Relative Permeability Effects on the Miscible CO2 WAG Injection Schemes Trough Compositional Simulations of Brazilian Small Scale Water-Wet Synthetic Pre-Salt Reservoir

Abstract

The present study aims to deepen the investigations of a previous study, in order to rank which variables are more critical to CO2-WAG process Enhanced Oil Recovery and which injection schemes and parameters are more effective when capillary pressure and relative permeability are adapted to Brazilian pre-salt scenario. The methods compared were Water Alternating Gas (WAG) and Hybrid Water Alternating Gas, considering saturation dependent hysteresis modeled through Larsen-Skauge model. A detailed and reliable Equation of State for oil with high pressure, and unusual natural carbon dioxide content (8.4%) was developed based in published data to match rheology and swelling measurements. The model was a small scale water-wet carbonate/stromatolite reservoir model with the same set of relative permeability and capillary pressure with hysteresis and entrapment data were modeled as basis for pre-salt Brazilian reservoir data. The optimization were analized including hysteresis and entrapment data from the literature. The reservoir model well constraints for each strategy were meant to be as realistic as possible, the recovery was optimized using an optimization tool, and the cases were analyzed comparatively. The most important effects were ranked and the best injection scheme and parameters were determined for the studied case in order to clear conclusions about the importance of fluid and petrophysical mechanisms for miscible final oil recovery. Among a series of petrophysical phenomena needing further investigations on their influence on oil recovery, relative permeability is of critical importance. Nonetheless, its representation in reservoir simulation is quite a complicated task. It may incorporate hysteresis effects (at least three different models that can be used), and it may present three-phase behavior (extrapolated from two phase relative permeability experimental data). Just like relative permeability hysteresis, there are at least four different models available to represent three phase relative permeability. In the present work, only Larsen and Skauge model was addressed. As a consequence of all complexities involved, for some subtle changes in the injection scheme, such as the injector operating restraints, make recoveries sensitive to hysteresis and passive of improvement. Simulations indicated that an incremental oil recovery of about 11%, in both WAG and HWAG schemes when hysteresis was considered compared to no-hysteresis case. Optimization of well operating conditions and constraints (bottom-hole pressure of producers and injectors, gas and water rate limits) has a significant effect on oil recovery and production rates.

State of the Art on Relative Permeability Hysteresis in Porous Media: Petroleum Engineering Application

“…At the front edge of the CO 2 slug, gas continues to displace the wetting phase during drainage, while at the rear edge, water displaces gas during imbibition, leading to gas phase fragmentation and subsequent trapping. As the slug migrates upwards, some CO 2 is trapped as residual immobile phase in pore spaces, effectively isolating CO 2 in the rock until the immobile gas dissolves over geological time [11,73,74,76,77,[85][86][87].…”