Scale control is conventionally achieved by use of chemical inhibitors, introduced to the production system either by squeeze treatment into the reservoir or continual injection into the well. However, as new field developments encounter more challenging production environments or when the associated economic impact of chemical intervention is large (e.g. subsea completed wells with poor chemical placement by bullhead treatments, or limited access to wet subsea wellheads), other methods of scale control such as sulphate removal must be considered during the front end engineering design (FEED) stage.
The Greater Plutonio development, lying in deep water (500 – 1600 metres) off the coast of Angola, will be produced via multi-well subsea facilities tied back to a floating production vessel, with reservoir pressure support being provided by seawater injection. Owing to the complex and inaccessible nature of the subsea production system, chemical control of sulphate scale was considered economically unfeasible. Scale control will therefore be achieved by means of sulphate removal prior to seawater injection. However, following conventional sulphate reduction towards 40 ppm there remains a residual scale potential, the impact of which on production is difficult to assess using thermodynamic scale prediction packages or normal laboratory conditions. Periodic squeeze treatments were however still anticipated. A detailed laboratory testing programme of dynamic and static scale formation tests coupled with dynamic core-blocking tests was undertaken to determine the effect of varying production conditions, organic acid content, flow regime and the nature of the underlying substrate on scale formation under field specific conditions for this mildly oversaturated system. The ultimate goal of the laboratory test programme was to define an optimal sulphate level in the injected seawater to prevent scale formation throughout the production lifetime of the field. By comparison of laboratory test results with predicted scale formation tendencies, an acceptable level of sulphate in the injected seawater was determined which would allow pressure support without causing scale formation. This allowed further detailed analysis of the costs associated with a residual scale risk at 40 ppm sulphate (or above) with the costs associated with upgrading the SRP facility to achieve lower levels of sulphate in the injection water. The laboratory work allowed an optimal sulphate concentration to be determined which was achievable through modification of the current design facilities and resulted in a significant cost saving in the development phase.