Drilling fluid, commonly known as “mud”, plays a vital role in the drilling process.
In fact, drilling mud has a number of key functions, including:
- Remove cuttings from the wellbore and allow them to separate at the surface.
- Control formation pressure and keep gas, oil and water from entering the wellbore to prevent a possible blowout.
- Coat the wellbore walls with a thin layer of filter cake to stop the wellbore from swelling or caving in.
- Lubricate and cool the drill bit as it rotates. Without drilling mud, the bit would literally burn up in a matter of minutes!
- Cuttings removed from the wellbore tell a story about the formations being drilled.
To accomplish all these functions on a vast range of well types, drilling mud is made up of a minimum of three or four products. For deeper or more challenging wells, as many as 10 to 15 products may be needed.
The specific recipe for each well is typically considered proprietary, but all drilling mud starts with a base fluid. In western Canada, fresh water is usually the base fluid, making up between 80% and 95% of the total fluid. In some areas of the world, sea water is used as a base. Other base fluids include low viscosity oils, synthetic oils, air and foam.
“Weighting agents” – heavy rock ground to dust sized particles – are added to the base to increase the fluid’s density. Also added into the mix are “viscosifiers” which thicken the fluid to support the weighting agents. The most common viscosifier is bentonite, a clay like substance but other products known as bio-polymers (used in salad dressings, ketchup and ice cream!) are also used to provide good suspension.
One of the best things about drilling fluid is that it is reused. A pump sends the mud down the wellbore and brings it back up again, along with cuttings from the formation. The cuttings are separated from the mud using a shale shaker. The cuttings then go into the reserve pit, while the clean drilling mud returns to the mud pits and is pumped back down the hole.
Great care is taken to handle drilling mud appropriately. Pipe wipers, mud buckets and vacuums recover excess mud for reuse. Centrifuges clean additional solids from used mud, reducing the amount of mud to be managed when the drilling process is complete. And technologies such as directional drilling and slimhole drilling reduce the overall amount of mud used.
After a well is drilled, drilling fluids are processed for safe disposal or recycling. Some muds are even approved to be spread on land to help regenerate poor quality soils.