Foams with Ultra-Low Interfacial Tensions for an Efficient EOR Process in Fractured Reservoirs

Abstract

Abstract We both evidence a new formulation for ultra-low IFT foams and a new coreflood set-up to demonstrate the efficiency of such a formulation in a model fractured reservoir. In fractured oil-wet carbonates reservoirs, recovery of oil from the matrix is limited by preferential flows in fractures, and the low permeability and wettability of the matrix. Injection of foams with ultra-low interfacial tension (IFT) may handle these challenges. On the one hand, foam can induce fluid diversion from the fractures to the matrix. On the other hand, ultra-low interfacial tensions with oil (< 10−2 mN/m at optimal salinity) greatly enhance the oil recovery as demonstrated by ASP (Alkali Surfactant Polymer) floodings in homogeneous reservoirs. Ultra-low Ift values can efficiently decrease the entry pressure drop of oil-wet cores. Thus we here investigate the injection of foam with ultra-low IFT properties to mobilize oil in the poorly-swept matrix of oil-wet fractured carbonates. First, obtaining both ultra-low IFT and stable foams in the same conditions is very often contradictory. Even a foaming solution with a domain of low-IFT generally exhibits a drop of foamability at the vicinity of the ultra-low IFT conditions. This study highlights a key criterion to obtain simultaneously both properties. The formulation requires an outstanding solubility (very low absorbance) at the vicinity of the ultra-low IFT domain. This condition is hardly encountered at the optimal salinity for common surfactant formulations and requires a wide screening of surfactants. From this criterion, we developed a mixture of surfactants which demonstrates simultaneously (60°C, 80g/L NaCl) a good foamability, foam stability and an ultra-low IFT with a crude oil. Second, we present here a new coreflood set-up - called WFR after Waterflood Fractured Reservoir - designed to model foam flows in fractured reservoirs. The foam is pre-formed by co-injection of nitrogen gas and surfactant solution in a glass bead pack. The foam continuously washes the upper end-face of a vertical core sample. This geometry simulates a foam flowing in a fracture. The resulting flows generated in the matrix and the low-IFT values gives a total oil recovery of 93% OOIP. The efficiency of the designed ultra-low IFT foam formulation is for the first time evaluated with this set-up to model representative conditions of a fractured system, and gives promising results.