Compositions of Anionic and Non-Ionic Surfactants within a Hybrid EOR Technology for Unconventional Hydrocarbon Reservoirs

It is well known that the development of unconventional reserves is quite complicated due to the poor reservoir porosity and permeability. The use of horizontal wells with multi-stage hydraulic fracturing remains one of the promising methods in use today for the development of such reserves. Subsequently, tertiary recovery methods popularly known as enhanced oil recovery (EOR) can then be carried out. In this paper, the compositions of anionic and non-ionic surfactants, potentially suitable for use in unconventional hydrocarbon deposits as EOR agents were investigated (on the example of one of the fields of Bazhenov formation). Also, attention was devoted to the assessment of the feasibility of co-injecting the surfactant solutions with a thermal agent (subcritical water) in a hybrid thermo-chemical EOR process. During the course of the study, 35 samples of industrial surfactants (individual and blends) were investigated. The compatibility of the surfactants with brine water, their stability under reservoir conditions (T>100 °C, P=25 MPa) for more than 14 days, and the effectiveness of the surfactants in reducing the interfacial tension (IFT) at the oil-brine boundary were the key factors in choosing the most appropriate compositions for use in the hybrid EOR. The ability of surfactants to decrease the IFT was investigated using a spinning drop tensiometer while the wettability alteration effect was estimated using a drop shape analyzer. Filtration experiment on oil-saturated core sample and evaluation of surfactant adsorption on rock surface were carried out with the best compositions. The results of the study show that the colloidal systems, represented by mixtures of anionic and non-ionic surfactants, have the best performance. The main components of these surfactant compositions are sodium salts of olefin sulfonates, derivatives of sulfonic acids C15-C20, and ethoxylated alcohols C6-C12. The results of measurements imply that certain compositions alter the initial rock wettability to become more water-wet and reduce the IFT between oil and water to a value of 0.051 mN/m. The adsorption of surfactant molecules on the rock was estimated to be 4 g/kg of rock, and the ultimate oil displacement rate increased due to surfactant injection from 8 % obtained during water flooding to 40.5 %. The possibility of using surfactants within the hybrid EOR technology was proven because the best surfactant mixture showed thermal stability at temperatures above 250 °C. Thus, we can conclude about the possibility of the use of some surfactant mixtures for the development of unconventional oil fields. Also, it is possible to combine the injection of surfactant solutions with the injection of thermal fluid, leading to the generation of synthetic oil in situ, thereby improving the reservoir properties of the rock and recovery of additional oil due to the effect of surfactants. This technology can be possibly applied for the development of unconventional reserves to increase the oil recovery ratio and make the process economically viable.