Practical Considerations in Ensuring Cost Minimization in the Design and Operation of Sulfate-Removal Systems

Abstract

The removal of sulfate from injected seawater for waterflood and pressure maintenance operations has become widely accepted for reservoirs with sulfate scaling potential. The technology has been found to be exceptionally useful in deep water and subsea operations where scale controls using scale inhibition squeeze treatments become prohibitively costly. Twenty five sulfate removal facilities are now operating or being built off the cost of West Africa, Brazil, and in the North Sea. Experience with these existing facilities has identified methods to minimize the size of the sulfate removal facility with accompanying weight and space savings which often exceed the capital cost of sulfate removal system itself. Similar savings and trends have been identified by the availability and successs of auxiliary pre-filtration equipment. Operation cost reduction and improved reliability have been achieved by selecting raw seawater intake reduction and improved reliability have been achieved by selecting raw seawater intake locations, the re-design of sulfate removal system. The careful selection and evaluation of sulfate removal system components, including fine filtration and deaerators, have resulted in significant savings by extending membrane life up to twice the warranty period and by reducing operation costs of 25% than originally projected. Introduction The key component of a sulphate removal membrane process is sulfate removal membrane module or element, the component of the process which performs the removal of sulphate from the incoming seawater. The membrane process, within the membrane module, is depicted in Figure 1 where sulphate ions are selectively repelled off a membrane surface while allowing most of the remaining components of seawater to pass through the membrane. This membrane separation is achieved by placing the sulphate removal membrane in a "spiral wound configuration" which provides 400 + ft2 of membrane area to be placed in a module or cylinder 8" in diameter and 40" in length. As shown in Figure 2, the seawater passes from one end of the membrane element to the other end through "feed channel spacers". As the seawater passes over the membrane, the sulphate ions are rejected or repelled off the surface and discarded. The remaining seawater, containing predominantly sodium chloride, necessary to maintain stable reservoir clays, passes through the membrane itself and is transported to a product water tube by the permeate or product water channel spacer. The sulphate free seawater is then collected and used for injection into the reservoir.