The Defining Series: Drilling Fluid Basics

In nonaqueous drilling fluids, commonly referred to as synthetic-base muds, the continuous phase may consist of mineral oils, biodegradable esters, olefins or other variants. Although typically more costly than aqueous drilling fluids, these systems tend to provide excellent borehole control, thermal stability, lubricity and penetration rates, which may help reduce overall cost for the operator.

In fractured rock or environments where the borehole will not support a column of water without significant fluid loss to the formation, drillers use air, mist or foam systems to help remove cuttings from the hole and maintain wellbore integrity.

Basic Functions

Drilling fluids are formulated to carry out a wide range of functions. Although the list is long and varied, key performance characteristics are the following:

Controlling formation pressures—Drilling fluid is vital for maintaining control of a well. The mud is pumped down the drillstring, through the bit, and back up the annulus. In open hole, hydrostatic pressure exerted by the mud column is used to offset increases in formation pressure that would otherwise force formation fluids into the borehole, possibly causing loss of well control. However, the pressure exerted by the drilling fluid must not exceed the fracture pressure of the rock itself; otherwise mud will escape into the formation—a condition known as lost circulation.

Removing cuttings from the borehole—Circulating drilling fluid carries cuttings—rock fragments created by the bit—to the surface. Maintaining the fluid's ability to transport these solid pieces up the hole—its carrying capacity—is key to drilling efficiently and minimizing the potential for stuck pipe. To accomplish this, drilling fluid specialists work with the driller to carefully balance mud rheology and flow rate to adjust carrying capacity while avoiding high equivalent circulating density (ECD)—the actual mud density plus the pressure drop in the annulus above a given point in the borehole. Unchecked, high ECD may lead to lost circulation.

Cooling and lubricating the bit—As the drilling fluid passes through and around the rotating drilling assembly, it helps cool and lubricate the bit. Thermal energy is transferred to the drilling fluid, which carries the heat to the surface. In extremely hot drilling environments, heat exchangers may be used at the surface to cool the mud.

Transmitting hydraulic energy to the bit and downhole tools—Drilling fluid is discharged through nozzles at the face of the bit. The hydraulic energy released against the formation loosens and lifts cuttings away from the formation. This energy also powers downhole motors and other hard-ware that steer the bit and obtain drilling or formation data in real time. Data gathered downhole are frequently transmitted to the surface using mud pulse telemetry, a method that relies on pressure pulses through the mud column to send data to the surface.

Maintaining wellbore stability—The basic components of wellbore stability include regulating density, minimizing hydraulic erosion and controlling clays. Density is maintained by slightly overbalancing the weight of the mud column against formation pore pressure. Engineers minimize hydraulic erosion by balancing hole geometry against cleaning requirements, fluid carrying capacity and annular flow velocity. The process of clay control is complex. Clays in some formations expand in the presence of water, while others disperse. To some degree, these effects can be controlled by modifying the properties of the drilling fluid. Regardless of the approach used, controlling the fluid's effect on the formation helps control the borehole and the integrity of the cuttings and leads to a cleaner, more easily maintained drilling fluid.