Browsing by Subject "Local capillary trapping"

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Investigation of buoyant plumes in a quasi-2D domain : characterizing the influence of local capillary trapping and heterogeneity on sequestered CO₂ – : a bench scale experiment
(2014-05) Sun, Yuhao; Bryant, Steven L.
Leakage of stored bulk phase CO₂ is one risk for sequestration in deep saline aquifers. As the less dense CO₂ migrates upward within a storage formation or in layers above the formation, the security of its storage depends upon the trapping mechanisms that counteract the migration. The trapping mechanism motivating this research is local capillary trapping (LCT), which occurs during buoyancy-driven migration of bulk phase CO₂ within a saline aquifer with spatially heterogeneous petrophysical properties. When a CO₂ plume rising by buoyancy encounters a region where capillary entry pressure is locally larger than average, CO₂ accumulates beneath the region. One benefit of LCT, applied specifically to CO₂ sequestration and storage, is that saturation of stored CO₂ phase is larger than the saturation for other permanent trapping mechanisms. Another potential benefit is security: CO₂ that occupies local capillary traps remains there, even if the overlying formation that provides primary containment were to be compromised and allow leakage. Most work on LCT has involved numerical simulation (Saadatpoor 2010, Ganesh 2012); the research work presented here is a step toward understanding local capillary trapping at the bench scale. An apparatus and set of fluids are described which allow examining the extent of local capillary trapping, i.e. buoyant nonwetting phase immobilization beneath small-scale capillary barriers, which can be expected in typical heterogeneous storage formation. The bench scale environment analogous to CO₂ and brine in a saline aquifer is created in a quasi-two dimensional experimental apparatus with dimension of 63 cm by 63 cm by 5 cm, which allows for observation of plume migration with physically representative properties but at experimentally convenient ambient conditions. A surrogate fluid pair is developed to mimic the density, viscosity and interfacial tension relationship found at pressure and temperature typical of storage aquifers. Porous media heterogeneity, pressure boundary conditions, migration modes of uprising nonwetting phase, and presence of fracture/breach in the capillary barrier are studied in series of experiments for their influences on LCT. A variety of heterogeneous porous media made of a range of sizes of loosely packed silica beads are used to validate and test the persistence of local capillary trapping mechanism. By adjusting the boundary conditions (fluid levels in reservoirs attached to top and to bottom ports of the apparatus), the capillary pressure gradient across the domain was manipulated. Experiments were conducted with and without the presence of fracture/potential leakage pathway in the capillary seal. The trapped buoyant phase remained secure beneath the local capillary barriers, as long as the effective capillary pressure exerted by the trapped phase (proportional to column height of the phase) is smaller than the capillary entry pressure of the barrier. The local capillary trapping mechanism remained persistent even under forced imbibition, in which a significantly higher hydraulic potential gradient, and therefore a larger gradient in capillary pressure, was applied to the system. The column height of buoyant fluid that remained beneath the local capillary barrier was smaller by a factor corresponding to the increase in capillary pressure gradient. Mimicking a breach of the caprock by opening valves at the top of the apparatus allowed buoyant mobile phase held beneath the valves to escape, but buoyant phase held in local traps at saturations above residual, and therefore potentially mobile, was undisturbed. This work provides systematic validation of a novel concept, namely the long-term security of CO₂ that fills local (small-scale) capillary traps in heterogeneous storage formations. Results from this work reveal the first ever unequivocal experimental evidence on persistence of local capillary trapping mechanism. Attempts to quantify the nonwetting phase saturation and extent of LCT persistence serve as the initial steps to potentially reduce the risks associated with long-term storage security.
