What Does Biodegradable Really Mean?
By Colin Rice · Colin Rice Exploration and Training (Pty) Ltd
A well designed drilling fluid is an indispensable part of any drilling operation however, circulating mixtures of chemicals through a borehole introduces a series of risks some of which are more difficult to manage than others.
This is the first article of Part 1 of our Technical Series on Drilling Fluids. Click here for an outline of the entire Technical Series on Drilling Fluids.
The risk of environmental damage is one of the more difficult risks to define and manage and so frequently, I see mining companies specify that all drilling fluid additives used on their projects must be “biodegradable” – the thought being that if the chemicals used are biodegradable then environmental risk is eliminated or at least reduced.
Many drilling fluid suppliers claim that many of their products are biodegradable but I have doubts about the validity of many of these claims. I believe therefore that mining companies should be extremely circumspect about accepting claims made by some suppliers. In this article, we examine what the term biodegradable means but we will also have a look at a far more important aspect of drilling fluid usage – the “toxicity” of drilling fluid additives.
When I started my career in the drilling industry as a young mud technician many years ago, three polymers were used that were considered biodegradable:
Guar gum: this is a naturally occurring polymer manufactured from the guar bean (India produces the major portion of the world’s guar gum). Guar gum is used as a thickener in many industries particularly the food industry - you probably eat a bit of it every day if you like tomato sauce or commercially produced salad dressings. Back in those early days, Johnson’s the well-screen manufacturer sold guar gum under the trade name “Revert” as a viscosifier for waterwell drilling. Its biodegradability in this application was a great advantage when developing the well.
Starch (as I recall we used to use potato starch): this is also naturally occurring, when placed in water the starch hydrates and the individual grains begin to swell. This made it useful in reducing filtrate loss in permeable formations.
Xanthan Gum: this is a fascinating natural polymer – it is produced by feeding sugar, glucose, or lactose, to a particular type of bacteria, the fermentation residue is then separated, dried and ground into a fine powder. Like guar gum, xanthan gum is widely used in foods, cosmetics and many other everyday products.
These polymers are naturally occurring and are clearly biodegradable – anyone who has ever used them will know how bad the sumps started to smell after a few days in the sun! In fact, we used to extend the life of these polymers by using a biocide to kill the bacteria that attacked them. All three of these chemicals are extremely expensive and, to my knowledge, are very seldom used in the mineral exploration industry today.
How valid are the claims by drilling fluid suppliers that their products are biodegradable?
A biodegradable product is “one that is capable of being decomposed naturally through the action of bacteria or other naturally occurring living organisms”. In other words, the product will disappear over a period of time.
This notion raises two important considerations: the time that it takes for the product to degrade and the products that are produced by the degradation process.
Different chemicals degrade at different rates - one would expect ambient temperature to affect the rate of degradation and the quality of the water that is used to mix the drilling fluid could also influence the rate of degradation for example, but we would expect that for a product to be called biodegradable, it should degrade in a reasonable period of time. One can argue, nonsensically, that plastic is degradable – only problem is that it takes hundreds of years for it to degrade.
A number of test methods have been developed to evaluate biodegradability and are modified to suit different types of chemical or product and also to suit a variety of biodegradation processes. Generally these test methods are based on a 28-day test period.
Two commonly used biodegradability tests are the OECD 301B and the OECD 302B standards. (OECD = Organisation for Economic Co-operation and Development).
The OECD 301B method is used to evaluate fuels, oils and surfactants in a liquid environment for example, and determines the production of carbon dioxide over a minimum of 28 days. The OECD 302B method measures the production of Dissolved Organic Carbon (DOC) or Chemical Oxygen Demand (COD) also over a 28 day period, however testing can be extended for up to 60 days.
Any claim about biodegradability made by a supplier should therefore refer to the test method that was used to evaluate biodegradability and this information should be included in the Material Safety Data Sheet (MSDS). If the documentation supplied does not reference the test method then it is probably safer to assume that the product is not biodegradable.
The question of drilling fluid biodegradability becomes quite complicated because many drilling fluids additives are in fact mixtures of two or more chemical components or compounds and each component or compound should be separately tested before any claim about biodegradability can be made.
It is clear that any claim that is made, of whatever nature, must be defendable and must be able to withstand rigorous interrogation – otherwise we should not make the statement. I therefore recommend that mining companies avoid the use of the term “biodegradable” altogether in their tender specifications and instead they should use the term “non-damaging”.
Essentially this means that the drilling fluid, in its constituent forms and in its mixed form must be non-toxic and must not have any long term impact on the environment.
This now raises the question of the toxicity of the additives. In the same way that products can be tested for biodegradability there are a number of standard tests for toxicity, the results of which will quote either a LC50 or a LD50 value.
The LD50 value is the amount of the material, given in one dose, that causes the death of 50% of the animals used in the test while the LC50 value is the concentration of the material in solution that will cause the death of 50% of the animals used in the test that were exposed to the solution.
Toxicity testing is costly and takes some time to conduct. Because toxicity testing involves animals, these tests have to meet very strict ethical protocols to ensure that test animals are not subjected to undue pain and suffering.
In the same way that claims about biodegradability must be supported by a test result, so too must claims about the toxicity of drilling fluid additives.
So, the use of drilling fluids is a bit more complicated than maybe you at first thought and raises an important question - how can a mining company manage the risks associated with using drilling fluid? We will answer this very important question in Part 3 of this Technical Series.