Numerical Analysis of Surfactant Application in Water-Wet Naturally Fractured Reservoirs

High heterogeneity and complex fluid flow behavior aggravates the recovery by conventional means from naturaly fractured reservoirs (NFRs). The last years have seen an increasing interest and a lot of laboratory and field tests were perfomed with the surfactants as Enhanced Oil Recovery (EOR) agents for this type of unconventional reservoirs. However, most of the attention was focused on application of the surfactants in mixed- to oil-wet conditions and not much is reported for the water-wet rocks. The aim of this study is to better understand and provide the quantitave analysis of the low interfacial tension (IFT) aided surfactant EOR mechanisms in water-wet NFRs. For this purpose, the previously published experiments are reproduced by means of numerical modeling employing commercialy available CMG simulator. Static and dynamic imbibition experiments in water-wet core samples performed by Schechter et al. (1994) and by Al-Quraishi (2004) respectively, showed incremental oil recovery effects of low IFT surfactants. Using the reported data and boundary conditions, conceptual numerical models are built for each of the experiments. In addition to numerical simulations, manual calculations of the Bond Number, which is not accounted for in the numerical simulator, has given a detailed insight on the balance between capillary and gravitational forces as well their contribution on oil desaturation. Simulation results of low IFT static imbibition experiments without initial water saturation have shown that the critical capillary numbers used for the matching of the experiments are orders of magnitude lower than those typically reported in literature. Calculation of the Bond number values also revealed that the observed low IFT incremental recovery effects cannot be explained by intensification of the gravitational forces. On the other hand, numerical analysis of dynamic imbibition experiments indicated considerable contribution of viscous forces towards incremental oil recovery, in contrary to conventional believe that the visous forces have limited effect on recovery from NFRs. The conducted numerical simulation study revealed that contribution of low IFT aided gravity forces on oil desaturation is minor. Overall the performed study revealed the weak contribution of the gravitational forces on oil desaturation in low permeability cores. Evaluation of the Bond numbers from the experiments, suggested that reduction to ultra-low IFT values can help to achieve the reasonable effect of gravity forces on recovery. However, the validation of this postulate requires implementation of further studies and laboratory experiments.