Chemical Foamers for Gas Well Deliquification
a report by
Martin J Willis
Industrial Technical Consultant, Gas Treatment, Nalco Company
In a world with an ever-increasing energy demand, the ability to deliver continuous production and enhance recovery is paramount to energy suppliers. When considering the production of natural gas there is one certainty: at some stage during the well’s production life-cycle it will suffer from liquid loading. Consider North America. It is reported that of the ~400,000 gas wells, 90% are suffering a degree of liquid impairment. The impact this has on production is significant and when considered in its entirety equates to a vast value of gas.
Liquid Loading, Management and Prevention
The term liquid loading relates to liquid accumulation in the wellbore, casing and tubing of a producing well. This phenomenon occurs in mature gas wells when production velocity decreases, resulting in a change of flow regime and, ultimately, a fall below the critical velocity, where there is insufficient energy to carry liquid to surface. Critical velocity is defined as the velocity at which a liquid droplet is suspended in vertical flow because the flow drag equals the force of gravity. Liquids will initially load the wellbore and casing, then the tubing, until eventually the hydrostatic head pressure is too great for the reservoir and flow ceases completely. To prevent and manage liquid loading artificial lift is required, either mechanical or chemical. Many forms of mechanical artificial lift are utilised in the industry, but there are scenarios where the application of chemicals is more favourable. This is especially the case for mature assets reaching their predicted lifetime, where any sort of capital expense is unfavourable.
Foamers
Historically, chemical foamers have been applied in the form of soap sticks, which are solid-state products that are dropped downhole. There are pros and cons to using such technology: cost and handling are advantages, but treatment volume, placement control and optimisation are problematic, especially if the well is deviated. Opinion on whether these soap sticks are effective is a point of great discussion, but the principle of introducing a foaming agent is widely accepted as an efficient means by which to support deliquification. Foams form when gas is dispersed in a liquid that has been modified with a chemical to have elasticity. Through surfactant application the surface tension of the liquid is reduced, resulting in a larger interfacial area. In the dynamic environment this generates foam that can be designed to remain stable and provide a way to remove liquid. Surfactants work through the gas, migrating to the interface where their hydrophilic and hydrophobic portions change the surface characteristics. Between each bubble that is formed is the lamella, within which water and condensate is held (see Figure 1).
The new age of liquid foamers brings novel technology to the market, offering significant benefits to operators. Liquid foamers can easily be customised to function in specific asset environments and offer great application versatility. Conventional surfactants used in these types of
© TOUCH BRIEFINGS 2010
Non-ionic foamers are typically polyoxyethylated compounds of phenols or alcohols, effective in a limited range of conditions. As temperature increases, solubility is reduced and they become sensitive to moderate and high levels of salinity. To enhance the performance of these products the actives are made more polar, often by sulphation or sulphonation, forming anionics. Anionic foamers are excellent water foamers due to their enhanced polarity; however, there are some application limitations. These foamers can be affected by high brine solutions and at elevated temperatures (130ºC+) can degrade to produce sulphuric acid as a by-product, which is an obvious corrosion concern. Cationic surfactants are common in the oilfield, predominantly as corrosion inhibitor actives. These actives have a good tendency to foam, especially condensate/brine mixtures, but unfortunately they are prone to forming emulsions. For this reason and the impact it has on processing, they are not the first choice in the industry. Amphoterics are the preferred actives. Due to their dual charge they are highly effective in acidic, neutral and basic conditions. They are stable at high temperatures (>175ºC) and tolerant of high salt content (10+wt%).
application fall into four main categories: non-ionic, anionic, cationic and amphoterics.1
These conventional products have been the market staples for many years, but due to committed research and development (R&D) in surfactant science, novel technology is now available that is able to address some of the application issues that have restricted treatment in the past. For example, a novel imidazoline-based patented chemistry2 has been designed to address condensate loading. Condensate acts as a defoamer for most conventional actives, suppressing performance. There are a vast number of loaded wells that are condensate-dominant, however, so an effective foamer has high potential. After extensive testing in the laboratory, the chemical has been successfully applied in the field, unloading wells at levels of >90% condensate.
The Five-step Lift Process for Foamer Selection
With a range of products available, application in the field requires a robust recommendation process. In the past foamer selection was considered more an art than a science – simply adding soap – but to ensure that a scientifically robust approach is used, Nalco utilises the five-step lift process: analyse, model, rank, recommend and execute.
Martin J Willis is an Industrial Technical Consultant for Gas Treatment with Nalco Company with responsibilities for corrosion mitigation, production optimisation and gas processing. He joined Nalco in 2006 in the Asset Integrity Technology group as a Senior Chemist before moving to his current role in late 2007. He is a member of the Royal Society of Chemistry, the Society of Petroleum Engineers (SPE) and the National Association of Corrosion Engineers (NACE), and has over 15 publications and patents. He has a master’s in chemistry from the University of Leeds.
E:
mjwillis@nalco.com
103
Natural Gas
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