Produced water re-injection with seawater treated by sulphate reduction plant: Injectivity decline, analytical model

Abstract

Commingled injection of seawater with produced water is an option for offshore fields submitted to waterflooding. It resolves the problems of produced water disposal and of the reservoir pressure maintenance. The presence of barium (strontium) in the formation water and consequent sulphate scaling with seawater flooding is a wide spread phenomenon. In this case, the produced water also contains some barium, which results in sulphate scaling in injection well during commingled injection with seawater. One of the ways around to avoid injectivity damage during the commingled injection of incompatible waters is using a sulphate reduction plant. Being mounted on sea platform, the plant significantly reduces sulphate concentration in seawater and allows using it for commingled produced water re-injection (PWRI). Low sulphate seawater injection together with PWRI causes low injectivity decline and makes it feasible to apply in offshore waterflood projects. Planning and design of commingled PWRI with low sulphate seawater injection includes choice of the ratio "produced water:seawater", determination of maximum allowed sulphate concentration in seawater after treatment, and injectivity decline prediction. The reliable design and plan is based on mathematical modelling with well-known model coefficients. In the current paper we derive an analytical model for the co-injection of low sulphate seawater with barium-containing produced water. Reagent and deposit concentrations along with the injectivity index are expressed by explicit formulae. It was found out that bulk sulphate deposition occurs in the 10-20 well radius neighbourhood. The injectivity decline is one order of magnitude higher than the productivity decline under similar reservoir conditions. Calculations for sulphate reduction plant have been performed for six fields of Campos Basin (Brazil). © 2009 Elsevier B.V. All rights reserved.