The effect of WACO2 ratio on CO2 geo-sequestration efficiency in homogeneous reservoirs

Various factors such as reservoir temperature, wettability, caprock properties, vertical to horizontal permeability ratio, salinity, reservoir heterogeneity, injection well configuration affect the CO2 geo-sequestration efficiency. Furthermore, it was previously investigated that CO2 storage efficiency can be improved by using water alternating CO2 (WACO2) technology. However, the effect of the WACO2 ratio (the ratio of the total amount of injected CO2 to the total amount of injected water) on CO2 storage efficiency has not been addressed adequately. Thus, in this paper, a 3D homogeneous reservoir simulation model has been developed to study the impact of the WACO2 ratio on CO2 mobility and CO2 trapping capacity using five different WACO2 ratios (i.e. 3, 2, 1, 1/2, and 1/3). For all WACO2 ratios tested, 9000 kton (kt) of CO2 were injected during 3 CO2 injection cycles (2 years each) and at an injection rate of 1500 kt per year. Each CO2 injection cycle was followed by a 2 years water injection cycle with injection rates of 500 kt/year, 750 kt/year, 1500 kt/year, 3000 kt/year, and 4500 kt/year for the 3, 2, 1, 1/2, and 1/3 WACO2 ratios, respectively. Then, this 12 years WACO2 injection period was followed by a 100 years post-injection period. Our results clearly indicate, after 100 years post-injection period, that the WACO2 ratio has an important effect on the CO2 migration distance, CO2 mobility and CO2 trapping capacity. The results demonstrate that lower WACO2 ratio leads to reduce the vertical CO2 plume migration and CO2 mobility. Furthermore, low WACO2 ratio enhances the capacities of capillary and solubility trapping mechanisms. Thus, we conclude that WACO2 has a significant impact on the geo-sequestration efficiency and less WACO2 ratios are preferabl

Implications of pore microgeometry heterogeneity for the movement and chemical reactivity of CO2 in carbonates

We studied the heterogeneity of natural rocks with respect to their pore-size distribution, obtained from mercury-intrusion capillary pressure (MICP) tests, at a scale about one-fifth of the standard plug size (2.5 cm). We investigated two Fontainebleau sandstone and two limestone samples. We found that at the scale of the MICP tests, heterogeneities are practically nonexistent. Still, there are large differences in the capillary curves from one rock type to another. Also, carbonate rocks, unlike Fontainebleau sandstone, show heterogeneities at a scale smaller than the scale used in MICP tests, as seen by the complexity in the mercury saturation versus pressure curves. We used this diversity between the capillary curves and this complexity within a single capillary curve to obtain information about the movement and chemical reactivity of CO2 in carbonates.The method consists of three steps: first, subdividing the carbonate pore system into microstructural facies, each of them having a specific range of pore throat size (e.g., tight micrite, microporous rounded micrite, small vugs, …); second, getting a characteristic value of their petrophysical properties (namely porosity, effective surface area, and permeability) from the collected MICP data; and third, computing, for experimental conditions corresponding to a transport-controlled system, the dimensionless Péclet and Damköhler numbers, expressed as a function of the aforementioned permeability and effective surface area. These numbers allowed us to infer the dominant process (i.e., diffusion, advection, or kinetics) controlling the dissolution/precipitation reaction induced by the carbonic acid. Because of heterogeneities in the pore microstructure, we found that either diffusion or advection is locally the dominant mechanism, which renders some zones (e.g., vugs or, to a lesser extent, microporous rounded micrite) chemically more reactive than others (e.g., tight micrite or spar cement)

Numerical simulation of reactive barrier emplacement to control CO2 migration

Long-term storage of anthropogenic CO2 in the subsurface generally requires that caprock formations will serve as physical barriers to upward migration of CO2. As a result, geological carbon storage (GCS) projects require reliable techniques to monitor for newly formed leaks, and the ability to rapidly deploy mitigation measures should leakage occur. Here, we develop a two-dimensional reactive transport simulation to analyse the hydrogeochemical characteristics of a newly formed CO2 leak entering an overlying reservoir and emplacement of a hypothetical pH dependent sealant in the vicinity of the leak. Simulations are conducted using the TOUGHREACT multi-component reactive transport code, focusing on the comparatively short time period of days to months following formation of the leak. The simulations are used to evaluate (1) geochemical shifts in formation water indicative of the leak, (2) hydrodynamics of pumping wells in the vicinity of the leak, and (3) delivery of a sealant to the leak through an adjacent well bore

