Drilling Fluid Functions and Characteristics

                  

The rotary method of drilling is distinguished by two factors;

           (1) The bit is rotated or hammered against the bottom of the hole,

(2) A fluid is circulated to the drill string through the bit and remove the drilled cuttings through the annulus.

The most common, fastest, and effective type of drilling in unconsolidated permeable formations is the mud rotary system. In consolidated rock formations, down hole hammer drilling using air, mist, or foam is the most advantageous and effective type of drilling. The properties of a drilling fluid useful in performance of the uninterrupted drilling operations. Most commonly the drilling fluid consists of bentonite, polymers, or chemicals added to make up water as treatments or drilling fluid enhancement materials are essential for efficient, productive, and safe drilling.

Functions of a Drilling Fluid:

●       Cool and lubricate the bit.

●       To Control pressure in the bottom hole.

●       To transport the cuttings from the subsurface to the surface.

●       It supports the borehole wall.

●       It cleans the bottom of the hole.

●       It is used to stabilize the borehole.

To achieve these functions, the following side effects must be minimize:

●       Reduce the Rate of Penetration in order not to damage the productive formation.

●       Swab and circulation pressure problems.

●       Loss of circulation.

●       Erosion of the borehole.

●       Sticking of the drill pipe against the walls of the hole.

●       Reduce the Wear of the Mud Pumps.

Characteristics of a Drilling Fluid

●       Viscosity

●       Velocity

●       Density

●       Gel Strength

●       Filtrate Control

●       Lubricity

Relationships between Characteristics and Functions

A. Cool and Lubricate the Bit and Drill String

                    As the drill bit and drill string rotate in the borehole, they become hot due to friction. A liquid fluid will transfer heat from the bit and drill string in order to cool it. A drilling fluid that has slickness or lubricity will lubricate the drill bit and drill pipe, reducing heat liberated from the bit and drill string.

B. Clean Beneath the Drill Bit

                   During circulation, the drilling fluid is ejected through jets of the drill bit at high velocity. This hydraulic force strikes the bottom of the hole, blowing cuttings from beneath the bit and starting them to lift up through the annulus where the drilling fluid can then transport them to the surface. If the bottom of the hole is not cleaned, the drill bit regrinds the cuttings, therefore reducing the rate of penetration.

C. Transport, Suspend and Drop Off Cuttings

                   Once the cuttings are cleaned from the bottom of the hole, it is the drilling fluid's job to transport them to the surface. To accomplish this, the drilling fluid must have a enough viscosity to transport the cuttings. In transporting cuttings, increased viscosity can accomplish this task at reduced velocity. At the same time, decreased viscosity will require an increase in velocity. When drilling stops, for whatever reason, the drilling fluid must be able to suspend the cuttings, not allowing them to fall back down the annulus where they can collect around the drill bit. Once the cuttings reach the surface, they must be dropped off and not recalculate. If the cuttings are not dropped off, they will cause wear on the pump and increase pressure on the formation.

D. Support the Walls of the Borehole

                   Lateral support of hole walls is removed as the bit drills a subsurface formation. Unless the drilling fluid prior to the casing being set replaces the support, the formation will collapse into the borehole. It depends on the characteristics of the drilling fluid and the nature of the formation being drilled, prevents this from occurring. If the formation is very firm and consolidated (granite), little support by the drilling fluid is required. If the formation is relatively firm and fairly consolidated (shale), sufficient support may be obtained solely from the density of the drilling fluid or drilling mud. If the formation is an unconsolidated permeable formation (sand), the drilling fluid must have sufficient density, and it must have the ability to form a thin deposition of cake on the walls of the borehole.

E. Control Pressure and Stabilize the Borehole

                  Two types of pressure are exerted on the borehole during drilling, formation pressure and hydrostatic pressure. Formation pressure can collapse the borehole if it is not overcome by hydrostatic pressure pushing back against the formation. Hydrostatic pressure is the density of the volume of drilling fluid pushing against the formation. In order to have hydrostatic pressure, the drilling fluid must push back against the formation with minimum penetration into the formation. In unconsolidated permeable formations, the hydrostatic pressure occurs when the weight of the fluid is in contact with the filter cake placed on the sides of the borehole by the drilling fluid. The filter cake and the hydrostatic pressure thereby control the formation pressure, reduce fluid loss and prevent caving, results in hole stabilization.

REFERENCES:

# John H. Berry (Hydrogeologist/Geologist)

by: Sameer Ahamed

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