Let’s take a look at some of the various purposes for the use of drilling mud:
As mentioned in the “Primary Well Control” lesson, drilling mud is used for pressure containment while drilling to ensure hydrocarbons do not freely flow through the wellbore. Oftentimes drilling mud occupies the well to cap hydrocarbons in their existing reservoir formation prior to production. However, it is important to note that reservoir and wellbore pressure must be accurately accounted for to ensure the correct usage and amount of drilling mud.
When using drill bits, the friction during rotation results in excess heat, and the increased temperature decreases the effectiveness of the drill bit. The circulation of drilling mud, reconditioned and cooled at the surface, can counteract the degradation of drill bit effectiveness.
The abrasion and mechanical rock breaking processes of the drill bit at the bottom of the wellbore create small pieces of rock known as “cuttings.” Circulation of the drilling mud from the drill pipe and jetted out of the bit helps move the cuttings away from the bit’s cutting surfaces to be transported to the surface via the annulus. At the surface the larger cuttings are screened out of the mud with the shale shaker, while finer grained rock particles and sand are removed with various other filtering processes before the mud is returned downhole for additional work.
Hydraulic Power Transmission
You may already be familiar with the idea that the hydraulic force of flowing water, released from behind a dam, can turn a turbine that spins the shaft in a generator to create electricity. In drilling, there is what is called a mud motor, which is a small turbine-like rotor inside the drill pipe that rotates due to the flowing action of the drilling mud. This rotor is connected to a shaft that rotates the drill bit to create the necessary cutting action for drilling.
When drilling under overbalanced conditions, the drilling mud will flow from the wellbore into the formation. The solids, predominantly the clay, will filter off onto the rock face of the wellbore wall, preventing drilling fluids from invading the formation, which can cause unwanted chemical reactions, particularly with water-based fluids in shale formations. Also, the hydrostatic pressure of the fluid pushing on the wellbore wall, when short of exceeding the fracturing pressure, provides mechanical support to the rock to prevent it from failing and falling into the wellbore.