No abstract

Polymer flooding is one of the most mature enhanced oil recovery (EOR) methods with many field implementations including those in China, Germany, Oman, and USA. The primary role of polymer is increasing the injected water viscosity, hence reducing the displacing front mobility and thereby improving the macroscopic sweep efficiency. Polyacrylamide, the main polymer used in EOR applications, achieve this increase in viscosity due to the large molecular size of its chains as well as the ionic repulsion between the chains. Those same properties result in significant interactions between the transported polymer molecules and the porous medium, including adsorption, mechanical entrapment, and hydrodynamic retention. Those phenomena, in turn, can lead to polymer losses, injectivity reductions and inaccessible pore volumes. Despite the maturity of polymer flooding, few implementations and research studies have targeted carbonates. Thus, a clear understanding of the magnitude and significance of those interactions and effects for carbonates is lacking. Those phenomena are critical for both numerical predictions and actual performances of polymer flood. Therefore, in this work we investigate thoroughly polymer losses, injectivity reductions and inaccessible pore volumes for a slightly viscous Arabian carbonate reservoir that exhibits high salinity and high temperature conditions. For this purpose, we perform single phase displacement experiments at reservoir conditions. Representative materials were used including simulated brines reflecting connate and injection brine salinities, dead crude oil, and aged reservoir plugs. Core plugs with a wide permeability range from 45.2 md to 12836 md were used for the tests. A pre-screened polyacrylamide was used at an injection concentration of 5,500 ppm. A 2,000 ppm tracer was added into the polymer solution to assess polymer interactions. The effluent polymer concentrations were determined by total organic carbon (TOC) method, and tracer concentrations were analyzed by gas chromatography (GC). Results showed that resistance factor (RF) tended to be higher for tighter samples. RF increased with increasing injection rate for lower permeability samples and decreased with increasing injection rate for higher permeability samples. Residual resistance factor (RRF) slightly decreased with increasing injection rate. RRF correlated well with pore size, with larger pore size corresponding to lower RRF. The effective in-situ viscosity of the polymer was constant at lower injection rates. However, at higher rates, the effective in-situ viscosity increased with injection, exhibiting a shear thickening behavior. Moreover, the polymer exhibited dynamic retention ranging from 0.155 to 0.530 mg/g-rock, and showed a decreasing trend for more permeable core sample. Finally, the studied carbonate constituted of 15.2% to 20.9% pore-volume that was inaccessible to the polymer. Those results besides being essential for numerical-based upscaling of polymer flooding, shed light on some of the similarities and differences between sandstones and carbonates when it comes to chemical EOR application.

A second stage of gravity settling with the addition of demulsifiers or clarifiers is commonly used in processing plants to further treat the separated produced water. In previous work, we demonstrated gravity settling lower efficiency in removing oil carryover from produced water compared to other processing techniques. Both centrifugation and filtration were found to significantly improve the separated water quality. In this work, we focus on centrifugation and further evaluate its efficiency in improving the quality of separated water for both water and chemical floods, specifically surfactant/polymer (SP) flooding. Samples were firstly prepared to imitate the separation plant projected feed and operations. Synthetic representative brines were prepared and used with dead crude oil to prepare the oil/water emulsions. Emulsion separation was conducted at different temperatures, as well as different concentrations of SP, and the demulsifier. The kinetics and efficiency of separation were thoroughly studied over two stages of separation: primary gravity settling and secondary centrifugation. We performed gravitational separation using bottle tests in order to firstly obtain the separated produced water for use in secondary water treatment studies and to secondly further investigate gravity settling kinetics and efficiency. Water quality, in terms of oil content, was then assessed through solvent extraction and UV analyses. Samples of the produced water separated by the primary gravity settling were then exposed to secondary centrifugation. Centrifugation was performed at different rotational speeds using a dispersion analyzer. Light transmission evolution in space and time was used to study kinetics, efficiency and mechanisms of secondary centrifugation. The results reconfirmed that a single-stage gravity settling is not sufficient to reduce oil carryover to acceptable levels for disposal and re-injection into oilfields. Secondary centrifugation yielded clear and significant improvement in water quality even in the presence of EOR chemicals. With centrifugation, the separation efficiency was a function of the rotational speed. Higher rotational speeds resulted in higher creaming velocities and faster separation. In addition, creaming velocities indicated that higher temperatures yield favorable effects on oil droplets migration and separation rates. This is possibly due to the lower density and larger bouncy at higher temperatures. Based on these results, we conclude that secondary centrifugation is very efficient and effective in improving the quality of separated water. In terms of the effects of investigated EOR formulations, SP addition caused minor but manageable reduction in separated water quality at a level that would not harm conventional disposal practices.

