Over the years environmental legislation has forced changes in the types of scale inhibitor molecule that can be deployed in certain regions of the world. These regulations have results in changes from phosphonate scale inhibitor to polymer based chemistry particularly in the Norwegian and UK continental shelf where phosphonates have either been on the substitution list or phased out for many applications. Over the past 10 years significant improvements in inhibitor properties of the so called “Green” scale inhibitors have been made. For one particular operator the squeeze application of this “green” scale inhibitor resulted in poorer than expected treatment lifetimes and significant operating cost due to the frequency of retreatment. To overcome the increasing operating cost an evaluation was made of the current treatment chemicals vs. the older more established phosphonate scale inhibitors. The results for the laboratory evaluation suggested that the older chemistry would extend treatment life and reduce operating cost. A case was made to the legislative authority and field applications started. The squeeze lifetimes for the “red” phosphonate chemistry were shown to be better than the “yellow/green” inhibitors. This paper presents the laboratory evaluation of the scale inhibitors, outlines the rationale for the change back to phosphonate scale inhibitors and illustrates the improvement observed via field results from the 2 wells treated so far.
The subject field is located in the Norwegian sector of the North Sea and came on stream in December 1998. The 8 production and 3 injection wells are tied into the wellhead platform and the oil processed at a Floating Production Storage and Offloading vessel (FPSO). The distance between the wellhead platform and the ship is 1 km. The water depth is 86 m. The oil is exported by tankers and the produced gas re-injected. Map of the field's location is shown in Figure 1. Water injection utilises seawater for reservoir support and is incompatible with the formation water. Over the years scale squeeze treatments have been applied to production wells to control sulphate scale. This is supplemented by application of scale inhibitor on the wellhead platform to prevent further deposition during transport of fluids to the FPSO and during its processing prior to discharge to sea.
Scale squeeze deployment is more challenging in this field than normal platform wells for the following reasons. The field is an unmanned wellhead platform and FPSO separated by approximately 1 km with flexible multiphase flowlines and umbilicals. This makes scale squeeze operations difficult due to the lack of space on the unmanned wellhead platform to locate chemical tanks and pumping equipment. To perform these operations, separate crews are deployed to both the wellhead platform and the FPSO which is only possible during periods of good weather due to helicopter landing issues on the FPSO and lack of accommodation and bedspace on the unmanned platform. Hardlines are taken from the water injection system through a manual choke to the wing valve on the target well.