Introduction

The injection of waters, both aquifer and seawater, is extensively used to support and maintain the production of oil and gas.

The main physical treatments of injection water are:

Filtration

De-oxygenation

Filtration is performed to remove particulates, macro-invertebrates and organic matter down to less than 5 microns in size. De-oxygenation of the water is required to reduce corrosion and aerobic bacterial growth. Primary oxygen removal is normally either by use of a vacuum tower or a gas stripping tower. These reduce the level of oxygen in the water from around 9ppm to less than 50ppb. Further oxygen removal is achieved by the addition of an oxygen scavenger downstream of the deaeration vessel.

Chemical Treatments

The main areas for the application of chemical treatments associated with water injection systems including the following:
 

Injectivity problems

Bacterial growth and proliferation

Filtration

Produced water re-injection

Oxygen removal

Erosion and drag reduction

Coagulants and Filter Aids - these chemicals are applied to improve the efficiency of the filtration system and aid removal of organic components, which can contribute to foaming of injection water. Coagulants and filter aids typically work at very low dose rates.

Oxygen Removal / Corrosion Control - achieved by deaeration towers and by the application of oxygen scavengers. It is vital that low oxygen concentrations are achieved to protect the topsides equipment, injection wells and subsea flowlines from oxidative corrosion.

Bacterial Control – achieved by chlorination treatment (electro-chlorination or hypochlorite addition) upstream of deaeration and batch biocide addition downstream of deaeration. Depending on which biocide is used, the biocide may interfere with the oxygen scavenger and therefore scavenger should be turned off during batch treatments.

Antifoam – generally applied upstream of deaeration if required. Roemex have designed non-foaming biocides for water injection treatments so as to prevent the requirement for antifoam addition. However if foaming is not dependent on biocidal constituents Roemex Antifoam RX-5505 is a chemical of choice being active at low concentrations and being environmentally friendly.