Huff-N-Puff Foam Injection in Naturally Fractured Carbonates Using Supercritical CO2

Abstract

Abstract Foam injection into naturally fractured reservoirs unequivocally has superior merits over pure gas injection. In this study, we present a novel foam generation strategy that incorporates the co-injection of surfactant and supercritical CO2 with huff-n-puff technique for the efficient diversion of the foaming gas into the rock matrix of fractured carbonates. A series of high pressure and temperature experiments were performed in fractured Indiana limestone cores with different permeabilities. First, gas injection was attempted to displace the oil from the fracture followed by foam flooding through the co-injection of a zwitterionic surfactant as a foaming agent and supercritical CO2. Then, a soaking period commenced at 200 psi above the preset pore pressure. Another co-injection of the foaming agent and scCO2 was carried out thereafter. The effect of the soaking pressure was evaluated by further utilizing 500 psi above the preset pore pressure in a subsequent soaking cycle. Similarly, this step was followed by a final foam injection to evaluate the potential increase in oil recovery. The incremental recovery by foam injection over gas flooding, i.e., 21-24%, was attributed to the foam invasion into the permeable pathways in the rock matrix while the selective diversion of surfactant solution and the increase in foam strength were the responsible mechanisms for oil recovery from the low-permeability core. In the latter case, the gradual evolution of foam strength was corroborated by the rising pressure drop across the fractured core. Besides, the imbibing surfactant solution across the fracture surface behind the foam front drove the crude oil production along the path ahead of the foam front, aligning with the mechanism of countercurrent imbibition. Subsequent soaking of the foam into the fractured cores managed to provide enough pressure and time for the invading scCO2 to develop hydraulic conductivity throughout the rock matrix mobilizing and relocating crude oil from previously inaccessible and deep pores toward fracture-neighboring pores. Subsequent foam injection produced this mobilized oil inducing an incremental oil recovery of 6% and 17% from the high- and low-permeability cores, respectively. Higher soaking pressure yielded even more oil recovery highlighting the role played by the scCO2 phase pressure in overcoming the threshold capillary pressure of rock matrix in mobilizing their resident oil and brine phases. As a result, the proposed foam huff-n-puff technique has proven to produce significantly higher oil recoveries from both high and low permeable limestones. Besides, tighter rocks with lower permeability would have the greatest benefit from the proposed procedure which indicates its promising potential performance in unconventional and ultra-tight reservoirs.