The Impact of Sulfate Level Reduction on Seawater Injection Economics: Using Sulfate Scale Precipitation Kinetics to Our Advantage

ADNOC Onshore plans to use seawater as alternative to aquifer water, its source of injection water for over 40 years. However, using seawater for injection introduces a sulfate scaling risk due to incompatibility with formation water. Sulfate in the seawater and cations in the formation water (Ca, Sr) are likely to precipitate, causing scaling and related flow assurance problems and formation damage. Sulfate can be removed from the injection water by means of desulfation, but sulfate removal to well below its scaling concentration is CAPEX intensive and negatively impacts seawater flooding economics. In this paper, the economic benefits of partial sulfate reduction are evaluated, by finding a balance between controllable scaling and costs for inhibition and sulfate removal.

Infinite Water: Implementation of Nanofiltration for Optimization of Vessel Stimulation Operations in the Offshore Greater Ekofisk Area, North Sea

The incorporation of a sulfate removal system onto a stimulation vessel has been shown to positively affect vessel utilization, increase efficiency in field development, and reduce freshwater consumption. Stimulation vessels have fixed storage and transportation volumes as well as a fixed total mass that can be loaded. Fresh water occupies the highest proportion of space and mass in most stimulation treatments, which imposes limitations on all other products that can be loaded out. Particularly for acid stimulation treatments, a compromise between the volumes of raw acid and fresh water must be made in order to achieve the best operational efficiency possible. Any method that can reduce, eliminate, or replace fresh water as a component in stimulation fluids will have a significant impact on vessel efficiency. One option is the use of seawater as the base fluid. However, seawater can cause problems for well production due to the high sulfate content in the water leading to the formation of mineral scale. The solution to this problem has been the installation of a sulfate removal system on the stimulation vessel. Driven by membrane nanofiltration, this system can produce up to 100 m3/hr of low sulfate water from seawater for well stimulation operations. By removing the scaling risk from seawater, this system enables the stimulation vessel to maximize the products it loads with the ability to produce low sulfate water as and when it is needed. The sulfate removal system can reduce SO4 content to 4.3 mg/l and reduce other ions present in seawater. With an output of 100 m3/hr and being installed independently from stimulation systems, the unit is able to produce water regardless of ongoing activities. In stimulation jobs, multistage ball drop operations are the most time-critical operations. In the analysis of hundreds of stages stimulated with water from the new nanofiltration system, the average stage completion time was 6 hours, which included ball loading, dropping, and displacement; diagnostic injection testing; and the main treatment. With an average water requirement of 600 m3, the vessel can keep up with water demand and remove water capacity from the utilization equation. The use of a compact nanofiltration system for SO4 removal has improved stimulation vessel operations where scale production is a key concern for operators. In addition to increasing vessel utilization and intervention efficiency, the system will lead to the elimination of approximately 68,000 m3 of fresh water being pumped every year for stimulation operations in the North Sea.

Mesoporous Poly(melamine-co-formaldehyde) Particles for Efficient and Selective Phosphate and Sulfate Removal

Due to the existence-threatening risk to aquatic life and entire ecosystems, the removal of oxyanions such as sulfate and phosphate from anthropogenic wastewaters, such as municipal effluents and acid mine drainage, is inevitable. Furthermore, phosphorus is an indispensable resource for worldwide plant fertilization, which cannot be replaced by any other substance. This raises phosphate to one of the most important mineral resources worldwide. Thus, efficient recovery of phosphate is essential for ecosystems and the economy. To face the harsh acidic conditions, such as for acid mine drainage, an adsorber material with a high chemical resistivity is beneficial. Poly(melamine-co-formaldehyde) (PMF) sustains these conditions whilst its very high amount of nitrogen functionalities (up to 53.7 wt.%) act as efficient adsorption sides. To increase adsorption capacities, PMF was synthesized in the form of mesoporous particles using a hard-templating approach yielding specific surface areas up to 409 m2/g. Different amounts of silica nanospheres were utilized as template and evaluated for the adsorption of sulfate and phosphate ions. The adsorption isotherms were validated by the Langmuir model. Due to their properties, the PMF particles possessed outperforming maximum adsorption capacities of 341 and 251 mg/g for phosphate and sulfate, respectively. Furthermore, selective adsorption of sulfate from mixed solutions of phosphate and sulfate was found for silica/PMF hybrid particles.

Offshore Water Treatment KPIs Using Machine Learning Techniques

New water treatment facilities in the Gulf of Mexico include a seawater Sulfate Removal Unit (SRU) to mitigate reservoir souring and scaling. The general industry sulfate target for offshore SRU is usually 20 mg/L or even 40 mg/L; however, some facilities may require <10 mg/L of sulfate in injection water, which makes water quality monitoring more critical and challenging. Current industrial practice relies on only pressure drop and a constant cleaning interval frequency to perform SRU maintenance which may result in reduced membrane life due to frequency cleaning or severe membrane fouling without the capability to predict fouling based on process conditions. The machine learning techniques applied will fill the gap and deliver a prediction model based on both simulation and real-time field data. This model will track and monitor the system key performance indicators (KPIs) including pressure, membrane fouling factor (FF), permeate sulfate concentration etc. The monitoring and prediction of these KPIs provide estimates on when the next maintenance procedure is required, track membrane system status for troubleshooting and actions, and optimize membrane performance by tuning operation conditions.

