Petroleum Africa
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Tuesday, May 30, 2023
By Samuel Ezewu, Senior Technical Manager
ChampionX Oilfield Solutions, Nigeria LTD
As the largest oil producer in Africa and the second largest gas producer, Nigeria is a highly active member of the global energy industry. Following concerted efforts by the federal government to boost the country’s crude oil production, the market is set to grow as the country aims to produce three million barrels per day by 2025.
While the decarbonization of oil and gas operations remains at the forefront of global environmental goals, ensuring this increased recovery is as efficient and economical as possible is a key priority, and optimum flow assurance is critical to achieving this.
Flow assurance issues are generally caused due to the formation of solids, such as hydrates, wax, scale and asphaltenes within the pipeline. A number of factors can affect the presence of these formations, such as fluid composition, pressure, flow rate and temperature. Particularly in mature oilfields, maintaining flow assurance can be challenging as water fraction increases. The use of water injection is a common method of increasing production, but it can also create issues as the presence of water in crude oil can lead to emulsions.
Without a proactive approach to flow assurance, solids can cause blockages within the pipeline, valve systems and surface equipment, resulting in significant project inefficiencies. This can also pose a risk to safety on board the asset. In serious cases, such obstructions can lead to lengthy shutdowns while the issue is remediated. A robust prevention strategy is the most efficient and cost-effective way to approach flow assurance and ensure production is optimized at all stages of the field’s life.
ChampionX, a global leader in highly engineered upstream and midstream chemical, mechanical, and digital solutions for the energy industry, recently worked with a Nigerian operator to develop a customized scale inhibitor monitoring program for the surface facilities and seawater flood systems of an FPSO, which was producing 230,000bpd of oil, 40,000bpd of water and 260,000bpd of seawater injection.
Proactive scale prevention
During production, increasing volumes of produced or effluent water can be generated during operations, so it is necessary to maintain a proactive approach to control the formation and maintain flow assurance and integrity of the production system. The potential for scale problems exist whenever the fluid produced from a reservoir contains water. As the well matures and more hydrocarbons are extracted from the reservoir, the water table in the reservoir rises, water begins to be produced, and the potential for scale deposition increases.
Chemical inhibition has been used over many decades to remediate potential scaling threats posed by different types of scale, including calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, iron sulfide, iron oxides and iron carbonate. However, with changing production fluid ratios, scale inhibitor rates must be monitored to ensure they are adequately preventing scale precipitation.
The conventional method of monitoring scale across several points in the FPSO’s seawater flood system and surface facilities is with the use of scale coupons and spools. Coupons are placed in the system, inserted through a valve or bull plug, and left for a given period to collect scale. Varying sizes of perforations in the rectangular metal coupon create turbulent areas of flow, which induces scale precipitation. Localized pressure drops at the perforations can stimulate loss of carbon dioxide, therefore creating carbonate scale formation. The weight of scale is determined by subtraction of the original weight of the coupon from its dried weight after removal.
The limitations of this approach lie in determining if the scale deposit observed in the electrostatic treater water outlets, for example, was formed earlier as dislodged scale trapped by the impact on the coupon surface or was due to active scale growth. Likewise, it is not possible to determine when actual scaling occurred between the period of its installation and retrieval or from the last visual inspection of the vessel or flowline.
While visual inspection of the production facility for inorganic scale deposits is the most definitive method of identifying the need for or effectiveness of a scale program, the discovery of inorganic deposits is usually made when some system upset has occurred, resulting in the impairment of production equipment or, in the worst case, loss of production. It is therefore imperative that production equipment is inspected whenever it is offline, or the opportunity presents itself. A sample of the deposit can be tested in the field or sent to a laboratory to determine its composition. A major disadvantage of physical inspection is that scale damage has already occurred by the time it is detected.
Scale inhibitor residuals measurement can be used to determine the amount of inhibitor in the process brine. However, this only measures how much inhibitor is present in the system, and not if the system is protected.
For this particular project, the scale inhibitor SCAL16082A, was being injected downhole into the subsea wells at 15 parts per million (ppm) concentration per well, based on water production, to control scale formation in the FPSO surface facilities and seawater flood systems. It was also injected into trains 1 and 2 of the produced water strainer outlets for the seawater flood system to mitigate scale formation at 5ppm.
Insightful data for real-time scale monitoring
Developed by ChampionX, DepCon was deployed to ensure the current scale inhibitor rates were adequately preventing scale deposit build-up through instantaneous or continuous measurements of fluids diverted through the device. These measurements were converted into a ‘mass per unit surface area’ to determine the severity of any deposition in the system, and the results were used to adjust system parameters, such as the chemical injection rate, to control scaling.
The portable and robust device uses quartz crystal microbalance (QCM) technology, which vibrates when an electrical potential is applied (piezo-electric effect). The vibration frequency is linearly proportional to the mass of a deposit on the metal surface of the QCM. Any decrease in frequency indicates deposition. The frequency change can be converted into micrograms of deposit per surface area per unit time.
Recent developments of the proprietary technology have produced a revised system that can be used for inline or sidestream operations at temperatures up to 177°C and 3,000 psi. In addition, the unit has a data logging capability so that it can be installed in a system and programmed to take measurements periodically. The data can then be downloaded using a laptop computer, which allows offshore personnel to have a more direct input into scale control programs.
Reducing costs and increasing flow
DepCon tests were conducted at seven locations, including two flotation units, the seawater flood booster pump and four hydrocyclone water units. The amount of deposit, which was determined from the frequency change of the resonator, varied as the treatment rate of the scale inhibitor was changed across the system.
When the current treatment rate of 5ppm was applied, there was very little or no scale deposition. However, when the injection rate was reduced to 3ppm, scale formation began to occur. The tests concluded that a protected system would show no increase in deposit with time.
The results of a technical audit recommended that the waterflood scale inhibitor should be changed from SCAL16542A to SCAL16082A, a lower cost, best-in-class application to maintain target concentration at 5ppm. By reducing the amount of inhibitor required, this effectively lowered the logistics requirements on the FPSO, thereby reducing operational costs.
Conclusion
Maintaining optimum flow has never been more important as operators look to gain greater economic return on existing wells. The consequences of poor monitoring and management of scale identification and deposition can include well plugging, localization of corrosion attack, and increased risk of safety hazards. Therefore, scale inhibitor rates must be monitored and adjusted to ensure they are adequately inhibiting scale formation to cost-effectively and efficiently address changing production fluid ratios.
About the Author
Samuel Ezewu has more than 28 years of experience in the oil and gas industry, particularly in upstream production chemicals. He joined ChampionX in 1992 and has spent the last ten years in product selection and plant trials and leading the troubleshooting and/or effective implementation of ChampionX programs. Samuel’s role is also integral to support the sales team in the delivery of customer value through robust technical support strategies.
He previously worked in offshore and onshore operations implementing and supporting ChampionX chemical programs at various customer locations, including: Chevron Nigeria Ltd, Esso Exploration and Production Nigeria Ltd, and Mobil Producing Nigeria Unlimited.
He has a BSc in Applied Chemistry from the University of Port Harcourt, Nigeria, a MSc in Analytical and Environmental Chemistry and a Master of Business Administration (MBA) both from the Delta State University, Nigeria.
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