Scale Inhibitor Squeeze Treatments Deployed From an FPSO in Deepwater Subsea Fields in the Campos Basin

Abstract

This paper describes field experience and lessons learned from scale-control operations in a deepwater subsea fielddevelopment in the Campos Basin, Brazil; specifically, from bullheading scale-inhibitor squeezes from the FPSO host, along the production flowlines, into four low-watercut, horizontal subsea wells, completed with sand control. The relatively small number of high-cost, highly productive wells, coupled with a very high barium-sulfate scaling tendency upon breakthrough of injection water, meant not only was effective scale management critical to achieve high hydrocarbon recovery, but even wells at low water cuts were deemed to be at sufficient risk to require squeeze application. Use of conventional, water-based squeezes have been known to cause significant damage to productivity in low-watercut wells, including those showing a fines-migration tendency, as was the case here. Hence, based upon risk mitigation, supported by an extensive programme of laboratory testing, it was decided that for the initial treatments only the mainflush would be water-based, with a mutual-solvent preflush and marine-diesel overflush. Other key challenges associated with treating from the host included the remote location of the wells, the potential to form hydrates, the cleanliness of the lines along which the treatment would pass, the achievement of effective placement over a long producing interval, as well as the need to deploy the chemical package via a support vessel adjacent to the FPSO. All had to be managed due to the high cost and low availability of a deepwater rig that could deploy the treatments directly to the subsea wellheads. The paper will explore in detail the issues associated with inhibitor-squeeze deployment in deepwater, subsea fields, many of which are currently being developed in the Campos basin, Gulf of Mexico and West Africa, and is a good example of best-practice sharing from another oil basin. INTRODUCTION Fields Description The fields are located in the Campos Basin offshore Brazil, approximately 145 km east of Macae,. The field is on the present -day continental slope, in water depths ranging from 700 to 850 meters. Development of fField X comprises of 6 horizontal producers, gravel -packed and completed with pre-packed screens, located centrally in the reservoir and 4 slantdeviated flank water injectors at the flanks. Seawater injection maintains pressure and increases recovery. The wells have been drilled from a central drilling location. The 6 production wells are located on 2 production manifolds and the 4 injection wells on a single injection manifold. Field Y is 5 km to the northwest of Field X, wereand was developed in a similar manner, with 2 horizontal producers completed as in Field X, producing to one manifold, gravel pack and completed with pre-pack screens, and 2 deviated water injectors tied back in 2 subsea manifoldsto another. Both fields (1 producer and 1 injection) and tied produce into the same FPSO,. The FPSO which has a production capacity of 81,000 bbl of oil per day and a storage capacity of 1.2 million barrels of oil. A 3third party company operates the FPSO. The field came on stream oin August 12th, 2003. Initial Aaverage production initiallywas some 60 kKbpd but this droppeding of to 50 kKbpd byin early 2005 due to early breakthrough of injection water breakthrough and well impairment. The reservoir temperature is approximately 90°C. Scale formation has been a production issue in these fields as they are supported by injection of seawater, which is incompatible with the formation brines, which contain up to 180 mg/l barium and up to 300 mg/l strontium ions. Wells with seawater breakthrough are scale squeezed using a phosphate-ester scale inhibitor to control sulfate and carbonate scale formation within the wells and flowlines; additional inhibitor is injected to the produced fluids once they reach the topside facilities.