Conventionally, scale mitigation is achieved using chemicalinhibitors either by squeeze treatment into the reservoir orcontinuous injection. However, with new fields encounteringincreasingly more challenging environments, or when theeconomic impact of chemical intervention by squeezetreatment is large (e.g. subsea fields with poor bullheadchemical placement), other methods of scale control such asthe use of low sulphate sea water (LSSW), must be consideredduring the front end engineering and design (FEED) stage of afield development. Nevertheless, for conventional sulphatereduction packages (SRP's) that reduce the sulphateconcentration in the injected sea water typically towards 40 –50 ppm, there remains a residual scaling risk and therequirement for periodic squeeze treatments.Previous work reported at the 2004 SPE OilfieldScale symposium (SPE 87465) examined the level of sulphatereduction required to mitigate the requirement for evenperiodic squeeze treatments against barium sulphate scale.This showed that sulphate levels of 20 ppm were required inorder to prevent scale formation under down hole productionconditions, although it was also demonstrated thatthermodynamically the system remained oversaturated withbarium sulphate.This paper expands considerably on this preliminary"field specific" case and examines the impact of LSSW on thescaling kinetics across a broad range of formation watercompositions (barium ranging from 150 ppm to 650 ppm) andat temperatures between 80oC and 120oC. The papertherefore investigates the relationship between scaling kineticsand thermodynamics in relatively mild scaling environmentsand illustrates that whereas extremely low levels of sulphatewould be required to completely prevent scale from athermodynamic viewpoint, the kinetics of scale formation mayprevent scale precipitation under down hole productionconditions, with additional continuous injection inhibitorapplied at wellheads to protect flow lines etc. In summary,this paper presents results from an extensive series of longterm dynamic flow and pseudo static performance testsdesigned to determine the relative impact of thermodynamicsand kinetics on the residual barium sulphate scaling risksassociated with the injection of LSSW for pressure support.
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