In Argentina’s Neuquen Basin, Vaca Muerta Shale, EOR represents a significant opportunity due to the large resource in-place, low ultimate recovery factors from primary depletion, a substantial basin wide infrastructure, a tremendous subsurface data set, and a knowledge base that has evolved over the life of the field. The purpose of this work is to evaluate the effect of different EOR technologies to improve the initial rate and the ultimate recovery of Vaca Muerta shale oil horizontal wells. In particular, we have focused on enhanced recovery chemical cocktails that could be added during the hydraulic fracturing process to improve initial producing rates and ultimate recovery. As opposed to traditional enhanced recovery operations that take place later in the producing life of a well or reservoir, enhanced recovery during the initial fracture stimulation could likely lead to more attractive economics by improving early time production and avoiding the fill-up time required to replace produced fluids in a depleted drainage volume or reservoir. While we do not discuss specific chemical formulations as part of this paper, we do detail our approach to designing stimulation EOR treatments for a future pilot test and also show the potential for these treatments improving recovery and economics based on reservoir simulation models.
Assuming that well productivity is driven by spontaneous imbibition, our initial improved recovery investigations previous improved recovery efforts in the field focused on alternating water injection, but this strategy proved unsuccessful as capillary pressure hysteresis drives this mechanism. Following these disappointing results, we started studying the Vaca Muerta from a rock microstructure standpoint. The microstructure variations in the Vaca Muerta we much more significant than those of conventional reservoirs. Often times, the microstructure varied widely at the scale of a few millimeters and many significant changes could be observed across a single core plug. This significant variation in microstructure is likely explained by the longer depositional time span of a shale reservoir depositional environment when compared to most conventional reservoir depositinal environments.
The Vaca Muerta shale has been long regarded as a water-wet shale because of the low percentage of frac water recovered during well production. We identified intercalations of possibly massive water-wet zones and strongly oil-wet zones in the Vaca Muerta kitchen zone. The oil-wet intercalations have high porosity and adsorption isotherms that could indicate more permeability than the water-wet zone. The water-wet intercalations are highly saturated with water, and similarly, the oil-wet intercalations are highly saturated with oil. The laboratory protocol indicates a large percentage of macro and meso-pores. The decane size is 0.7 nm. Thus, the liquid phase is present in pore sizes above 1 nm and above.
The goals of our EOR cocktail design were to make favorable changes in interfacial tension, viscosity, and wettability. Intercalations of high porosity high permeability zones in which the injection of a mutual solvent that reduces viscosity could also change wettability in oil-wet/water-wet Vaca Muerta, improving matrix connectivity and initial oil rate.
Lab results and reservoir simulation show that these changes alone could increase initial oil rates by 20%. Because Vaca Muerta’s organic porosity is a weak point in the rock fabric, which turns out to be the entry zones for chemicals, it is possible that the injected chemicals could also induce a connectivity cascading effect that could increase permeability and further increase the initial rates above those percentages.
Using the results from our laboratory and reservoir simulation studies, a four-well field pilot test has been designed. In this future pilot, we will test different injection concentrations, while keeping the total mass constant. In this manner, we will estimate the volume contacted by the chemicals.
