Chen, Szu-Ying (University of California at Santa Barbara) | Kaufman, Yair (University of California at Santa Barbara) | Kristiansen, Kai (University of California at Santa Barbara) | Howard, A. Dobbs (University of California at Santa Barbara) | Nicholas, A. Cadirov (University of California at Santa Barbara) | Seo, Dongjin (University of California at Santa Barbara) | Alex, M. Schrader (University of California at Santa Barbara) | Roberto, C. Andresen Eguiluz (University of California at Santa Barbara) | Mohammed, B. Alotaibi (Saudi Aramco) | Subhash, C. Ayirala (Saudi Aramco) | James, R. Boles (UCSB) | Ali, A. Yousef (Saudi Aramco) | Jacob, N. Israelachvili (UCSB)
Waterflooding via injection of chemistry-optimized low-salinity – also, low ionic strength/concentration – waters, such as SmartWater, is becoming increasingly attractive for improved oil recovery, especially in carbonate reservoirs. In this manuscript, we describe the results from a series of experiments and theoretical modeling to determine the mechanisms that govern the
We measured various interrelated crude-oil(
The results presented in this manuscript are of practical significance to provide a better understanding of SmartWater flooding mechanisms in carbonates at multiple length scales, including subnano-, nano-, micro-, and macroscopic scales. The new fundamental understandings presented in this study will also guide the optimization of SmartWater flooding processes in other reservoir systems.
Smart water and low salinity waterflooding has been established as an effective recovery method in carbonate reservoirs by demonstrating a significant incremental oil recoveries in secondary and tertiary modes compared to seawater injection. Therefore, understanding of multiphase flow phenomena in reservoir rocks is critical to optimize injected water formulations for substantial increase in oil recovery. Characterization of fluid-fluid and fluid-rock interactions have been extensively conducted at micro- and macroscopic scale, attempting to reveal the underlying mechanisms responsible for wettability alteration. Indeed, routine methods for assessing macro-wettability of fluids on rock surfaces (contact angle) include the sessile drop and captive bubble techniques. However, these two techniques can provide different contact angle depending on rock surface heterogeneities, roughness and drop size. Thus, contact angle measured at macroscale can only be used to characterize the average wettability and a direct visualization at nanoscale is needed to identify oil and brine distribution in the carbonate matrix and wettability state at the pore scale. The application of ion-beam milling techniques allows investigation of the porosity at the nanometer scale using scanning electron microscopy (SEM). Imaging of carbonate porosity by SEM of surfaces prepared by broad ion beam (BIB) and under cryogenic conditions allow to investigate preserved fluids inside the rock porosity and, combined with energy dispersive spectroscopy (EDS) identify crude oil and brine distributions and quantify carbonate-oil interfaces and wettability state. The experiments have been conducted on carbonate rock samples aged in crude oil and saturated with brines at high and reduced ionic strength. This study established an experimental protocol using Cryogenic high resolution broad ion beam (Cryo-BIB SEM) equipped with energy dispersive spectroscopy (EDS). The results show that ion-BIB milling provides a smooth surface area with large cross-section of few mm2. High resolution imaging analysis allowed identification of the different phases, chemical mapping and distribution of oil, brine within the porous matrix. Segmentation of rock-oil-brine interface allowed an estimation of the in-situ contact angle and showed the effect of injected salinity brine on the 2D contact angle and more accurate description of the carbonate wettability at nanoscale.
Accurate assessment of remaining oil saturation and sweep efficiency greatly depends on the implemented monitoring program, which requires the integration of all available geoscience and engineering data, by effective analysis using statistical and reservoir simulation methods. This will allow improvedunderstanding of sweep, validation of recovery factor and identifying new development opportunities.
Comprehensive reservoir surveillance is also a critical factor for effective reservoir management in achieving optimal hydrocarbon recovery. Monitoring programs encompass the deployment of up-to-date reservoir saturation tools and techniques capable of delivering high-quality data. There are many complications to be considered such as mixed salinity environments, reservoir heterogeneities, tools with limited depth of investigation and mud invasion effects. These challenges must be considered for a successful reservoir saturation monitoring program. Therefore, the value established by an integrated program involves the use of the most efficient approach in analyzing the acquired saturation data and overcoming the field challenges.
