Microplastic pollution in the oil and gas sectorBy Dr Henry Craddock, HC Oilfield and Chemical Consulting - 05/06/2018
The issue of plastic pollution, particularly in the marine environment, has received widespread public attention. The oil and gas sector releases microplastics into the environment through the degradation of plastics and polymers used in a range of applications.
The main plastic and polymer types used in the oil and gas sector fall into two categories:
- Engineering plastics which are highly durable – a considerable advantage over commodity type disposable plastics – but not degradable
- Various polymers used as chemical additives across a wide range of applications, from drilling to enhanced oil recovery.
A main route of degradation of plastics and polymers is photolysis or photo-oxidation in the presence of oxygen or ozone (although photolysis can occur in the absence of oxygen). Photo-oxidation is a chemical change that reduces the polymer's molecular weight. The effect is facilitated by radiant energy such as UV or artificial light. This process is the most significant factor in the weathering of polymeric materials such as plastics, and is of particular concern to policy makers and regulators in the offshore oil and gas industry as it aids in the generation of microplastics, which are known to be responsible in the marine environment for the concentration of persistent organic pollutants (POPS).
Polymeric chemical additives are not usually exposed to UV light, as they are mainly applied ‘downhole’ – into the reservoir, in the wellbore, subsea flowlines and transport pipelines. However, at points of discharge to the environment they may encounter light, and photolysis along with other degradation process may occur. In offshore operations, photolysis can occur to a depth of several metres, provided the water is reasonably clear.1 This is particularly so in shallow basins such as the Gulf of Mexico and the Arabian Gulf, where photolysis can be extensive and complex, involving surface layers, the water column and sediments.
A difficulty in studying microplastic pollution is that, until now, there has been no clear and accepted definition of a microplastic, although work has begun to examine this in light of the need for regulatory control.2 Such definition provides legal certainty, and enables the consistent monitoring of trends in microplastic pollution and evaluation of the effects of policy measures. There appears to be a growing consensus that microplastics have a size of <5mm. In addition to size, which could be as small as 5um, physio-chemical properties need definition and the description of a microplastic requires refinement, such as they are derived from synthetic polymers, are insoluble in water and not degradable. These are important features when considering measures to reduce microplastic emission. In June 2017 the European Commission published a working definition, and referenced this in another report on Intentionally added microplastics in products, published in October 2017.3
The so-called oxo-plastics, or oxo-degradable plastics, are conventional plastics which include additives to accelerate fragmentation into very small pieces, triggered by UV radiation or heat exposure. The intention is to allow conversion of the fragments into small-chain organic chemicals such as ketones, alcohols, carboxylic acids and low molecular mass hydrocarbon waxes. These compounds are biodegradable by bacteria which are ubiquitous in terrestrial and marine environments. However, there is concern that these fragments will degrade into plastic particles, and finally microplastics, with potentially similar properties to microplastics originating from degradation of conventional plastics.
The timescale for complete biodegradation is much shorter for oxo-plastics than for conventional plastics which, in normal environments, are very slow to biodegrade and cause persistent pollution. For this reason, some countries in the Middle East4 have banned plastics unless they are upgraded with oxo-biodegradable technology.
As can be seen, there is an obvious dichotomy of policy direction regarding the use of oxo-plastics. This is particularly important not only for industrial use in the oil and gas sector, but for societal needs generally, as we strive to introduce the best possible environmental protection whilst simultaneously maintaining modern 21st century living standards.
Meanwhile, in the oil and gas sector, the need for measured and considered assessment of the use of plastics and polymers needs to be weighed against the operational impact of a wealth generating industrial sector. There appears to be a somewhat reactionary move towards policy implementation despite the work of scientific advisors being far from complete.