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
