Introduction

The recovery of oil from wells is often helped by sea water flushing, especially on offshore works.

That sea water must satisfy various physical chemical characteristics to be compatible with the permeability of the source rock and with the composition of the produced water; moreover it must not corrode the steel piping network used for flushing.

This implies a multiple stage treatment line; in which, roughly speaking, the first stages provide the bio-physical purification (chlorination, filtration) and the following anti-corrosion treatment (degassing, conditioning).

In fact considering all the possible interferences between the treatment steps, it becomes necessary to set up a line of treatment not merely as a sequence of individual actions, but rather as an optimum combination avoiding any harmful interaction. Thus, the following list of treatments applicable offshore describes only a chronological sequence:

  • sea water

  • mechanical filtration or screening

  • fine filtration

  • anti acid conditioning.

  • degassing by gas stripping or vacuum and complete deoxygenation

  • anticorrosion and antifouling condtioning.

The severe restrictions related with offshore works, regarding space and operational conditions, have led these last years to look for a reduction in the size of the two bulkiest units in the treatment line, namely the filters and the degassers, without jeopardizing the operation.

For onshore works, space is generally not limited, but the coastal waters are often highly loaded in suspended solids, so that an intermediate stage of clarification may be necessary between the two filtration steps.

As usual in matters of water treatment, a perfect knowledge of the physical-chemical characteristics of the water is mandatory, hence the considerable development of pi1ot and test plants, which enable people to collect on site all the actual data related to the water, with the advantage when operating in the open sea of being free from all the accidental contamination and pollution changes so common along the shores and in the estuaries.

Characteristics of raw water to treat

As examples, let us take the characteristics of Mediterranean and north Sea: Table (Available in full paper)

Measurements generally show a large dispersion which does not appear on simplified charts as those given on Fig. 1,2,3. The counting of particulates is done with the Coulter Counter, which gives discontinuous measurements for particulates of 2 microns and even 1.25 microns. This device is used on pilot plants and for the commissioning of plants, while a continuous measure of turbidity in J.U. made automically and semi continuously is enough to control the operation of the filtering plants.

Sea Water Chlorination

Besides its bactericidal and algicidal action, chlorine distributed continuously assumes other important functions:

  • coagulation of the phvtoplancton, an essential condition for efficient filtration downstream

  • dislacement of the hypobromite in the sea water to bromine, enhancing its disinfection.

Chlorine doses required range from 1 to 3 mg/l, out of the estuaries where pollution is higher.

Chlorine can be generated as sodium hypochlorate by direct electrolysis of salt water containing at least 10 to 12 g/l of sodium chloride.

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