A Stimulation Treatment of a Sub Sea Well Using a Scale Dissolver

Abstract

The Stattjord East and North are two sub sea fields tied in to the Stattjord C platform on the Stattjord Field. A well, on the Stattjord East satellite was damaged during the gravel pack operation. The damage is believed to be rust particles due to use of dirty drill collars during placement of the pre pack gravel. The productivity index, PI, of the well after clean up of the perforations, i.e. prior to gravel packing, was calculated as 742 Sm3/d/bar. After gravel packing and bean up of the well, the PI was calculated as 55 Sm3/d/bar. The well observed increased productivity to about 80 Sm3/d/bar before its decline prompted the use of a scale dissolver to remove the damage. The challenge was to pump the scale dissolver 7.8 km from the platform out to the wellhead template through a 2.5 inch methanol line. At the time of performing the stimulation treatment the oil rate of the well was 776 Sm3/d with a PI of 11 Sm3/d/bar. The maximum pumping rate during the operation was 800 litres per minute. After the scale dissolver had been soaked in the gravel pack for 12 hours, the well was put on production and it delivered 2600 Sm3/d of oil. The PI was increased to 55 Sm3/d/bar. A decline in production was observed, and after a year a second squeeze was performed successfully. The PI increased this time from 23 to 59 Sm3/d/bar and the oil production increased with 1000 Sm3/d. For the first time in the Norwegian North Sea, successful stimulation treatments of a sub sea satellite well has been achieved by pumping a scale dissolver through a 2.5″ methanol line into a 5.5″ completed well. The evaluation of the data has shown that the plugging process is complex and difficult to interpret. However, it can be concluded that injection of fluids has improved the gravel pack productivity. The erosion potential has also been minimised due to reduced draw down across the gravel pack. The damage mechanism is most likely a combination of rust, brine and fines plugging the wire wrapped screens and the poorly filled perforation tunnels. The first stimulation operation had a payback time of 2 days and the second operation 4 days.