Sea Water Desulphation – Optimisation of Scale Treatment and Reduction of OPEX & CAPEX

Abstract

One barrel of seawater has to be injected into the reservoir in order to be able to produce the same amount of oil. In order to avoid problems of souring and/or incompatibility with reservoir water, desulphated seawater is injected. In this paper, we have completely reconsidered the desulphation process with the objective of producing more water while optimising the quantity and type of scale inhibitors, with a priority given to green chemicals. The desulphation process established since 1992 has not been modified since. Our new philosophy adapts to the constraints of the field life. Theoretical and simulations studies have been carried out on risks of scale deposition on the membrane (polarisation layer) taking into account parameters and physical laws based on fluid mechanics, electroneutrality and material transfer. The behavior of membrane and risk of deposition depends on temperature, pressure, flux, tangential flow, potential accumulation of scale on the membrane and spacer; site and laboratory pilots have been used. Tube blocking tests have been carried out in order to select the scale inhibitors. Results showed that it is possible to operate beyond 80% of recovery with the help of a scale inhibitor. 83% recovery appears to be a maximum limit. Without the use of a scale inhibitor it is possible to obtain 75% of recovery under certain circumstances. These results have enabled us to issue operational recommendations on TOTAL operating fields, on projects under development and on our future projects. The increase of the recovery and / or decrease in the concentration of scale inhibitor will conduct to less chemical discharged to the sea (via the concentrate). In addition, the selection of scale inhibitors has allowed selection of several biodegradable products, with better efficiency than currently used products. Of the 11 products tested, four have been selected. Reduction of OPEX: The consumption of scale inhibitor could be reduced by 2 or 3 and even suppressed for certain operating conditions. Selection tests have allowed us to choose most appropriate chemical from a technical, economic and environmental friendly point of view. Reduction of CAPEX: The increase of recovery has allowed us to reduce the dimensioning of the whole pretreatment of the nanofiltration as the flowrate is the parameter that influences the most the cost of a water treatment plant.