Dynamics of supercritical CO₂ foam in porous media with CO₂ soluble surfactuants

CO₂ flood has become a routine technology for enhanced oil recovery worldwide. However, the problem of poor sweep efficiency has been frequently encountered in CO₂ flooding due to the presence of preferential flow paths such as high permeability zones or fracture networks. The objective of this work is to improve robustness of supercritical CO₂ foam as a mobility controlling agent, surfactant utilization, and reservoir sweep efficiency in CO₂ flooding. The focus will be the unconventional foam process with proprietary non-ionic CO₂ soluble surfactants. The properties of these surfactants including solubility in CO₂, partitioning between CO₂ and water phases, and static adsorption on rock are directly measured versus temperatures and pressures. For the first time the unconventional foam process with CO₂ soluble surfactant has been systematically studied from the viewpoint of foam improved CO₂ flood and compared with conventional foam (stabilized with a widely used conventional CO₂ insoluble surfactant. Both Silurian carbonates and Berea sandstones were used in this study. The effect of reservoir heterogeneity such as stratification and natural fractures are also investigated. For the first time, foam behavior is probed in fractured system with variable fracture apertures, foam qualities, injection rates and rock permeabilities. Micro-scaled model is also developed to visualize foam propagation in fracture flow. All the measured surfactant properties are then used to model foam transport on a field scale using a commercial reservoir simulation. The advantages of CO₂ soluble surfactants are quantified for different injection strategies. Optimization of surfactant partition coefficient for field-scale foam process is performed to determine the variation of surfactant partitioning effect. A new method is proposed, which can be used in conjunction with the cloud point measurements, to obtain information directly on the soluble portion of a given sample. The partitioning of nonionic hydrocarbon surfactants between water and brine and CO₂ as a function of electrolyte concentration, temperature and pressure are also investigated. These functional relationships have been rarely found in the literature. The solubility of the non-ionic surfactant in CO₂ increases with pressure and gradually decreases with temperature. The partitioning of this surfactant between CO₂ and water phases is much more sensitive to temperature than pressure. Strong foam development in Silurian cores is observed for both non-ionic and anionic surfactants while the adsorption of the latter surfactant is almost three times higher. The foam with CO₂ soluble surfactants is much more robust in water displacement due to both reduced surfactant adsorption and surfactant partitioning. However, the adsorption magnitudes of the non-ionic and anionic surfactants are very close in sandstone, but foam with CO₂ soluble surfactants still outperforms that with conventional surfactants because of surfactant partitioning. A new experimental method to determinate dynamic CO₂ foam in artificially fractured carbonate cores at high pressure is presented. The rheology of foam in fractures is well distinguished from that in matrix. Foam propagation decreases with increasing fracture aperture at very low shear rate. However, this relationship is strongly influenced by matrix permeability. A strong correlation between foam performance and injection quality was not observed an increase in total injection rate promote foam development. In agreement with the above experimental observations, foam with CO₂ soluble surfactant on a field scale can improve injectivity, surfactant transport, and foam propagation in regions where CO₂ mobility control is needed. The benefit of co-injection and alternating injection could be significantly enhanced with CO₂ soluble surfactant. The ability of surfactant to partition between CO₂ and water phase reduces the effect of gas and water slug sizes on foam performance. A new injection strategy where surfactant is injected in CO₂ without water injection gives the highest sweep efficiency. This particular foam process shows best performance under constant rate injection mode. Foam performance and surfactant transport is governed not only by constrained injection strategies, but also surfactant partition coefficient. Both experiments and simulations demonstrate that good surfactant transport with CO₂ soluble surfactant improves foam robustness once local surfactant concentration exceeds a critical concentration. The CO₂ soluble surfactant and its injection strategies presented in this work not only improve the robustness of foam performance but also reduce the overall cost of foam process. The optimal partition of the surfactant between the CO₂ and aqueous phases minimize the loss of expensive surfactant in water that is never contacted with CO₂. The results from this study enable us to tailor properties of CO₂ soluble surfactants (i.e. partition coefficient and solubility in CO₂) to a wide range of reservoir conditions and optimal injection strategies. This novel CO₂ soluble surfactant concept diversifies injection strategies with respect to operational constraints, broadening the application of foam process.