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Abstract Preventing scale precipitation is one of the main issues for maintaining well productivity on the Veslefrikk Field 1.An oil soluble scale inhibitor treatment was pumped that reduced the initial productivity index (PI) from 21 Sm[3]/d/bar to 3 Sm[3]/d/bar. Reperforation only restored some of the productivity (from 3 to 7 Sm3/d/bar) indicating deeper near wellbore damage. A small hydraulic fracture stimulation was then performed, creating a propped fracture with a half-length of approximately 6 m. The PI then increased to 30 Sm[3]/d/bar, a 43% increase from the initial value. The stimulation was performed under pressure through a 2 7/8" work string run on Rig Assist Snubbing (RAS) with pumps and blenders rigged on the pipe deck.
Introduction The Veslefrikk Field is located in block 30/3 on the Norwegian sector of the North Sea. The field is developed with a fixed wellhead platform with 24 well slots, connected to a floating production facility.Original recoverable reserves were estimated to 36 million Sm[3] of oil to be developed during 10 years of production starting from 1989.
Today approximately 45 million Sm[3] has been produced and still close to 10 more million Sm[3] is planned to be produced.
The producing reservoirs are in the Brent Group, the Intra Dunlin Sand and the Statfjord Formation (Figure 1), each containing a different fluid system. The main pressure support is from sea water injection.
Although the ion composition of the Veslefrikk formation waters are not extreme compared to other North Sea formation waters, severe scale precipitation has occurred. Both CaCO[3] and BaSO[4] have been identified, enhanced by high reservoir temperature, 125º C (CaCO[3]), and commingled production (BaSO[4]). A preventive scale control strategy[1] mainly based on scale inhibitor treatments has been implemented.
Pre stimulation well history Well 30/3-A-24 T2 was initially perforated in two 3 meter intervals in the upper part of the Oseberg Formation. This is relatively clean, moderate porosity sandstone with permeabilities in the 100 – 1000 md range. Calcite cemented and shaly layers have been observed in some areas of the field, but the lateral communication is good. To allow for zone selective water shut-off (straddle) or zone selective fracturing, the two perforation intervals were separated by 8 meter.
The well was put on production on the 19th of May 2002, and a step rate test indicated a liquid productivity index (PI) of 21 Sm[3]/d/bar. Formation water breakthrough occurred after 2–3 days of production, and the subsequent well tests showed decreasing PI. This was interpreted to be due to deposition of calcium carbonate in the perforation tunnels and in the liner (confirmed by later wireline interventions), and a scale inhibitor squeeze was therefore initiated.
Previous scale squeezes in low water cut wells on the field have often resulted in decreased PI and increased water cut, probably due to relative permeability effects. To avoid this it was decided to deploy a scale inhibitor dissolved in an organic solvent (Oil Soluble Inhibitor, OSI). When these chemicals get in contact with the connate water, the inhibitor will transfer to the connate water and remain in the pores. After water break-through the inhibitor is activated and protects the produced water from scale precipitation. This was the first OSI to be performed on Veslefrikk, the supplier reported a success rate of almost 100% (approximately 30 jobs), and the field specific laboratory tests and core floods showed acceptable results.
The OSI treatment was performed the 30th of May 2002. During the pumping sequence high pressures were observed for a longer period than expected, but the squeeze was nevertheless completed. Fracturing pressure was never exceeded. It soon became obvious that the treatment had caused severe formation damage. Well tests showed that the PI had decreased to 3 Sm[3]/d/bar as a result of the OSI squeeze, Figure 2.
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