It has long been established that microorganisms can and do affect the properties of drilling fluids. This topic will be addressed in the following four sections:

  1. The Nature Of Micro-organisms And The Contamination Of Drilling Fluid.

  2. Microbial Mechanisms Which Affect Drilling Fluids.

  3. Implications of Microbial Contamination.

  4. Identification And Control Of Microbial Problems.


There is probably no natural water environment, no matter how harsh, which does not contain some form of micro-organism. There are literally hundreds of thousands of species of micro-organisms (1) and new species are discovered at the rate of more than 1,000 per year (2). Natural populations can number from a few hundred organisms per litre of fluid to well in excess of a billion per litre. Microorganisms are a formidable force and are collectively capable of destroying nearly every organic in existence. The role of micro-organisms is essentially the reduction of complex matter to a more simple form, and the return of this energy to the environment as the building blocks of life.


Drilling fluids are constantly exposed to large numbers and types of micro-organisms. Although drilling fluids and their additives were once thought to have low susceptibility to microbial attack, we know now that most drilling fluids, by their very nature and complex organic structure are ideal environments for a variety of micro-organisms.

Natural gums, carboxymethylcellulose, liqnosulphonates, tannins, lignins and many other compounds which are added to muds are all found to be susceptible to biodegradation (5). Even the synthetic polymers such as polvacrylamides are not immune to attack (6).

There are many vectors for drilling fluid continuation. the source water used to prepare the mud, wind blown dust and dirt, rain, human contact, and possibly even some of the materials which are used to prepare the mud. Once the contamination occurs, a number of other factors determine just how serious the problem will be.

  1. The temperature of the re-circulating mud.

  2. The composition of the water used to make up new mud.

  3. The chemical nature of the mud system itself.

  4. The length of time that is required to drill the hole.

  5. The type of micro-organisms which become established.

Given sufficient time, a population will eventually be established which can have a serious affect on the properties of the drilling fluid.

There are a number of methods by which biological activity san alter a drilling fluid. One mechanism of indirect attack is the excretion of metabolic byproducts. The liberation, for example, of organic acids in a drilling mud may result in a lowering of the pH and subsequent loss of fluid character. The production of H2S by sulphate reducing bacteria may affect some drilling fluids, in addition to potential increased corrosion of drilling equipment.

A second mechanism might be microbial attack of a mud constituent without actual alteration of the chemical properties of that constituent.

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