by Xueying Lu and Mohammad Lotfollahi
CO2 sequestration in subsurface often suffers from poor volumetric sweep efficiency due to low gas viscosity, low gas density, and formation heterogeneity. This study investigates CO2 mobility control techniques of Water Alternating Gas (WAG) and Surfactant (or Nanoparticle) Alternating Gas (SAG) to increase CO2 storage capacity in Cranfield via field-scale simulations and optimization.
UT in-house parallel compositional reservoir simulator, IPARS, can model both compositional flow and geomechanics. IPARS incorporates advanced three-phase relative permeability (UTKR3P) and hysteresis (UTHYST) models  with foam models  to more accurately simulate WAG and SAG processes . The hysteretic relative permeability model captures local capillary trapping during cyclic injection of liquid and gas. Foam-assisted CO2 mobility control technique is examined to investigate eminent level of CO2 capillary trapping by applying a foam model.
|Figure 1. Comparison of injection schedules for continuous CO2 injection, water alternating gas injection, and surfactant alternating gas injection cases.|
Figure 1 shows three designed injection schemes for field scale CO2 sequestration in Cranfield. Same amount of CO2 is injected at the end of each injection scheme. The simulations were run on Stampede2 with 64 cores for each case.
Figure 2-5 show CO2 saturation profiles in the reservoir with a view from the bottom during the continuous CO2 injection, WAG without hysteresis modeling, WAG with hysteresis modeling and SAG, respectively. It is clearly demonstrated that including the observed hysteresis in alternating injection processes significantly increases the performance of these processes. Alternative injection of water or surfactant with CO2 can suppress gas gravity override and reduce gas channeling through high permeable streaks. During SAG process, foam is generated in the high permeability streaks and upper layers with higher CO2 flow rates and diverts the CO2 flow into low permeability regions and bottom layers, leading to more efficient areal and vertical sweep efficiency. In this study, CO2 storage volume increased by 20.1% and 40.2% of total CO2 injection volume – compared to continuous CO2 injection- during four-cycle WAG and SAG processes, respectively.
|Figure 2. CO2 saturation during continuous CO2 injection, view from the bottom.|
|Figure 3. CO2 saturation during water-alternating-gas injection without hysteresis modeling, view from the bottom|
|Figure 4. CO2 saturation during water-alternating-gas injection with hysteresis modeling, view from the bottom.|
|Figure 5. CO2 saturation during surfactant-alternating-gas injection, view from the bottom.|
SAG processes is optimized for injection schedule using UT optimization toolbox with Genetic Algorithm . The number of cycles and length of each cycle are optimized with an objective function equals to cumulative CO2 storage volume in 20 years. Figure 6 shows the evolution of the objective function. A trend of increase and convergence is achieved in 10 generations. Optimized SAG achieved 15% more CO2 storage volume and 60% less surfactant and water consumption comparing to basecase SAG.
|Figure 6. Evolution of the objective function using the genetic algorithm optimization.|
This work on numerical simulation and optimization of gas mobility control techniques during CO2 sequestration is currently in preparation  and a talk was recently given on the topic at the 2017 SIAM Geosciences Conference in Erlangen, Germany. Our simulation results suggest that foam has promising applications in increasing CO2 storage volume during field scale CO2 sequestrations. The results also demonstrated the capacities of our well developed toolsets for screening and designing field scale CO2 sequestration operations. In the near future, the team will work on sensitivity analysis of optimization parameters, and foam flow modeling in a coupled compositional- geomechanics framework.
 Lu, X., Lotfollahi M., Ganis B., Min B., & Wheeler, M. F., Numerical simulation and optimization of gas mobility control techniques during CO2 sequestration in Cranfield, in preparation, 2017
 Beygi, M. R., Delshad, M., Pudugramam, V. S., Pope, G. A., & Wheeler, M. F. (2015). Novel three-phase compositional relative permeability and three-phase hysteresis models. SPE Journal, 20(01), 21-34.
 Lotfollahi, M., Kim, I., Beygi, M. R., Worthen, A. J., Huh, C., Johnston, K. P., Wheeler, M. F. & DiCarlo, D. A. (2017). Foam Generation Hysteresis in Porous Media: Experiments and New Insights. Transport in Porous Media, 116(2), 687-703.
 Min, B., Wheeler, M. F., & Sun, A. Y. (2017). Parallel Multiobjective Optimization for the Coupled Compositional/Geomechanical Modeling of Pulse Testing. In SPE Reservoir Simulation Conference. Society of Petroleum Engineers.