The Effect of Oil on Foam for Enhanced Oil Recovery

Abstract

Foam has unique microstructure in pore networks and reduces gas mobility significantly, which improves considerably the sweep efficiency of gas injection. Foam injection is thus regarded as a promising enhanced oil recovery (EOR) technology. One key to success of foam EOR is foam stability in presence of oil. This thesis seeks to understand fundamentally both steady-state and transient foam flow with oil in porous media through theoretical analysis and coreflood measurements. A quantitative modeling study is conducted to illustrate how the two algorithms ("wet-foam" model and "dry-out" model) represent the effect of oil on foam. Experimental observations evidently show that the two foam regimes without oil also apply to foam with oil, i.e. high- and low-quality regimes depending on foam quality. Oil affects both regimes with a stronger effect on the high-quality regime. Model fitting to data shows that currently applied implicit-texture (IT) foam models are suitable to represent foam flow with oil; both wet-foam model and dry-out model are needed to describe the effect of the oil on the two foam regimes. Three-phase fractional-flow theory together with the wave curve method (WCM) is applied to understand foam displacements with oil. Theoretical solutions suggest foam displacement cannot bank up an oil bank with oil saturation greater than an upper limit for stable foam. A critical phenomenon, i.e. that some injection conditions correspond to more than one possible foam states as predicted by the IT model, has been analyzed with fractional-flow theory and the WCM. We show how to determine the unique displacing state; the choice of the displacing state depends on initial state. Fundamentally, a boundary curve in ternary saturation space is defined that captures the nature of the dependence of the displacement on initial state. In addition, a new capillary number definition for micromodels is derived from a force balance on a ganglion trapped by capillarity. The definition in particular accounts for the impact of pore geometry and its validity is verified using two-phase flow data in micromodels. Based on current findings, some open questions concerning foam-oil interactions in porous media are defined and summarized at the end of this thesis.