The synergy between various enhanced oil recovery (EOR) processes has always been raised as a potential optimization route for achieving a more economic and effective EOR application. In previous work, beside the established positive impact of SmartWater on polymer rheology, we have demonstrated polymer positive impact on SmartWater effects through measurements of surface potential and contact angles. The results were also supported by forced oil-displacement experiments. In this work, we further investigate this SmartWater/Polymer synergy focusing on the possible impact of SmartWater on polymer floods specifically polymer injectivity, retention and acceleration. For this purpose, a set of thoroughly designed single-phase displacement experiments were performed at reservoir conditions. The results demonstrate that SmartWater has: (1) a slight negative impact on both polymer and chase water injectivity, (2) a positive impact on polymer retention, and (3) a negligible impact on polymer, hence oil bank, acceleration. Given the minor negative impact on injectivity, we conclude that SmartWater/Polymer flooding exhibits an overall favorable synergy that reduces chemical consumption and improves sweep.

We investigated the utility of a modified Washburn method, combining results of two sorption experiments, to study wettability alteration processes. The main objective in deploying this technique was to avoid the difficulty of direct contact angle measurements in surfactant solutions. In such cases, the ultra-low interfacial tension between the crude oil and surfactant tends to cause the oil drop to spread prematurely rendering the measurement unreliable if possible at all. A carbonate-rock, dead crude oil, synthetic brines including SmartWater, and five surfactants were used in this study. The surfactants included: an anionic alfa olefin sulfonate, a cationic quaternary ammonium salt, an amphoteric surfactant, a nanosurfactant, and a nonionic ethoxylated alchohol. Hexane was also used as a reference completely wetting fluid. For sorption experiments, the rock was powdered. The powder with size between 80 and 100 mesh was compacted in the sample holder. In each experiment, a given fluid was raised to the bottom of the powder pack and allowed to rise into the powder by capillarity. A sensitive balance was used to measure fluid imbibition into the powder until no more fluid imbibes. Beside sorption experiments, surface and interfacial tension measurements were made. For benchmarking, we relied on previous findings reported in the literature. Plotting the square of fluid mass against time gave a slope which enabled the calculation of contact angle in air. The results (slope) observed for the completely wetting fluid (hexane) provided the rock constant. Sorption results of crude oil coupled with the previously determined rock constant enabled estimation of the Wasburn oil/air contact angles using the modified Washburn equation. Sorption results with brine and surfactant solutions coupled with the rock constant and oil/air enabled estimation of the Washburn water/air contact angles using the modified Washburn equation. Based on water/air and oil/air contact angles, values of the conventional oil/water contact angle were estimated. For surfactant solutions, interfacial tension between oil and surfactant-free brine were used to approximate contact angles that were otherwise undefined. This approach, enabled a robust and rapid estimation of the effects of various processes on contact angles (hence wettability alteration). Based on the benchmark previous findings, the sorption approach yielded acceptable results especially for screening purposes. The results demonstrated the potential of the Smartwater recipe and the nonionic surfactant for wettability alteration. However, it is recommended to rely on the sorption method direct measurements including sorption rates to establish sorption-based wettability indices and eliminate the intermediary and probably unnecessary contact-angle estimation step. In conclusion, the use of sorption to obtain contact-angle estimates provide a novel rapid and robust procedure for evaluation and screening of wettability-alteration agents. This eliminates direct contact angle measurements that are often cumbersome. It specifically eliminate the difficulties of attaching an oil drop onto a rock surface immersed in surfactant solutions.