Sweep efficiency in miscible enhanced oil recovery processes

Solvent injection can enhance oil recovery by developing multicontact miscibility with the oil and improving microscopic displacement efficiency. However, oil bypassing due to viscous fingering, heterogeneity, and gravity segregation can reduce macroscopic displacement efficiency and lead to low oil recovery.;Ideally, controlled field tests are needed to evaluate sweep improvement methods. Field tests are expensive and not well controlled. Additionally, initial condition cannot be replicated in subsequent tests. Simulation of field scale displacements can be unreliable due to inadequate representation of reservoir heterogeneity and complex displacement mechanisms. There is a need to conduct laboratory sweep efficiency studies with multicontact miscible fluids at reservoir conditions to evaluate various sweep improvement techniques. Reservoir condition laboratory tests can be used to calibrate numerical simulators and evaluate qualitative changes in sweep efficiency.;A high pressure quarter five-spot cell was constructed to study multicontact miscible (MCM) displacement of a moderately viscous oil (-78cP) in continuous and water-alternating-gas (WAG) injection schemes. Experiments were conducted at reservoir condition using actual fluids. Vapor extraction (VAPEX) experiments were performed in the high pressure cell using an MCM solvent. Sweep efficiency was deduced from in-situ saturations and compositions estimated with fully compositional three-dimensional reservoir simulators run in history match mode.;Calibrated one-dimensional numerical studies confirmed minimum miscibility pressure (MMP) between ethane and a medium viscosity oil. Sweep efficiency in continuous gas injection followed by water flood displacements decreased with increasing reservoir pressure above the MMP. WAG injection improved recovery relative to continuous gas flood by nearly 7% of the original oil in place (OOIP). Recovery decreased with increasing WAG slug size. Increasing WAG ratio increased sweep efficiency and oil recovery, approaching a limit at ∼2. Simulated sweep and oil recovery were improved with WAG over continuous injection for field-scale models with random heterogeneity.;Surfactant-alternating gas (SAG) displacements were modeled using a 3D compositional reservoir simulator. Sweep efficiency in SAG displacement of a viscous oil by MCM solvent with aqueous surfactant solution and ethane was similar to WAG. Foam diverts flow into lower permeability layers that are in capillary communication.