This paper presents a comprehensive approach that was implemented on in situ data acquired from a carbonate reservoir that has operated continuously for several decades with pressure support from peripheral water injection. The technique capitalizes on the wealth of data acquired both from saturation and production logs. The prime objectives of this technique are to evaluate remaining oil saturation, remaining unswept oil column and displacement, and vertical/areal sweep efficiency. The strength of this methodology is the capability of efficiently quantifying and mapping remaining oil saturation. This helps in identifying "sweet spots" behind the flood front and thereby guiding future development activities for maximizing hydrocarbon recovery.
In most reservoirs around the world, paleo oil exists below the free water level and is considered residual oil to natural/geological waterflood. This non-trivial resource of residual oil will not flow by primary or secondary recovery means but requires carefully designed enhance oil recovery (EOR) methods to mobilize it. To date, there is no detailed analysis of paleo oil in the literature simply because it is difficult to obtain a reservoir sample.
This study provides a comprehensive paleo oil analysis for samples obtained from reservoir sponge cores. The oil in the sponge core was extracted, analyzed, and compared to main pay zone oil (MPZ). Critical data have been unveiled on paleo oil characterization through fundamental studies on oil quality, fingerprint, filling history, available hydrocarbon components and compounds, and molecular level characterization. It was found that the global composition and overall quality of paleo oil is very similar to the MPZ oil. However, the differences between the two oils were only apparent when the study was further extended to include molecular level analysis and available hydrocarbon components and compounds. These differences may define the appropriate the EOR methods to mobilize this oil and explain trapping mechanisms caused by fluid properties.
Gas chromatography studies revealed that paleo oil extracts have the same Pristane/Phytane ratio as the MPZ oil suggesting that they are of the same origins and share the same source rock. Further analysis showed a good match of the Terpane biomarkers between paleo oil extracts and MPZ oil but with slightly less maturity levels. Paleo oil quality was compared to MPZ oil using Pyrolytic Oil Productivity Index (POPI) analysis which indicated same API range as the MPZ oil and same light volatile, thermally distilled and cracked components. Paleo and MPZ oils were also analyzed using nuclear magnetic resonance (NMR) to qualitatively test the similarity of the oil components and measure their apparent viscosities. Both oils have shown very comparable viscosity measurements and NMR signatures. The simulated distillation analysis showed that lighter components in paleo oil are less abundant than MPZ oil while medium to heavy components are relatively similar. Fourier Transform Ion Cyclotron Resonance (FT-ICR) study, which zoomed into the heavier components, revealed that paleo oil has less aromaticity than MPZ oil and lacks aromatic sulfur and di-sulfur compounds, negligible amount of nitrogen compounds, and no resin type components.
This study provides in depth information about oil extracted from the residual oil zone, which doesn't flow by primary or secondary recovery means. Up to our knowledge, there is no available information in the literature that explains the components, compounds, quality and behavior of this oil because it is hard to obtain reservoir samples. This data shed light on possible trapping mechanism caused by fluids in place. The study also employed several methods and tools to confirm the conclusions and ensure repeatability of results.
SmartWater flooding through injection of chemistry optimized waters by tuning individual ions is recently getting more attention in the industry for improved oil recovery in carbonate reservoirs. Most of the research studies described so far in this area have been limited to studying the interactions at rock-fluids interfaces by measuring contact angles, zeta potential, and adhesion forces. The other widely reported interfacial tension data at oil-water interfaces do not consider the formation of interfacial monolayer and the interfacial tension is estimated as an average parameter relying on the properties of two individual bulk phases. As a result, such measurements have serious shortcomings to provide any details on complex microscopic scale interactions occurring directly at the interface between crude oil and water to understand the SmartWater flood recovery mechanism.