Biological sulfate removal with low-cost carbon sources using cold-acclimated bacteria

The main goal of this study was to develop a cost-efficient biological method for the removal of sulfate from mining effluents in cold conditions. A consortium of cold-tolerant sulfate-reducing bacteria (SRB) was tested at 6 °C regarding the utilization of economically viable, low-cost carbon sources, i.e., whey, conditioned sewage sludge, and peat, in the removal of sulfate from synthetic mining water. Succinate was used as a reference carbon source. Of all the studied low-cost carbon sources, conditioned sewage sludge proved to be the most efficient. Nuclear magnetic resonance (NMR) spectroscopy revealed that sewage sludge contained propionic acid, which proved to be utilizable by SRB under cold conditions. Peat both adsorbed the sulfate and acted as a nutrient source in the sulfate reduction process. When whey was used as a carbon source, only a slight decrease in sulfate concentration was detected. Succinate was found to work in a truly predictable and efficient way as a carbon source in biological sulfate reduction, even at the lowest concentration tested. The use of conditioned sewage sludge increased the bacterial diversity in liquid cultivations significantly. However, the number of SRB was highest in the succinate cultivations.

Subsea Sulfate Removal and Low Salinity Plant Membrane Life Prediction for IOR and EOR

Subsea water treatment technology for water injection provides a solution for an optimized water injection strategy on both green and brownfield applications. There are no space and weight constraints on the seabed, which enables simple, flexible, and reliable designs. A full-scale sulfate removal and low salinity plant designed for seabed operation with minimum maintenance intervention has been in operation for ten months without any chemical cleans. This paper reviews the performance results used to predict the life and longevity of a subsea sulfate removal unit (SRU) for a field application in the North Sea. The ability to reliably predict the potential risks and serving life of a system is crucial for the success of subsea processing equipment. Results from a test conducted over ten months evaluated the key design parameters such as SRU membrane permeate flux, recovery rate, and analyzed membrane fouling behavior during the test period. Lessons learned from the testing are incorporated into the model for more accurate and reliable membrane life prediction. Field data was analyzed to extrapolate aging factors that are used in the membrane design projection software. The projection software simulated aging performance of the membrane over the membrane lifespan under subsea operation conditions, and the results were applied to determine operating philosophy, maintenance, and intervention interval for the subsea plant. Lessons learned from this field study were discussed and used as guidelines for the next phase full-scale design and practice. This novelty field practice establishes a break-through step towards full implementation of subsea seawater treatment and injection for increased oil recovery (IOR) and enhanced oil recovery (EOR) purposes. This firsthand data helps the operators to optimize the operation on the seabed, which minimizes the downtime and demonstrates promising advantages of CAPEX and OPEX saving in the long run.

Innovation to Reduce Operation Downtime in Sulfate Removal Offshore Applications

Sulfate removal in injection water is standard practice to prevent scaling and souring in subsea oil reservoirs. Nanofiltration membranes have been used to this purpose since 1987, when FilmTec™ SR90-400 elements were installed in an offshore platform in the North Sea. The most pressing concern in this type of systems is membrane fouling, with the associated reduction in effective plant operation time and shorten element lifespan caused by the standard Clean-in-Place (CIP) protocols. The object of this research has been to test the latest developments in biofouling-resistant sulfate removal membranes to achieve oil and gas (O&G) industry requirements. Improved chemistry and improved module engineering have enabled the production of new membrane elements that represent the next-generation in sulfate removal nanofiltration. Next-generation sulfate removal membranes have been trial-tested. In pilot testing, target performance was validated in terms of productivity, permeate quality and fouling resistance. The results of this testing indicate that improvements in membrane chemistry and module engineering have resulted in a 63% decrease in pressure drop and a much slower fouling trend over the total of 6 elements. This significant improvement should allow an important reduction in the number of cleanings, which the authors have estimated to be of 50%. Moreover, sulfate rejection values are in the range of 99.9% (below 1 ppm of sulfate in the permeate), providing great injection- quality water. Full-scale testing in a production site in the Atlantic Ocean was done to validate pilot testing results, showing a continued operation of 100 days without any need for a clean-in-place (CIP) procedure. The results obtained in the extensive testing carried out on these new antifouling elements, show that the improvements implemented in its design have the ability to improve the operation of Sulfate Removal Units (SRU). These improvements are the results of reducing maintenance costs and downtime on offshore platforms, resulting in increased operation and improved productivity.