Kinetic Hydrate Inhibitor Removal, Recovery and Reuse from Produced Water and Rich MEG Streams

One of the key challenges with transport of natural gas in long subsea multiphase flowlines is preventing gas hydrates (ice like plugs) formation in the flow lines. Traditionally this risk is mitigated by injecting large volumes of Mono Ethylene Glycol (MEG) into the flow lines. Sometimes MEG injection is needed continuously, meaning that it must be regenerated. MEG systems are large with numerous components including pipelines for transportation and distribution, storage tanks, and a regeneration system, which are expensive from a CAPEX and OPEX perspective. The high cost of MEG systems is forcing the industry to consider alternatives. One of the alternatives to MEG is the use of low dosage hydrate inhibitors (LDHI). LDHIs include Kinetic Hydrate Inhibitors (KHI) which delay/prevent the nucleation and growth of hydrates. The major advantage of KHI over MEG is that the volume required is much lower under similar conditions, which potentially reduces CAPEX and OPEX. However, KHIs are not regenerated and the most common KHIs are non-biodegradable. Produced water (PW) containing non-biodegradable KHI cannot be released to the environment and must be injected into disposal wells. The KHI polymer may precipitate and reduce injectivity by plugging the pores in an injection well. KHIs also cause process upsets due to polymer precipitation, resulting in increased OPEX and loss of valuable hydrocarbons. An efficient process for extraction and separation of KHI will address these issues. KHI removal can be achieved by means of extraction with an immiscible extraction fluid that is mixed into the rich MEG or produced water (aqueous phase). With correct mixing and selection of an appropriate extraction fluid, KHI will partition into the extraction fluid, which can then be separated from the aqueous phase. NOV Process & Flow Technologies has run a joint industry project (JIP) with two Oil&Gas operators to demonstrate how this process could be brought to an industrial scale. The project was partly funded by the Research Council of Norway. Through bench scale- and pilot scale tests, near 100% KHI removal has been demonstrated for some KHIs, provided the correct extraction fluid was used and it was mixed with the water phase in the right way and at the right temperature. Further, it was demonstrated on bench scale that KHI removed with this extraction method showed hydrate inhibition properties that was comparable to the original KHI formulation. Note that this was demonstrated on commercially available KHIs that are currently in use in the field. A concept study was done, whereby a commercial process with relevant equipment was costed and expected process efficiency was estimated. Even with conservative assumptions on process efficiency and losses during reuse, it was found that it was possible with 80% reduction in KHI consumption. The main negative finding was that the process would need large amounts of extraction fluid. Even if the extraction fluid is relatively harmless (PLONOR), the volumes required will be slightly larger than the original KHI consumption without a recovery process. Hence, the overall chemical consumption will not reduce with this process. However, there will be less need for non-biodegradable chemicals and the need for the much more costly KHI chemical can be reduced by 80%

The principal of KHI removal by solvent extraction and even reuse of KHI was successfully demonstrated. However, the problems with extraction fluid emulsions and the need for large quantities of extraction fluid, which would be lost upon reuse, means that this technology is not very promising for field application. NOV therefore currently have no concrete plans for further development of the technology. NOV do however remain open for further investigations targeted for specific fields that may be more suitable for this technology. For example, dry gas fields (no condensate) could be more suitable since the extraction fluid will not be lost upon reinjection. I.e., there is a real potential to save chemicals. Such fields, in combination with the use of KHI, are very few though.