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

Abstract

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.