Laboratory experiments and modeling to evaluate critical CO₂ saturation for geologic carbon storage
(2024-05) Ubillus, Jose Eduardo ; DiCarlo, David Anthony, 1969-; Ni, Hailun
During geologic carbon storage, the flow regime is strongly capillary- and buoyancy-dominated as the CO₂ plume migrates away from the injection well and during the post-injection redistribution period. Small-scale geological heterogeneities in the targeted storage formations can affect CO₂ saturation and cause a substantial increase in trapped CO₂ during this flow regime. These heterogeneities can retain a significant amount of CO₂ because of capillary heterogeneity trapping, also known as local capillary trapping. Sand tank laboratory experiments have shown that the value of this parameter can vary greatly depending on the type (i.e., geometry) and degree (i.e., grain size contrast) of heterogeneity. This work aims first to investigate the impact of small-scale heterogeneities on trapping performance using sand tank experiments and then examine how heterogeneity-induced trapping affects field-scale simulation results. Various realistic ripple deposits with varying grain size contrast and wettability were produced using an automated glass bead packing system in a meter-scale slab chamber to study the impact of small-scale heterogeneities trapping performance. A surrogate fluid pair was used to mimic the properties of in-situ brine and supercritical CO₂. Using a light transmission system, we measure the infiltration patterns, capillary heterogeneity trapping, and overall trapping performance. The experimental results showed variations in trapped saturation and capillary heterogeneity trapping exhibited when the ripple bedform architecture was altered (10% to 20% increment). Similar growth in trapping performance is observed when grain size contrast increases. Additionally, wettability changes (water-to-oil-wet) increased nonwetting saturation and capillary heterogeneity trapping up to 5% and 10% to 20%, respectively. It is crucial to adequately upscale the effect of small-scale heterogeneities; otherwise, it can lead to inaccurate field-scale CO₂ storage simulations. Capillary heterogeneity trapping buoyancy-driven flow can be captured through a parameter known as the critical CO₂ saturation. This parameter, which represents the first nonzero value on the CO₂ drainage relative permeability curve, can inform field-scale numerical simulations. We conducted numerical full-physics simulations to investigate the influence of critical CO₂ saturation on plume dynamics distribution and trapping mechanisms. Three sets of 2D field-scale simulations were run in a synthetic composite confining system with varying critical CO₂ saturations from 0.1 to 0.4. The first set was implemented without hysteresis and dissolution effects to capture only the impact of critical saturation on residual trapping. The second set included dissolution to understand the role of critical saturation in solubility trapping. Lastly, simulations involving both hysteresis and dissolution were performed to study the influence of different trapping mechanisms on CO₂ retention and plume migration. Furthermore, we study the impact of critical saturation when different established capillary pressure models are used. The Brooks-Corey (BC), Modified Brooks- Corey (MBC), and van Genuchten (VG) models are employed. While neither the BC nor VG models consider critical saturation in their formulation, the MBC model does incorporate the critical saturation value as a direct input. The simulation showed that the plume size and lateral extent decreased as critical saturation increased, keeping the plume confined within the pore space domain boundaries. Moreover, movable gas decreased over time as critical saturation increased. As expected, hysteresis boosted CO₂ residual trapping due to imbibition. However, the impact of imbibition trapping diminished with increasing critical saturation since a more significant proportion of CO₂ had already been trapped during the drainage process thanks to the effect of capillary heterogeneity trapping. Additionally, the VG capillary pressure model contributed more to residual and solubility trapping than the MBC model. These results indicate that the shape of the capillary pressure curve near the critical saturation plays a significant role in trapping mechanisms. Regardless of the capillary pressure model, an increase in critical saturation causes the CO₂ plume dynamics distribution and trapping performance to converge. Critical CO₂ saturation significantly affects the plume size, lateral extent, and trapping mechanisms from the field-scale simulations. Therefore, these results provide insightful information about the behavior of CO₂ migration and trapping in heterogeneous geologic formations.
