1. Principle Functions
The principle functions of a drilling fluid are those that require continued
observation and intervention by the drilling fluid specialist. Usually every
day that a rig is drilling, a mud report form is issued. This document lists
the current properties as tested by the fluids specialist. These numbers and
vigilant observation of the drilling operations, allow the specialists to adjust
the properties to optimize its functionality.
In rotary drilling, the principal functions performed by the drilling fluid
are:
1. Prevent the inflow of fluids—oil, gas, or water—from permeable rocks
penetrated and minimize causing fractures in the wellbore. These functions are controlled by monitoring the fluid’s density (mud weight) and
the equivalent circulating density (ECD). The ECD is a combination of
the hydro-static pressure and the added pressure needed to pump the fluid
up the annulus of the well-bore.
2. Carry cuttings from beneath the bit, transport them up the annulus, and
permit their separation at the surface. The fluids specialist must manipulate
the viscosity profile to ensure good transport efficiency in the well-bore
annulus and to help increase the efficiency of solids control equipment.
3. Suspend solids, particularly high specific gravity weight materials.
The effective viscosity and gel strengths of the fluid are controlled to
minimize settling under either static or dynamic flow conditions.
4. Form a thin, low-permeability filter cake that seals pores and other
openings in permeable formations penetrated by the bit. This is done by
monitoring the particle size distribution of the solids and maintaining the
proper well-bore strengthening materials.
5. Maintain the stability of uncased sections of the borehole. The fluid
specialist monitors the mud weight and mud/well-bore chemical reactivity
to maintain the integrity of the well-bore until the next casing setting
point is reached.
2. Associated Functions
These functions intrinsically arise from the use of a drilling fluid. The mud
specialist does not necessarily routinely monitor the fluid for properties
affecting these functions, or has no control over them.
1. Reduce friction between the drilling string and the sides of the hole.
2. Cool and clean the bit.
3. Assist in the collection and interpretation of information available from
drill cuttings, cores, and electrical logs.
3. Limitations
In conjunction with the above functions, certain limitations—or negative
requirements—are placed on the drill fluid. The fluid should:
1. Not injure drilling personnel nor be damaging or offensive to the
environment.
2. Not require unusual or expensive methods to complete and produce the
drilled hole.
3. Not interfere with the normal productivity of the fluid-bearing formation.
4. Not corrode or cause excessive wear of drilling equipment.
4. COMPOSITION OF DRILLING FLUIDS
All drilling fluids systems are composed of:
● Base fluids—Water, Nonaqueous, Pneumatic
● Solids—Active and Inactive (inert)
● Additives to maintain the properties of the system.
Additives in a drilling fluid are used to control one or more of the
properties measured by the drilling fluids specialist. These properties can be
classified as controlling:
● Mud weight—Specific Gravity. Density
● Viscosity—Thickening, Thinning, Rheology Modifiction
● Fluid loss—API Filtrate, Seepage, Lost Circulation, Wellbore Strengthening
● Chemical reactivity—Alkalinity, pH, Lubrication, Shale Stability, Clay
Inhibition, Flocculation, Contamination Control, Interfacial/Surface
Activity, Emulsification
