Pore-Scale Imaging of Tertiary Low Salinity Waterflooding in a Heterogeneous Carbonate Rock at Reservoir Conditions

Abstract

We investigated pore-scale oil displacement and rock wettability in tertiary low salinity waterflooding (LSW) in a heterogeneous carbonate sample using high-resolution three-dimensional imaging. This enabled the underlying mechanisms of the low salinity effect (LSE) to be observed and quantified in terms of changes in wettability and pore-scale fluid configuration, while also measuring the overall effect on recovery. The results were compared to the behavior under high salinity waterflooding (HSW). To achieve the wetting state found in oil reservoirs, an Estaillades limestone core sample was aged at 11 MPa and 80°C for threeweeks. The moderately oil-wet sample was then injected with high salinity brine (HSB) at a range of increasing flow rates, namely at 1, 2,4, 11, 22 and 42 µL/min with 10 pore volumes injected at each rate.Subsequently, low salinity brine (LSB) was injected following the same procedure. X-ray micro-computed tomography (micro-CT) was usedto visualize the fluid configuration in the pore space.A total of eight micro-CT images, with a resolution of 2.3 µm/voxel, wereacquired after both low salinity and high salinity floods.These high-resolution images were used to monitor fluid configuration in the porespace and obtain fluid saturations and occupancy maps. Wettabilitywascharacterized by measurements of in situ contactanglesand curvatures. The results show that the pore-scale mechanisms of improved recovery in LSW are consistent with the development of water micro-dropletswithin the oil and the expansion of thin water films between the oil and rock surface. Before waterflooding and during HSW, the measured contact angles were constant and above 110°, while the meancurvature and the capillary pressure values remained negative, suggesting that the HSB did not change the wettability state of the rock. However, with LSW the capillary pressure increased towards positive values as the wettability shifted towards a mixed-wet state. The fluidoccupancy analysis reveals a salinity-induced change in fluid configuration in the pore space. HSB invaded mainly the larger pores and throats, but with LSW brine invaded small-size pores and throats.Overall,our analysis shows that a change from a weakly oil-wet towards a mixed-wet state was observed mainly after LSW, leading to an incremental increase in oil recovery. This work established a combined coreflooding and imaging methodology to investigate pore-scale mechanisms and wettability alteration for tertiary LSW in carbonates.It improves our understanding of LSW asan enhanced oil recovery (EOR) method for potential field-scale applications. The data provides a valuable benchmark for pore-scale modelling as well as an insight into how even modest wettability changes can lead to additional oil recovery.

Relevance of zeta potential as a tool for predicting the response of controlled salinity waterflooding in oil–water-carbonate systems

“…The same COBR system has been previously studied at pore-scale network using X-ray microcomputed tomography to obtain further insights on the mechanism of EOR by low salinity waterflooding, either in secondary mode [45] or in tertiary mode [46]. Using low-salinity brine in secondary mode, the evolution of oil recovery in the study of Selem et al [45] is quite comparable to the one obtained in our study (Figure 4b).…”

“…The composition of the prepared solutions is presented in Table 1 and is globally comparable to the brines used in other studies [34,41,45,46]. Here, the objective was not to optimize the composition in term of ionic composition and concentration to improve the recovery factor, but to induce a change in zeta potential polarity and to compare two different injection scenarios with model solutions.…”

“…Using low salinity brines in tertiary mode, Selem et al have observed a 9 % increase in oil recovery after switching from high salinity to low salinity brine, but after the injection of several tens of PVs [46]. They evidenced the rapid formation of water micro-droplets in oil and the expansion of water films on the rock surface during injection of low salinity brine, so these phenomena are proposed to be the underlying mechanisms of wettability alteration [46].…”

“…Using low salinity brines in tertiary mode, Selem et al have observed a 9 % increase in oil recovery after switching from high salinity to low salinity brine, but after the injection of several tens of PVs [46]. They evidenced the rapid formation of water micro-droplets in oil and the expansion of water films on the rock surface during injection of low salinity brine, so these phenomena are proposed to be the underlying mechanisms of wettability alteration [46]. However, the fluid occupancy maps' change due to low salinity injection was different for pores and throats: the changes after low salinity waterflooding were insignificant for the small throats while in small pores the oil was partially replaced by water, which was attributed to wettability change.…”

Pore-scale processes in tertiary low salinity waterflooding in a carbonate rock: Micro-dispersions, water film growth, and wettability change

“…However, only low salinity brine was injected during the waterflood. Here we extend this work to tertiary LSW, where both high and low salinity brines were injected, and build on some previous preliminary results [58] allowing for a comparison of the two LSW modes.…”