It was observed that higher oil recovery could be obtained when low-salinity (LS) water flooded a core of high-salinity initial water about 15 years ago. Such low-salinity waterflooding benefit or ...effect has drawn the oil industry attention since then. In the recent years, many researchers conducted laboratory corefloods, and several companies carried field tests. The objectives of these efforts were (1) to conform the benefits and (2) find the mechanisms of such benefit. Although most of the results confirmed the positive effect, some results showed no benefit. Many mechanisms have been proposed, but there is no consensus of the dominant mechanism(s). The oil industry is continuing the effort to discover the effect. This paper is to provide a critical review of the results and to summarize the achievements of the industry׳s effort. This paper aims to provide the status of the art. The information provided in this paper hopefully will help to speed up our further efforts to explore this effect. The following contents are reviewed: (1) history of low-salinity waterflooding; (2) laboratory observations; (3) field observations; (4) working conditions of low-salinity effect; (5) mechanisms of low-salinity waterflooding; and (6) simulation of low-salinity waterflooding.
In this paper, the mechanisms proposed in the literature and their validity are discussed.
•Provide a comprehensive review of lab results, field observation and all proposed mechanisms.•Discuss the validity of each mechanism.•Provide the personal view of possible effects of low-salinity waterflooding in real field-scale.
This book bridges the gap between theory and practice in a range of real-world EOR settings. Areas covered include steam and polymer flooding, use of foam, in situ combustion, microorganisms, "smart ...water"-based EOR in carbonates and sandstones, and many more. Oil industry professionals know that the key to a successful enhanced oil recovery project lies in anticipating the differences between plans and the realities found in the field. This book aids that effort, providing valuable case studies from more than 250 EOR pilot and field applications in a variety of oil fields. The case studies cover practical problems, underlying theoretical and modeling methods, operational parameters, solutions and sensitivity studies, and performance optimization strategies, benefitting academicians and oil company practitioners alike.
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•An experiment based on centrifugation and NMR was conducted to test the fluid movability in shale.•The saturation of fluid in shale during the centrifugation simulated.•The ...movability of bound fluid in nanopores was larger than the theoretical value.
The occurrence states of oil in the kerogen and structural characteristics of shale are unfavorable for the production of shale oil. Understanding the movability of oil and water at different occurrence states or in different pore structures is important for the development of shale oil reservoirs. Therefore, a high-speed centrifugation experiment based on the nuclear magnetic resonance technique was conducted for shale samples saturated with oil and water to test the movability of oil and water in the shale. To describe the effect of nanopores and the direction of capillaries on the movability of oil and water in the shale, a mathematical model based on the variational diameter capillary model was developed. The results indicated that a larger amount of bound oil (the adsorption-dissolution oil in the kerogen and the oil in the nanopores) was present in the shale, which is 3.12–5.44 times the amount of bound water (the adsorption oil on the interlayer of clay and the oil in the nanopores). The movability of free water (up to 88.8%) in the shale was larger than that of free oil (up to 68.1%) and the movability of free fluid (up to 88.8%) in the shale was significantly larger than that of bound fluid (up to 12.4%). According to the model, most of the capillary diameters of the shale gradually increased in the direction of the centrifugal force and weakened as the lamination developed. The movability of bound oil and water in the nanopores was larger than the theoretical value.
Background
Diet and exercise during pregnancy have been used to prevent gestational diabetes mellitus (GDM) with some success.
Objective
To examine the effectiveness of lifestyle intervention on GDM ...prevention and to identify key effectiveness moderators to improve the prevention strategy.
Search strategy
Pubmed, Scopus, Cochrane, and cross‐references were searched.
Selection criteria
Randomised controlled trials (RCTs) evaluating lifestyle interventions during pregnancy for GDM prevention.
Data collection and analysis
Two independent reviewers extracted data. A random‐effects model was used to analyse the relative risk (RR) and 95% confidence interval (95% CI). Meta‐regressions and subgroup analyses were used to investigate important moderators of effectiveness.
Main results
Forty‐seven RCTs involving 15 745 participants showed that diet and exercise during pregnancy were preventive of GDM (RR 0.77, 95% CI 0.69–0.87). Four key aspects were identified to improve the preventive effect: targeting the high‐risk population; an early initiation of the intervention; the correct intensity and frequency of exercise; and gestational weight gain management. Although 24 RCTs targeted women who were overweight or obese, body mass index (BMI) failed to predict the effectiveness of an intervention. Instead, interventions are most effective in high‐incidence populations rather than simply in women who are overweight or obese. Furthermore, exercise of moderate intensity for 50–60 minutes twice a week could lead to an approximately 24% reduction in GDM.
