Clariant Oil Services Removes Sulphide Scale via Dissolver Soak for UK North Sea PWRI System

Benefits

• Increase in water injection performance
• Increased oil production
• Effective hydrocarbon removal for deposit clean-up
• Effective removal of sulphide scale deposit
• Bullhead and soak or low dose continuous injection deployment options

Background

A field operating within the UK North Sea experienced a decline in water injection following the start-up of Produced Water Re-Injection (PWRI). The onset of water break through resulted in sour gas production, which was not planned for during the design phase. A rapid decrease in performance was also witnessed, the source being attributed to the formation and subsequent re-injection of FeS, formed through reaction of the H2S with the system metallurgy. Upon investigation and analysis, the following conditions were found:

• The water injection rate declined from 28,000 bpd to 22,000 bpd within 6 months
• A caliper run identified significant restrictions in 5” injector tubing
• Significant quantities of solids were removed from injector wells
• The solids recovered were identified via XRD analysis as iron sulphide, iron carbonate and iron hydroxide

Following the intervention, a chemical treatment was still required to remove the deposits that had formed in the lower tubing, perforations and near well bore.

Clariant Oil Services Solutions and Value Delivered

Due to the previously high levels of oil-in-water present and that were re-injected, hydrocarbons were expected to be present on the deposits. Iron sulphide deposits are also known to be preferentially oil-wet, therefore a solvent package was required up-front of any dissolver treatment.

Clariant Oil Services identified the mutual solvent ESP2000 for this purpose. The product offers improved performance over standard hydrocarbon solvents, without risking changing the wettability characteristics of the reservoir and damaging the formation. After treatment, the application of ESP2000 resulted in the effective removal of the hydrocarbons known to be present, thereby allowing increased penetration of the selected scale dissolver into the Fe-based deposit.

The next chemical treatment involved a pre-flush with ESP2000 to remove any hydrocarbons entrained in the deposits. This was flushed into the reservoir followed by a 3- to 6-hour soak of neat MULTITREAT 5537, a proven sulphide scale dissolver that is used worldwide.

Clariant Oil Services’ MULTITREAT 5537 quickly and effectively chelates the Fe-based scales without the production of H2S that would be formed through application of traditional acid-based dissolver products. The use of MULTITREAT 5537 thereby allowed the fluids to be displaced into the reservoir and, as such, avoided the requirement to backflow the well and dispose of the waste products.

Following the soak, the fluids were displaced into the reservoir using treated drill water. This treatment was preformed twice to ensure effective deposit removal. To determine the effectiveness of the well intervention a Pressure Fall-off (PFO) test was performed.

The results identified that KH improved from 31,000 md ft to 37,000 md ft. This is an improvement in injectivity index of around 20%. The skin remained almost unchanged. Interpretation of the analysis identified that the increase in injectivity was related to the removal of solids within the perforations and near well bore, and not simply due to removal of solids alone from the tubing.

Following the success of this treatment, the use of Clariant Oil Services’ ESP2000 and MULTITREAT 5537 chemical treatments have been qualified for use on further well interventions by the operator.

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