Experimental laboratory evidence of enhanced production by spontaneous imbibition via the addition of surfactants into completion fluids, as well as field observations, indicate a significant improvement in EUR with the use of surfactants to improve oil recovery from unconventional liquid reservoirs (ULR). During a hydraulic fracture treatment, the surfactant molecules interact with the rock surface, altering its wettability and interfacial tension. The wettability alteration of the rock surface from oil-wet to water-wet enables the spontaneous imbibition of water into the matrix, which expels the oil out of the pore space towards the fractures. Several laboratory and numerical studies have investigated the effectiveness of surfactant-assisted spontaneous imbibition (SASI) on various ULR. However, the understanding of surfactant selection for the optimization of enhancing recovery in ULR is not well studied.
Capillary pressure is the dominant force for spontaneous imbibition process. Contact angle (CA) and interfacial tension (IFT) are essential terms in the Young-Laplace capillary pressure equation as well as in published scaling analysis of the spontaneous imbibition process. With the large amount of data released on SASI, it is natural to develop a correlation between the two properties to the recovery factor. However, no work has been conducted to investigate the relationship of contact angle and IFT on ultimate recovery by spontaneous imbibition in ULR. In this manuscript, a compilation of CA, IFT, and spontaneous imbibition experiments from two of the most prolific shale reservoirs is presented to give an insight into the relationship between the three variables. Then, based on the observed trends and correlations, a new scaling model for SASI in ULR is proposed. The ultimate goal is to develop a surfactant selection method based on scaling analysis results and laboratory data for optimal performance in ULR.
A total of 35 independent SASI correlated experiments data were compiled, which includes CA, IFT, recovery factor, porosity, core plug dimensions, and capillary pressure calculated from the Young-Laplace equation. Experimental procedure on each data point followed the robust data gathering methodology that was already developed for the past four years. The reliable procedure ensures the representability of the reservoir condition in the laboratory measurements to provide an accurate description of the effectivity of different surfactants on a corresponding oil/water/rock system. Two systems were analyzed and assembled into three groups, Wolfcamp, Eagle Ford A, and Eagle Ford B. An inversely proportional correlation between CA and recovery factor was observed, while on the IFT and recovery factor analysis, a less apparent correlation was found. Theoretically, a directly proportional correlation between capillary pressure and recovery factor can be expected due to spontaneous imbibition that is primarily dominated by capillary forces, which is consistent with experimental data analysis. The high dependence of recovery factor on contact angle and the less significant effects from IFT lead us to conclude that a substantial wettability altering surfactant is highly preferred to enhance through SASI. In addition, based on the observed correlation, a new dimensionless scaling equation fully accounting for the effect of surfactant addition was developed to generalize the flow behavior of SASI.