Local capillary trapping and permeability-retarded accumulation during geologic carbon sequestration
(2017-09-13) Ren, Bo; Lake, Larry W.; Bryant, Steven L.; DiCarlo, David A; Daigle, Hugh C; Meckel, Timothy A
Safe storage of CO2 in saline aquifers depends on CO2 migration rate, accumulation, and trapping inside saline aquifers that have intrinsic heterogeneity. This heterogeneity can be in both capillary entry pressure and permeability. The former heterogeneity causes local capillary trapping while the latter results in permeability-retarded accumulation. A main objective of this dissertation is to understand how both local capillary trapping and permeability-retarded accumulation secure CO2 storage. We establish a fast simulation technique to model local capillary trapping during CO2 injection into saline aquifers. In this technique, modeling efforts are decoupled into two parts: identifying trapping in a capillary entry pressure field and simulating CO2 flow in a permeability field. The former fields are correlated with the latter using the Leverett j-function. The first part describes an extended use of a geologic criterion originally proposed by Saadatpoor (2012). This criterion refers to a single value of ‘critical capillary entry pressure’ that is used to indicate barrier or local traps cells during buoyant flow. Three issues with the criterion are the unknown physical critical value, the massive overestimation of trapping, and boundary barriers. The first two issues are resolved through incorporating viscous flow of CO2. The last issue is resolved through creating periodic boundaries. This creation enables us to study both the amount and clusters of local capillary traps in infinite systems, and meanwhile the effects of reservoir heterogeneity, system size, aspect ratio, and boundary types are examined. In the second part, we adapt a connectivity analysis to assess CO2 plume dynamics. This analysis is then integrated into the geologic criterion to evaluate how injection strategies affect local capillary trapping in reservoirs. We demonstrate that reservoir heterogeneity affects the optimal injection strategies in terms of maximizing this trapping. We conduct analytical and numerical modeling of CO2 accumulations caused by both permeability hindrances and capillary barriers. The analytical model describes CO2 buoyant migration and accumulation at a low permeability region above a high-permeability region. In the limiting case of zero capillary pressure, the model equation is solved using the method of characteristics. The permeability-retarded accumulation is illustrated through CO2 saturation profiles and time-distance diagrams. Capillary trapping is subsequently accounted for by graphically incorporating the capillary pressure curve and capillary threshold effect. The relative importance of these two types of accumulations is examined under various buoyant source fluxes and porous media properties. Results demonstrate that accumulation estimate that account for only capillary trapping understates the amount of CO2 accumulated beneath low permeability structures during significant periods of a sequestration operation.
Local capillary trapping in geological carbon storage
(2012-08) Saadatpoor, Ehsan, 1982-; Bryant, Steven L.; Sepehrnoori, Kamy, 1951-
After the injection of CO₂ into a subsurface formation, various storage mechanisms help immobilize the CO₂. Injection strategies that promote the buoyant movement of CO₂ during the post-injection period can increase immobilization by the mechanisms of dissolution and residual phase trapping. In this work, we argue that the heterogeneity intrinsic to sedimentary rocks gives rise to another category of trapping, which we call local capillary trapping. In a heterogeneous storage formation where capillary entry pressure of the rock is correlated with other petrophysical properties, numerous local capillary barriers exist and can trap rising CO₂ below them. The size of barriers depends on the correlation length, i.e., the characteristic size of regions having similar values of capillary entry pressure. This dissertation evaluates the dynamics of the local capillary trapping and its effectiveness to add an element of increased capacity and containment security in carbon storage in heterogeneous permeable media. The overall objective is to obtain the rigorous assessment of the amount and extent of local capillary trapping expected to occur in typical storage formations. A series of detailed numerical simulations are used to quantify the amount of local capillary trapping and to study the effect of local capillary barriers on CO₂ leakage from the storage formation. Also, a research code is developed for finding clusters of local capillary trapping from capillary entry pressure field based on the assumption that in post-injection period the viscous forces are negligible and the process is governed solely by capillary forces. The code is used to make a quantitative assessment of an upper bound for local capillary trapping capacity in heterogeneous domains using the geologic data, which is especially useful for field projects since it is very fast compared to flow simulation. The results show that capillary heterogeneity decreases the threshold capacity for non-leakable storage of CO₂. However, in cases where the injected volume is more than threshold capacity, capillary heterogeneity adds an element of security to the structural seal, regardless of how CO₂ is accumulated under the seal, either by injection or by buoyancy. In other words, ignoring heterogeneity gives the worst-case estimate of the risk. Nevertheless, during a potential leakage through failed seals, a range of CO₂ leakage amounts may occur depending on heterogeneity and the location of the leak. In geologic CO₂ storage in typical saline aquifers, the local capillary trapping can result in large volumes that are sufficiently trapped and immobilized. In fact, this behavior has significant implications for estimates of permanence of storage, for assessments of leakage rates, and for predicting ultimate consequences of leakage.