Membrane Filtration of Seawater for Oil Reservoir Injection

Abstract

Abstract In the recovery of hydrocarbons from offshore oil reservoirs seawater injection is frequently applied. Unfortunately, the high concentration of sulphate in the injection water may cause problems such as scaling and souring of oil reservoirs. Membrane filtration using nanomembrane can be applied to reduce the sulphate concentration. Nanomembranes have a high rejection of divalente anions and organic molecules with molecular weight above 200. In the present study the separation performance of a NF70-2570 nanomembrane was evaluated. The removal efficiencies obtained for sulphate. magnesium and calcium were greater than 93 %,47 % and 37 % respectively. The rejection of monovalent anions such as chloride and bicarbonate were in the order of 10–30 % and 30–60 % respectively. The water flux through the membrane decreased from 36.8 1/m2h to 20 1/m2h at T=26°C after 64 hours of filtration. The required membrane area for the production of 500 m3/day will be in the order of 1000 – 1400 m2. Introduction The presence of hydrogen sulphide (H2S) in produced liquids and gases has become an increasing problem for many North Sea operators. In most cases hydrocarbon reservoirs were initially designated as "Sweet", i.e. containing no hydrogen sulphide. Consequently the production systems were built without considering the corrosive effects of hydrogen sulphide. The generation of hydrogen sulphide is in many cases thought to be a result of the activity of sulphate reducing bacteria (SRB) (Cord-Ruwisch et al, 1987). SRB is a group of specialized microorganisms that live in anoxic aqueous environments. The main nutrients for SRB are simple organic acids and molecular hydrogen. The nutrients are oxidized through reduction of sulphate to sulphide (hydrogen sulphide). Seawater has a high concentration of sulphate and therefore provide favourable conditions for the growth of SRB when injected into oil reservoirs. In addition to souring, the high concentration of sulphate may also cause problems due to the formation of scales such as BaS04 and SrS04. The injection water or the producing wells are usually treated with chemicals to reduce the potential of souring and scaling. The application of chemicals in water flooding projects is not only expensive but also the environmental concerns are of increasing importance. In addition, the actual effects of biocide treatment in porous media have been questioned (Bakke et al, 1989). An alternative way of controlling the activity of SRB is to control the biological factors that favours SRB. One of these factors is the sulphate concentration in the injection water. A reduction of the sulphate concentration will reduce the problems due to scaling and it will also limit the activity of sulphate reducing bacteria and consequently the potential of souring (Bakke et al, 1992). Membrane filtration may be applied as a means of reducing the sulphate concentration in seawater.