiments. In this manuscript we focus on the development of geochemical model that describes the alteration of both the cement and the caprock. In our experiments, chemical alteration of the cement significantly exceeded any dissolution of carbonate minerals within the caprock and fracture geometry played no role on the extent of reaction. The experimental data was used to calibrate a numerical model of wellbore-caprock interfaces coupling reaction-front chemistry, fluid flow and transport of dissolved species. The geochemical model adopts an idealized representation of the cement chemistry in which appropriate equilibrium conditions are enforced at a series of discrete reaction fronts. The equilibrium conditions are coupled by diffusive transport between the fronts, which also determines the rate of front propagation. Despite its simplicity, the calibrated model accurately reproduces the reaction-zone growth and effluent chemistry for the range of experimental conditions considered and allowed key parameters to be confirmed or calibrated. These include the use of portlandite, calcite, and analcime solubility as equilibrium controls at specific reaction fronts within the cement; the use of constant effective diffusivity for each alteration zone; and diffusive growth of the alteration layers.

Abstract A comprehensive risk-based monitoring assessment methodology is developed to (i) incorporate background data for thresholds and monitoring design, (ii) use stochastic leakage simulations and monitoring modeling for risk scenarios given uncertainty, (iii) apply statistical methods to combine multiple monitoring techniques and provide decision support for evaluating proposed monitoring plans, and (iv) facilitate implementation in the National Risk Assessment Partnership (NRAP) integrated assessment model (IAM) framework. This methodology is illustrated using a case representative of the High Plains aquifer, considering the stochastic leakage events simulated using reactive transport simulations. The resulting groundwater quality changes were reflected in three groundwater monitoring parameters (pH, TDS and benzene concentrations), which were used to calculate the corresponding detection probability for each simulation, based on the background distributions and the selected thresholds. The consequent detection probability was then used to evaluate the monitoring well density and the proposed network designs. Beyond the detectability, the earliest response time and spatial coverage constraints of a monitoring network design are considered, and the role of adaptive monitoring strategies for compliance monitoring and incorporation of expert judgment on likely leakage locations is considered.

Abstract We present an approach for managing geologic CO2 storage using wells that serve three sequential purposes (1) characterization and monitoring, (2) brine production, and (3) CO2 injection. Two reservoirs are deployed in tandem: (1) a CO2-storage reservoir and (2) a brine-storage reservoir. This approach provides data that can be analyzed prior to CO2 injection, enabling proactive reservoir management, which reduces cost and risk. We analyze a range of brine-disposition options, including 100% reinjection in a nearby brine-storage reservoir and cases where a portion of the produced brine is used to generate water, with the residual brine reinjected in the nearby reservoir. We also consider the option of time-shifting the parasitic load of brine production and injection, so that those operations can utilize excess energy from electric grids during periods of over-generation.

Abstract Well integrity is a prerequisite for safe geological storage of CO 2 . Both active and abandoned wells are man-made channels between the storage reservoir and the atmosphere that can become leakage paths over time. The CO 2 injection wells are of special concern, as they are exposed to low temperatures and strong temperature variations. In oil/gas wells this is known to threaten well integrity, and it should therefore be investigated how the sealing ability of well barriers is affected in the temperature range relevant for CO 2 injection wells. This has been the main focus in a research project entitled “Ensuring well integrity during CO 2 injection” carried out in the period 2014-2016. This paper summarizes the major findings obtained within the project, and presents research-based recommendations for construction and operation of CO 2 injection wells.

AbstractThe Final-Phase Weyburn geochemical research program includes explicitly integrated yet conceptually distinct monitoring, modeling, and experimental components. The principal objectives are to monitor CO2-induced compositional evolution within the reservoir through time-lapse sampling and chemical analysis of produced fluids; to document the absence (or presence) of injected CO2 within reservoir overburden through analogous monitoring of shallow groundwater and soil gas; to predict intra-reservoir CO2 migration paths, dynamic CO2 mass partitioning among distinct trapping mechanisms, and reservoir/seal permeability evolution through reactive transport modeling; to assess the impact of CO2-brine-rock reactions on fracture flow and isolation performance through experimental studies that directly support the monitoring and modeling work; and to exploit a novel stochastic inversion technique that enables explicit integration of these diverse monitoring data and forward models to improve reservoir characterization and long-term forecasts of isolation performance.