Advanced Technologies for Monitoring CO2 Saturation and Pore Pressure in Geologic Formations: Linking the Chemical and Physical Effects to Elastic and Transport Properties

Ultrasonic P- and S-wave velocities were measured over a range of confining pressures while injecting CO2 and brine into the samples. Pore fluid pressure was also varied and monitored together with porosity during injection. Effective medium models were developed to understand the mechanisms and impact of observed changes and to provide the means for implementation of the interpretation methodologies in the field. Ultrasonic P- and S-wave velocities in carbonate rocks show as much as 20-50% decrease after injection of the reactive CO2-brine mixture; the changes were caused by permanent changes to the rock elastic frame associated with dissolution of mineral. Velocity decreases were observed under both dry and fluid-saturated conditions, and the amount of change was correlated with the initial pore fabrics. Scanning Electron Microscope images of carbonate rock microstructures were taken before and after injection of CO2-rich water. The images reveal enlargement of the pores, dissolution of micrite (micron-scale calcite crystals), and pitting of grain surfaces caused by the fluid- solid chemical reactivity. The magnitude of the changes correlates with the rock microtexture – tight, high surface area samples showed the largest changes in permeability and smallest changes in porosity and elastic stiffness compared to those in rocks with looser texture and larger intergranular pore space. Changes to the pore space also occurred from flow of fine particles with the injected fluid. Carbonates with grain-coating materials, such as residual oil, experienced very little permanent change during injection. In the tight micrite/spar cement component, dissolution is controlled by diffusion: the mass transfer of products and reactants is thus slow and the fluid is expected to be close to thermodynamical equilibrium with the calcite, leading to very little dissolution, or even precipitation. In the microporous rounded micrite and macropores, dissolution is controlled by advection: because of an efficient mass transfer of reactants and products, the fluid remains acidic, far from thermodynamical equilibrium and the dissolution of calcite is important. These conclusions are consistent with the lab observations. Sandstones from the Tuscaloosa formation in Mississippi were also subjected to injection under representative in situ stress and pore pressure conditions. Again, both P- and S-wave velocities decreased with injection. Time-lapse SEM images indicated permanent changes induced in the sandstone microstructure by chamosite dissolution upon injection of CO2-rich brine. After injection, the sandstone showed an overall cleaner microstructure. Two main changes are involved: (a) clay dissolution between grains and at the grain contact and (b) rearrangement of grains due to compaction under pressure Theoretical and empirical models were developed to quantify the elastic changes associated with injection. Permanent changes to the rock frame resulted in seismic velocity-porosity trends that mimic natural diagenetic changes. Hence, when laboratory measurments are not available for a candidate site, these trends can be estimated from depth trends in well logs. New theoretical equations were developed to predict the changes in elastic moduli upon substitution of pore-filling material. These equations reduce to Gassmann’s equations for the case of constant frame properties, low seismic frequencies, and fluid changes in the pore space. The new models also predict the change dissolution or precipitation of mineral, which cannot be described with the conventional Gassmann theory

Determination of safe mud window considering time-dependent variations of temperature and pore pressure: Analytical and numerical approaches

Wellbore stability is a key to have a successful drilling operation. Induced stresses are the main factors affecting wellbore instability and associated problems in drilling operations. These stresses are significantly impacted by pore pressure variation and thermal stresses in the field. In order to address wellbore instability problems, it is important to investigate the mechanisms of rock–fluid interaction with respect to thermal and mechanical aspects. In order to understand the induced stresses, different mathematical models have been developed. In this study, the field equations governing the problem have been derived based on the thermo-poroelastic theory and solved analytically in Laplace domain. The results are transferred to time domain using Fourier inverse method. Finite difference method is also utilized to validate the results. Pore pressure and temperature distributions around the wellbore have been focused and simulated. Next, induced radial and tangential stresses for different cases of cooling and heating of formation are compared. In addition, the differences between thermo-poroelastic and poroelastic models in situation of permeable and impermeable wellbores are described. It is observed that cooling and pore pressure distribution reinforce the induced radial stress. Whereas cooling can be a tool to control and reduce tangential stress induced due to invasion of drilling fluid. In the next step, safe mud window is obtained using Mohr-Coulomb, Mogi-Coulomb, and modified Lade failure criteria for different inclinations. Temperature and pore pressure distributions do not change the minimum allowable wellbore pressure significantly. However, upper limit of mud window is sensitive to induced stresses and it seems vital to consider changes in temperature and pore pressure to avoid any failures. The widest and narrowest mud windows are proposed by modified Lade and Mohr-Coulomb failure criteria, respectively