Conclusion
The best strategy to prevent GDM is to target the high‐risk population predicted by risk evaluation models and to control the gestational weight gain of women through intensified diet and exercise modifications early in their pregnancy.
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Four key effectiveness moderators of lifestyle interventions for GDM prevention.
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Four key effectiveness moderators of lifestyle interventions for GDM prevention.
It is generally believed that a higher efficiency of flowback of the invaded fluid will result in a reduced formation damage and a higher oil rate. However, in water-wet formations having flow ...characteristics close to those seen in low permeability formations, high flowback does not always result in high oil production rates. In such reservoirs, the strength of capillary gradient across the matrix-fracture interface controls the amount of invaded-fluid flowback. This paper discusses two factors which impact the strength of capillary gradient: depth of fracture fluid invasion and presence of moderate interfacial tension (IFT) surfactant in the fracture fluid. Microfluidic-chip or otherwise called Lab-on-a-Chip based experiments are conducted in this work to highlight the discussed factors. For a water wetting chip, it is observed that as the depth of fracture fluid invasion into the matrix increases, flowback efficiency is improved while the late-time oil production rate decreases. Additionally, it is found that the presence of a moderate IFT-reducing surfactant in the fracture fluid aids in reducing the invasion-induced formation damage compared to a fracture fluid without surfactant. The work presented strongly impacts the oil industry in contemporary times when hydraulic fracturing is significantly prevalent. The results obtained provide a better understanding of the flow behavior of in-situ and invaded fracture fluids during production phase of the operation.
Surfactant enhanced oil recovery (EOR) includes surfactant flooding and surfactant stimulation. The main functions of surfactants are to reduce interfacial tension and wettability alteration. This ...paper is to review the EOR technology related to surfactant injection. The reviewed topics include the following:
• Surfactant EOR mechanisms
• Factors affecting interfacial tension
• Trapping number
• Screening criteria
• Laboratory work
• Numerical simulation work
• Summary of pilot and large-scale applications
• Surfactants used
• Salinity gradient
• Surfactant EOR in carbonate reservoirs
• Surfactant EOR in shale reservoirs
We have performed more than 300 atmospheric simulations of the 1991 Pinatubo eruption using the AER 2-D sulfate aerosol model to optimize the initial sulfur mass injection as a function of altitude, ...which in previous modeling studies has often been chosen in an ad hoc manner (e.g., by applying a rectangular-shaped emission profile). Our simulations are generated by varying a four-parameter vertical mass distribution, which is determined by a total injection mass and a skew-normal distribution function. Our results suggest that (a) the initial mass loading of the Pinatubo eruption is approximately 14 Mt of SO2; (b) the injection vertical distribution is strongly skewed towards the lower stratosphere, leading to a peak mass sulfur injection at 18–21 km; (c) the injection magnitude and height affect early southward transport of the volcanic clouds as observed by SAGE II.
Surfactant enhanced spontaneous imbibition is an effective method to enhance oil recovery in shale reservoirs, however the mechanism is still unclear. This work attempts to investigate the effect of ...added surfactants on the spontaneous imbibition into shale rocks using NMR tests. Contact angles and IFT are measured to evaluate the efficiency of different surfactants in terms of wettability alteration and IFT reduction. According to the experimental results, the studied anionic surfactants perform better on the wettability alteration of shale rocks than studied nonionic surfactants, and the key factor to enhance the shale oil recovery may be wettability alteration.
CO2 miscible displacement is an efficient method for enhanced shale-oil recovery. Understanding the movability of shale oil in different states of occurrence is important for CO2 miscible ...displacement. Therefore, a nuclear magnetic resonance (NMR)-based CO2 miscible displacement experiment is conducted on oil-saturated shale and sandstone. To describe the processes involved in the measurements, a mathematical model based on the Maxwell–Stefan equation is developed; it considers the heterogeneity of shale and the competitive adsorption and absorption of CO2–oil mixtures in organic matter. The results show that the porosity of immobile oil in the shale samples (4.0% and 4.3%) is lower than the porosity of free oil (8.0% and 10.2%). Compared with sandstone, a larger CO2-injection time yields a smaller recovery factor in the case of shale, and the recovery factor of immobile oil is much less than that of free oil in the shale. According to the model, the order of oil production in the laminated shale during CO2 miscible displacement is as follows: free oil in the inclusions, free oil in organic matrix-clay, and adsorption and absorption of oil in organic matter. The oil production rate is associated with the heterogeneity of shale (R 0) and the competitive adsorption and absorption of CO2-oil mixture in the organic matter (ω0, k ads).
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