Oil-water pore occupancy in the Vaca Muerta source-rocks by NMR cryoporometry

Abstract

Abstract The main motivation of this work is to determine quantitatively the pore occupancy of water and oil in some source rocks. The pore occupancy might be imposed by differences in chemical composition, wettability behavior and sorption of fluids in organic and inorganic porosities and will dramatically affect the hydrocarbons flow through the porous medium. We address this issue using NMR cryoporometry in two phase situations complemented by other NMR measurements. Using oil-wet samples originating from the Vaca Muerta formation, oil saturation as large as 70% were obtained by simply immersing fully water saturated samples into decane. At 100% water saturation, the measured pore size distributions indicate two main peaks around 2 and 80 nm. After immersion into oil, the pores occupied by water in the presence of decane are mostly around 2 nm and less. The pore sizes occupied by decane in the presence of water can then be deduced by subtracting the distributions obtained at 100% water saturation and after spontaneous drainage. Oil is mainly located in pores around 80 nm. These measurements are confirmed by high resolution SEM images showing the presence of porosity in the organic matter (sponge-like structure). Most of the oil can be associated with the porous organic matter network. These observations are complemented by the determination of the porosity below 2 nm and the water connectivity. From D2O diffusion experiments at 100% water saturation, we observed that about 19% of the total porosity is non-exchangeable with D2O. From the measurements of the liquid signal at −29 °C, we observed that on average 58% of the total porosity is located in pores smaller than 2 nm.