Estimation of rock frame weakening using time-lapse crosswell: Frio brine pilot project

CO2 injection into subsurface reservoirs leads to pressure and saturation changes. Furthermore, CO2 -brine-minerals interaction could result in dissolution or reprecipitation of rock frame-forming minerals. Observed time-lapse seismic associated with CO2 injection into poorly consolidated sandstone at the Frio CO2 injection site (Texas, USA) could not be predicted using classical rock-physics models (i.e., models involving elastic changes in the rock frame due to saturations and/or pressures changes only, and assuming no changes in the rock microstructure). That, and the changes in the fluid chemistry after CO2 injection, suggests that the assumption of a constant rock microstructure might be violated. Using high-resolution time-lapse crosswell data, we have developed a methodology for estimating changes in the rock frame by quantifying the rock-frame drained moduli before and after CO2 injection. Based on rock microstructure diagnostics, we found that the changes in the drained frame elastic properties are due to the changes in the grain contact-cement percentage. The reduction in contact-cement percent is found to be variable throughout the reservoir, with a maximum near the injection well, down to 0.01% from the initial 0.1% contact cement; this results in more than 40% reduction in the drained frame shear and bulk moduli. CO2 saturation was estimated using this model for uniform and patchy saturation cases. Our rock-physics analysis may allow improved interpretation of time-lapse seismic for CO2 saturation in the context of other poorly consolidated sandstones with similar geomechanical properties. Having the P- and S-wave velocity time-lapse data is key to improve saturation estimates with this analysis method

Determination of safe mud window considering time-dependent variations of temperature and pore pressure: Analytical and numerical approaches

Wellbore stability is a key to have a successful drilling operation. Induced stresses are the main factors affecting wellbore instability and associated problems in drilling operations. These stresses are significantly impacted by pore pressure variation and thermal stresses in the field. In order to address wellbore instability problems, it is important to investigate the mechanisms of rockefluid interaction with respect to thermal and mechanical aspects. In order to understand the induced stresses, different mathematical models have been developed. In this study, the field equations governing the problem have been derived based on the thermo-poroelastic theory and solved analytically in Laplace domain. The results are transferred to time domain using Fourier inverse method. Finite difference method is also utilized to validate the results. Pore pressure and temperature distributions around the wellbore have been focused and simulated. Next, induced radial and tangential stresses for different cases of cooling and heating of formation are compared. In addition, the differences between thermo-poroelastic and poroelastic models in situation of permeable and impermeable wellbores are described. It is observed that cooling and pore pressure distribution reinforce the induced radial stress. Whereas cooling can be a tool to control and reduce tangential stress induced due to invasion of drilling fluid. In the next step, safe mud window is obtained using Mohr-Coulomb, Mogi-Coulomb, and modified Lade failure criteria for different inclinations. Temperature and pore pressure distributions do not change the minimum allowable wellbore pressure significantly. However, upper limit of mud window is sensitive to induced stresses and it seems vital to consider changes in temperature and pore pressure to avoid any failures. The widest and narrowest mud windows are proposed by modified Lade and Mohr-Coulomb failure criteria, respectively. 2017 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND licens

The rock physics basis for 4D seismic monitoring of CO2 fate: Are we there yet?

Monitoring, verification, and accounting (MVA) of CO2 fate which are the three fundamental needs in geological sequestration are discussed. The primary objective of MVA protocols is to identify and quantify the injected CO2 stream within the injection/storage horizon and any leakage of sequestered gas from the injection horizon, providing public assurance. Changes in the elastic properties of the reservoir induced by the injection of CO2 can be various, affecting the properties of the fluid, those of the rock frame, or both. Seismic reservoir monitoring has traditionally treated the changes in the reservoir rock as a physical-mechanical problem, that is changes in seismic signatures are mostly modeled as functions of saturation and stress variations and/or intrinsic rock properties. To enhance the effectiveness of time-lapse seismic studies, CO2-optimized physical-chemical models involving frame substitution schemes must be developed to account for the type and magnitude of reductions caused by rock-fluid interactions at the grain/pore scale