Reasons for contamination of drilling fluid in oil well drilling operations

While an oil well is being drilled, the drilling fluid is circulating from the active tanks to the bottom of the well, once it reaches the bottom of the well it fulfills a series of functions due to a series of chemical and physical properties that are monitored so that all these functions are performed in the best way, Once the drilling fluid reaches the bottom of the well, it returns through the annular space and rises to the surface to complete the circulation cycle and return to the active tanks. If we analyze the route taken by the drilling fluid, we can realize that the drilling fluid is exposed to be contaminated by external agents that may come from the reservoir or even be contaminated by unwanted chemical agents on the surface and that are induced by human error.

The negative influence that this scenario of drilling fluid contamination has on drilling operations is that the drilling fluid when contaminated causes the physical and chemical properties for which it was prepared to be lost and cannot fulfill the functions it has to make the drilling to be carried out in an optimal way. The most important properties that can change if the drilling fluid is contaminated are: density, viscosity and filtration.

All drilling fluids can become contaminated, however due to its susceptible characteristics it is the water based drilling fluid that tends to become contaminated faster and faster. The good thing when the mud is contaminated is that it can be treated to recover its initial physical and chemical characteristics under which it was prepared, but for this it is essential to know how to identify where the drilling fluid contamination comes from and with which material it is contaminated.

When the drilling fluid becomes contaminated with solids

The drilling fluid can be contaminated with solids from the lithological formation while drilling the hole, these solid contaminants alter its properties, one of the properties that is mostly altered is the density of the mud, as it is known that density is equal to the mass between the volume, then the greater amount of formation solids are incorporated into the fluid will increase its density, the solution for this contamination in water-based fluids, simply is to lower the density by adding more water.

It can also happen that by mistake the operators in charge of the control and supervision of the drilling fluid properties unintentionally add solid material such as barite or bentonite, which also increases the mud density and perhaps alters other physical and chemical properties.

Calcium ion contamination

Calcium ion is an important contaminant for freshwater-based sodium clay-treated mud systems. Calcium ion tends to replace sodium ions on the clay surface through base exchange, thus causing undesirable changes in mud properties such as rheology and filtration. It also renders diluents added to the mud system ineffective. The treatment depends on the source of the calcium ion. For example, sodium carbonate (soda ash) is used if the source is gypsum or anhydrite. Sodium bicarbonate is the preferred treatment if the calcium ion is from lime or cement.

This type of contaminants has its origin for example when it is being commented, as cement when it comes in contact with the drilling fluid contaminates fresh water based drilling fluids, the problem with this contamination is that the calcium ion replaces the sodium ions on the surface of the clay with which the water based drilling fluid has been prepared causing undesirable changes in some properties such as rheology and filtration of the drilling fluid.

It is important to highlight that properties such as rheology, which is altered by calcium ion contamination, are very important, since the rheological properties of the drilling fluid are the ones that allow an efficient cleaning of the well while drilling. In the event that the calcium ion contamination alters the filtrate, it can have negative consequences such as an excessive amount of solids passing into the formation and causing damage to the formation.

Conclusion

There are several contaminating agents, they can be solid, liquid or gaseous, the important thing is that any external agent that is added to the drilling fluid will work as a contaminant, since it will alter the original physical and chemical properties with which it was prepared and for which it must fulfill a specific function.

It is important to mention that being able to identify the type of external agent that contaminates the drilling fluid and knowing whether the contaminating external material comes from the geological formation or is added by mistake at the surface, can allow us to know what chemical agent can be added to the drilling fluid to correct the degree of contamination.

References

Nyland, T., Azar, J.J., Becker, T.E. et al. 1988. Additive Effectiveness and Contaminant Influence on Fluid-Loss Control in Water-Based Muds. SPE Drill Eng 3 (2): 195-203. SPE-14703-PA