There are many projects that leverage water injection or waterflood (WF) as a secondary recovery tool to increase oil production returns. Many of these projects’ economic viability is intrinsically linked to the ability to inject large volumes of water into the reservoir as a means of oil drive and pressure maintenance. For these fields, it is vital that the integrity of the water injection system is maintained through fit-for-purpose material selection and asset integrity management processes.
During water injection, water is injected directly into the reservoir to sweep oil from the formation and/or maintain pressure in the reservoir as shown by Figure 1. While treated seawater is commonly used for injection, special filtration or sulfate removal is sometimes employed to reduce risk in the reservoir and production system. This article will focus on seawater injection.
The seawater is injected via a seawater injection (SWI) system. Major components of a SWI system include:
- Seawater lift pumps – draws the seawater from the water source into the SWI System.
- Filters (Coarse/Multi-Media/Cartridge) – seawater passes through these to remove smaller suspended solids and particulates.
- Dearation unit – filtered water passes through this to remove any dissolved oxygen.
- Water injection pumps – injects water directly or through a water injection flowline into the reservoir.
Inherent in the design and requirements of a water injection system are several integrity management threats:
- Oxygen corrosion
- Bacterial growth and microbiologically-induced corrosion (MIC)
- Hypochlorite-induced corrosion
- Erosion due to solid pick-up
All of the mentioned threats need to be mitigated through appropriate material selection and integrity management processes.
Fit-for-Purpose Integrity Management of SWI Systems
When determining appropriate means to mitigate the risks to the SWI system, all aspects of the plan should be gathered and worked into an actionable plan that is based on an appropriate risk-based assessment and should focus on the fit-for-purpose material selection and integrity management (IM) processes. These processes include monitoring and surveillance and chemical treatment during operation.
When selecting the materials for SWI, there are two strategies typically used for design:
- Use internally bare carbon steel with best practice asset management processes that must be followed, particularly regarding oxygen and bacterial control. This option requires relatively low capital expenditure (CAPEX) but is high in operational expenditure (OPEX) due to the need for strict IM processes.
- Use lined carbon steel or corrosion resistant alloy (CRA) materials. These materials include stainless steel, copper-based alloys, composites, titanium, and high-nickel alloys. While these materials require less monitoring and maintenance and thus necessitate relatively low OPEX, they have a generally higher CAPEX due to material cost.
Integrity Management Processes
Integrity management processes that can be considered include monitoring and surveillance and chemical treatment. Both allow the SWI system to be able to maintain its integrity. A formal risk assessment is typically conducted to determine which process is most appropriate to use; however, it is typically best to use a combination of these processes.
Monitoring and surveillance is an option to maintain the topsides, subsea, and downhole portions of the SWI system. The subsea and downhole portions are typically considered too costly for the SWI system; however, the topsides portion needs to be monitored, particularly if bare carbon steel is used. For topsides, the best practices for a monitoring and surveillance strategy include the following:
- There should be several locations for sampling and these sampling points should evaluate the parameters outlined in Table 1.
- Sample points before, in-between, and after filter medias allow the ability to sample for solids, for bacteria, and to understand the effectiveness of the filters.
- Sample points before and after the dearation unit allow bacteria counts and oxygen concentration changes to be determined through the unit.
- A combination of monitoring options such as galvanic probes, corrosion coupons, and an electrical resistance probes should be used.
- The coupon and probes should be positioned in the 6 o’clock position in horizontal pipes, if possible, because this allows the coupon and probes to also monitor for under deposit corrosion and MIC in certain situations.
- It is best to have these monitoring options before the injection pumps to have a good understanding of the corrosion effects on the SWI system.
- Access fittings for these monitoring options should follow installation best practices to ensure safety of the system and the personnel using them.
- A combination of surveillance options (e.g. visual inspection with ultrasonic testing, radiographic testing, magnetic particle testing, etc.) should be used based on risk assessment findings and strategy.
- Visual inspection may make another surveillance option such as ultrasonic testing easier to carry out. However, it does require an experienced inspection engineer in detecting the types of potential damage to components.
Chemical treatment is another option to maintain the SWI system. The chemicals should be injected topsides and the following chemicals should be considered:
- Oxygen Scavenger
Hypochlorite is critical to controlling biological growth, macro-fouling, and corrosion in the SWI system. Typically, it is used on a continuous basis, but can be used as a batch treatment. Any residual hypochlorite will react rapidly with oxygen scavenger. If oxygen scavenger is planned to be used on a SWI system, additional batch biocide treatments are recommended after the oxygen scavenger injection point. Hypochlorite should be injected upstream of the seawater lift pumps.
Oxygen scavenger is critical to controlling the threat of oxygen corrosion in the SWI system. Typical chemistries for oxygen scavenger include ammonium bisulfate and sodium sulfite. Dosage rate is critical to the use of oxygen scavenger as over-dosage can result in the deposition of scale at the injection points. It is recommended for high-rate injection scenarios where oxygen scavenger is being used for fully-aerated seawater without other means for mechanic dearation that a spool piece with a parallel injection quill is used just in case a plug was to occur. Another issue to note with oxygen scavenger is that typical oilfield biocides such as tetrakis hydroxymethyl phosphonium sulfate (THPS) and glutaraldehyde greatly decrease the reaction rate of oxygen scavenger .
Biocide is typically batch-treated into the SWI system to protect the parts of the system that are not exposed to hypochlorite. Biocides are more environmentally sensitive and expensive compared to hypochlorite. Biocides can also cause additional foaming in dearation units; however, it is important to have the biocide upstream of the dearation unit as a lack of biocide in this component could leave an opening for bacterial growth.
Chemical Treatment Locations
The integrity management processes including both monitoring and chemical treatment for SWI systems based on preferred locations is shown in a typical SWI system in Figure 2.
Seawater injection is a vital tool for the economic success of some projects and those projects need to ensure that the seawater can be effectively delivered from the seawater source to the reservoir. Maintaining the integrity with appropriate material selection and integrity management processes will ensure this.
- Moore, J.; Keasler, V.; Bennett, B., Compatibility of Tetrakis (Hydroxymethyl) Phosphonium Sulfate (THPS) and Ammonium Bisulfite (ABS). NACE Corrosion 2010, San Antonio, TX. 2010