Abstract

Carbonate or sulphate scaling is usually mitigated by using chemical inhibitors that alter the growth mode of the scale mineral, but with halite scaling the apparent absence of a suitable inhibitor has meant that periodic removal with low-salinity water has been favoured. We report laboratory testing and the first field application of a chemical inhibitor for halite scaling. The inhibitor was field-tested in the Leman gas field (North Sea), where halite scaling occurs in gas-compression equipment due to evaporation of water from brine droplets carried over from the gas-water separator. The scale reduces the efficiency of both compressor and cooler, necessitating regular costly cleaning of the internals. After a recent system clean out, the cheap, non-toxic halite inhibitor was dosed continuously for 3 months into the production stream downstream of the gas-water separator. Whilst halite deposition was not completely suppressed, an increase in compressor efficiency was observed for all compression stages compared with recent non-inhibited data. A significant improvement in gas-cooler efficiency and a reduced tendency to block gas-flow meters, were also observed. This indicates that the inhibitor has considerable potential for cost-effective control of halite scaling, possibly as an alternative to periodic removal or in combination with it.

Introduction

Scale deposition is a widespread problem that causes production deferment, necessitates costly intervention, and can also compromise safety systems. It can be particularly severe when highly saline (typically>200,000 mg/litre) or even salt-saturated formation water is involved, because of the potential to generate large quantities of halite scale (rock salt, NaCl), normally by evaporation of water into the gas phase. Although halite deposits tend to be easier to remove than most other scales - most commonly by periodic washing with low-salinity water -, the rate of build-up can be orders of magnitude greater,1 and thus very frequent intervention may be required to limit hydrocarbon deferment and prevent equipment malfunction.2

Unlike carbonate and sulphate scales, which can be controlled by the use of threshold inhibitors that influence crystal nucleation and/or growth, preventing the build-up of halite scale has only been achieved by continuous dilution of the fluid stream with water upstream of where deposition occurs. Whilst successful, this can require significant amounts of low-salinity water to be available, which preferably is chemically compatible with the produced water and has been deoxygenated to limit corrosion. Sea water is often unsuitable without addition of large amounts of inhibitor chemicals against sulphate scale, because high-salinity formation waters frequently contain extreme levels of barium, strontium and calcium ions, and are hence highly incompatible with the high concentration of sulphate in sea water.3 Moreover, the introduction of even small amounts of additional liquid can be highly detrimental to the performance of some equipment, such as gas compressors.

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