What Level of Sulphate Reduction is Required to Eliminate the Need for Scale Inhibitor Squeezing?

Abstract

Abstract Scale deposition is often controlled using chemical scale inhibitors. However, for high scale severity, sulphate reduction in the injected seawater may provide a more attractive economic solution to the problem. This study uses data based on Marlim Leste (Campos Basin, offshore Brazil) reservoir brine compositions and conditions for both reservoir modelling and experiments.Using reservoir modelling, supersaturation and precipitation potentials in the system are first calculated assuming no precipitation reactions in the reservoir. Sensitivity calculations are then performed to investigate the effect of brine mixing both deep in the reservoir and at the production well.Supersaturation and precipitation levels are then recalculated assuming in situ stripping has reduced [Ba[2+]] and/or [SO[4][2-]] levels at the wellbore. Static (uninhibited) BaSO[4] precipitation experiments have been conducted to determine the limiting sulphate level that removes the requirement for applying squeeze treatments.A simple rate law is found and this is used to define "safe operating envelopes" where the barite precipitation is sufficiently slow that we expect no squeeze treatment is required.Some risk is associated with these envelopes due to uncertainties in the determination of the rate constant, k, for barium deposition.Based on further kinetic analysis using data on the safe envelopes, we establish what rate the barite deposition may occur thus identifying where the system would deposit barite slowly and where it would reach its full equilibrium deposition limit quickly.This work presents both a methodology and also some analytical modelling tools for establishing, on a sound technical basis, the answer to the question: What level of sulphate reduction is required to eliminate the need for scale inhibitor squeezing? Introduction Scale inhibitors (SI) have been used extensively to prevent mineral scale deposition in oilfields[1].Nowadays, new field developments present further challenges; e.g. when there is limited access to wet subsea wellheads especially with the development of long subsea tiebacks, or when difficulties arise from SI placement in remote ultra-deep horizontal wells2.Complicated well structures also pose problems with squeeze treatments as does the chemical analysis of return fluids where producing streams co-mingle.[3] Thus, the problems arising from scale formation require an even more rigorous assessment than previously and this should be incorporated into the Front End Engineering Design (FEED) in the field development.An alternative scale control strategy is to reduce the sulphate level in the injected seawater before its deployment, using a sulphate reduction plant (SRP).This would reduce or avoid scale formation when the low sulphate seawater (SW) mixes with the reservoir formation water (FW) during production. Other options may be to combine desulphation with squeeze treatments, or to utilize alternative sources of injection brine such as from aquifers or produced water. Several operators have assessed their fields to determine which of the various scale control methods are most technically viable and economically attractive.[2,4,5]