Non Aqueous Foam for Improving Hydrocarbon Miscible Flooding in Water Sensitive Tight Oil Formations
Authors: Sie Chao-yu , Nguyen Quoc 
Published: October 2019

Abstract
Enhanced oil recovery from tight carbonate formations has been challenging due to its high reservoir heterogeneity, unfavorable wettability to water, and low reservoir permeability which can restrict the application of traditional water-based EOR methods with mobility control (e.g. polymer). With the increasing availability of wet gas due to the rise of shale production over the last decade, we introduce a novel foam EOR utilizing the raw mixture of constituents of Natural Gas Liquids Mixture (NGLM) composed principally of ethane, propane, butane, and natural gasolines. This process involves the injection of NGLM with non-condensable gas (i.e. nitrogen) and non-aqueous foam stabilizing additive (such as surfactant) to simultaneously maximize the displacement efficiency based on its miscible nature and the sweep efficiency due to the mobility control as provided by the NGLM-based foam. The objectives of this study are to (i) investigate the feasibility of this non-aqueous-foam-enhanced miscible hydrocarbon flooding in sub-10-mD carbonate cores, and (ii) evaluate the effect of the non-aqueous foam on miscible displacement.
The proof-of-concept study of non-aqueous foam assisted miscible displacement were conducted by performing a series of core floods in heterogeneous carbonate cores with sub-10-mD permeability. The effect of foam on oil recovery performance was evaluated based on the ultimate recovery factor and oil recovery rate. The effect of injection strategy (continuous drive injection and alternating injection) on non-aqueous foam propagation and overall recovery were evaluated and compared. The propagation of foam and the mobilization of the unswept oil were monitored based on the measured pressure drops of the core.
It was found that non-aqueous foam-assisted miscible flooding can achieve promising ultimate recovery factor while significantly reducing the amount of injected NGLM. For the reference case (continuous NGLM injection), an ultimate recovery factor of 87.6% was achieved after 2.06 PV of NGLM injection. High ultimate recovery factor may be attributed to (i) the miscibility between NGLM and oil, and (ii) gravity stabilization (top-down injection). For continuous nitrogen drive (0.33 PV of NGLM slug followed by a nitrogen drive), a low ultimate recovery factor of 47.4% was observed due to the acceleration of the injectant breakthrough as caused by the extremely unfavorable mobility ratio between nitrogen and crude oil. With the non-aqueous foaming agent, the ultimate recovery factor for the continuous nitrogen drive was increased by 15.7%. Comparison between the overall and sectional pressure drops in these two cases indicates that the in-situ generation of non-aqueous foam significantly delayed the injectant breakthrough and recovered the unswept oil in the sections closer to the producer. For alternating injection, the ultimate recovery factor of the case with the foaming agent (87.5%) is 17.8% higher than that without the foaming agent. Most importantly, the ultimate recovery of the foam-enhanced NGLM-alternating-nitrogen is similar to that of the continuous NGLM injection with only 0.83 PV of NGLM consumed. Overall pressure drops in two foam-enhanced cases (continuous drive injection and alternating injection) indicate that foam generation and propagation are much stronger in the alternating injection case which leads to the higher ultimate recovery factor and sweep efficiency. This could be attributed to the "foam dryout" effect caused by the continuous desaturation of the mobile liquid phase in the case of single liquid slug injection. Lastly, NGLM efficiency (defined as the volume of crude oil recovered divided by the volume of injected NGLM) was calculated to evaluate the commercial vitality of this process.
This work demonstrates for the first time that non-aqueous-foam-asissted hydrocarbon miscible flooding is technically feasible in sub-10-mD rocks where conventional mobility control options are quite limited. The experimental results show that the ultimate recovery factor of hydrocarbon miscible flooding in low permeability carbonate cores can be significantly improved by non-aqueous foam with lower consumption of NGLM.