In this study, two novel interfacial instruments of interfacial shear rheometer and surface potential sensor were used to study microscopic scale interactions of various individual water ions at both air-water and complex crude oil-water interfaces. The measured interfacial rheology data indicated totally different interfacial behavior at crude oil-water interface when compared to air-water interface due to presence of crude oil functional groups. Viscous dominated response was observed at crude oil-water interface for all brine compositions. These interfaces behaved like a viscous fluid without exhibiting viscoelastic solid like properties. Lower interfacial viscous modulus was observed for certain key ions such as calcium, magnesium, and sodium. The interfacial viscous modulus was found to be substantially much higher for sulfates, besides exhibiting some elasticity. The surface potential was gradually decreased by replacing seawater with calcium only brine. The better surface activity with seawater can be attributed to adsorption of more key water ions at the surface.
The interesting results observed with certain water ions at fluid-fluid interfaces are expected to work in tandem with rock-fluids interactions to impact oil recovery in SmartWater flood. At first, they play a role to control the accessibility of active water ions to approach the rock surface, interact with it and subsequently alter wettability. Next oil droplets adhering to the rock surface will be detached and released due to favorable interactions occurring at rock-fluids interfaces. The interfacial film between oil and water can then quickly be destabilized due to less viscous interfaces observed with certain ions to promote drop-drop coalescence and easy mobilization of released oil droplets. This coalescence process is sequential and it would continue until the formation of small oil bank.
This is the first study that showed added importance of fluid-fluid interactions in SmartWater flood by using direct measurements on individual water ions at crude oil-water interface. In addition, a new oil recovery mechanism was proposed by combining both the interactions occurring at fluid-fluid and rock-fluids interfaces. The new fundamental knowledge gained in this study will provide an important guidance on how to synergize water ion interactions at fluid-fluid interfaces with those at rock-fluids interfaces to optimize oil recovery from SmartWater flood.
The importance of tuning injection water chemistry for upstream is getting beyond formation damage control/water incompatibility to increase oil recovery from waterflooding and different improved oil recovery (IOR)/enhanced oil recovery (EOR) processes. The water chemistry requirements for IOR/EOR have been relatively addressed in the recent literature, but the key challenge for field implementation is to find an easy, practical, and optimum technology to tune water chemistry. The currently available technologies for tuning water chemistry are limited, and most of the existing ones are adopted from the desalination industry, which relies on membrane based separation. Even though these technologies yield a doable solution, they are not the optimum choice to alter injection water chemistry in terms of incorporating selective ions and providing effective water management for large scale applications. In this study, several of the current, emerging, and future desalination technologies are reviewed with an objective to develop potential water treatment solutions that can most efficiently alter injection water chemistry for SmartWater flooding in carbonate reservoirs.
Standard chemical precipitation technologies, such as lime/soda ash, alkali, and lime/aluminum based reagent, are only applicable for removing certain ions from seawater. The lime/aluminum based reagent process looks interesting, as it can remove both sulfates and hardness ions to provide some tuning flexibility for key ions included in the SmartWater. There are some new technologies under development that use chemical solvents to extract salt ions from seawater, but their capabilities to selectively remove specific ions need further investigation.
Forward osmosis and membrane distillation are the two emerging technologies, and these can provide good alternatives to reverse osmosis seawater desalination for the near-term. These technologies can offer a better cost-effective solution where there is availability of low grade waste heat or steam. The two new desalination technologies, based on dynamic vapor recovery and carrier gas extraction, are well suited to treat high salinity produced water for zero liquid discharge (ZLD). These technologies may not be able to provide an economical solution for seawater desalination. Carbon nanotube desalination, graphene sheet-based desalination, and capacitive deionization are the three potential future seawater desalination technologies identified for the long term. Among these, carbon nanotube based desalination is much attractive, although the technology is still largely under research and development.
This review study results show that there is no commercial technology yet available to selectively remove specific ions from seawater in one step and optimally meet desired water chemistry requirements of SmartWater flooding. As a result, different novel schemes involving selected combinations of chemical precipitation, conventional/emerging desalination, and produced water treatment technologies are proposed. These schemes represent both approximate and improved solutions to selectively incorporate specific key ions in the SmartWater, besides presenting the key opportunities to treat produced water/membrane rejects and provide ZLD capabilities in SmartWater flooding applications. The developed novel schemes can provide an attractive solution to capitalize on existing huge produced water resources in Saudi reservoirs to generate SmartWater and minimize wastewater disposal during field-wide implementation.