Types of Drilling Fluid

The composition of a particular drilling fluid system depends on the actual re-quirements of the individual well or well section. Wells are drilled through dif-ferent formations, which require difdif-ferent mud properties to achieve optimum

1The ability of a fluid, such as cement or drilling mud, to develop gel strength over time when not subject to shearing, and then to liquefy when agitated.

penetrations and stable borehole conditions. Economics, additives (cost and availability), temperature profile, contamination, and environment are some im-portant factors that determine the design of the drilling fluid. The basic classifi-cation of drilling fluids is (sub–classificlassifi-cations exist):

1. Water–base fluids

(a) Clear water and native muds

(b) Inhibitive water–based muds – calcium muds (c) Dispersed muds – lignosulfonate muds (d) Non–dispersed muds – KCL/Polymer muds (e) Flocculated muds

(f) Salt–saturated muds

2. Oil–base fluids and Emulsion fluids (a) Oil–in–water emulsion

(b) Water–in–oil emulsions 3. Synthetic Fluids

4. Aerated fluids

(a) Gaseous fluids (air, nitrogen, CO2, natural gas) (b) Foams

(c) Mists

7.2.1 Water–Base Fluids

Water–base fluids are any drilling fluid in which the continuous phase, where some materials are in suspension and others are dissolved, is water. Thus any water–base fluid system consists of a water phase, inert solids, reactive solids phase, and chemical additives. Each of these parts contribute to the overall fluid properties. The individual contributions are:

water: create initial viscosity,

inert solids: (low-gravity solids like sand and chert, and high-gravity solids like barite and lead sulfides) produce required mud weight,

reactive solids: ( low-gravity solids like bentonite and attapulgite clays) cause additional viscosity and gel,

chemical additives: (thinners [phosphate, chrome, lignosulfonate, lignites, sur-factants] and thickeners [lime, cement,polymers]) provide control to vis-cosity, yield point, gel strength, fluid loss, pH, filtration behavior, etc.

Caustic soda (NaOH) is used to keep a high pH required to control corro-sion, hydrogen embrittlement and the solubility of Ca2+ and Mg2+.

Curtin University of Technology Department of Petroleum Engineering

Master of Petroleum Well Engineering Drilling Engineering Fundamentals

7.2.1.1 Clear Water and Native Muds

To drill compact formations which are normally pressured², fresh water and salt–saturated water can be used as drilling mud. Native muds are the result of mixing water and clays or shales from the cuttings drilled. Here the clays or shales are dissolved by the water and returned to the surface. Clear water and native muds are the cheapest mud systems since no additional material is needed to form the mud. They are also environmentally best accepted.

7.2.1.2 Inhibitive Water–Base Muds – Calcium Muds

When swelling and hydration of clays and shales are expected, inhibitive water–

base muds can be used. Calcium muds are best suited to drill formations that contain gypsum and hydrite. A subclassification of inhibitive water–base muds distinguishes seawater muds, saturated saltwater muds, lime muds, and gypsum muds.

7.2.1.3 Dispersed Muds – Lignosulfonate Muds

Dispersed muds are used when the following characteristics are required:

• relative high mud weight (larger than 14 ppg),

• tolerance to moderately high formation temperatures,

• low filtration loss

• high tolerance for contamination by drilling solids.

Some of the disadvantages of dispersed muds are sloughing of shales and formation damage due to dispersing of formation clays in the presence of lig-nosulfonate.

Dispersed mud systems consist of:

• fresh or salty water,

• bentonite,

• lignosulfonate,

• caustic soda

• colloidal polymers (carboxymethylcellulose or stabilized starch).

In general, these mud systems exhibit better control of viscosity, higher solids tolerance, and better control of filtration than non–dispersed muds.

2Formation with pore pressure gradient equal to brine density

7.2.1.4 Non–Dispersed Muds – KCL/Polymer Muds

To drill sloughing shales and water sensitive rocks such as productive sands, which are prone to formation damage, fresh water non–dispersed muds are used. Commonly, non–dispersed muds are associated with low mud weights and low solid concentrations.

Non–dispersed mud systems consist of:

• fresh water or brine,

• potassium chloride (KCl),

• inhibiting polymer,

• viscosifier,

• stabilized starch or carboxymethyl cellulose,

• caustic soda,

• lubricants.

Low–solids polymer mud systems are widespread in the industry since they offer advantages like increased penetration rate, hole stability, shear thinning ability, hole cleaning with maximum hydraulics, and lower equivalent circula-tion density over convencircula-tional deflocculated muds. Besides these advantages, they also have disadvantages like instability at temperatures above 250 ^◦F, ir-reversible absorption of the polymer on clay, higher dilution, the requirement of adequate solids removal equipment, and the fact that they are more corrosive.

7.2.1.5 Flocculated Muds

Flocculated muds generally causes a dynamic increase in filtration, viscosity, and gel strength. Flocculation refers to a thickening of the mud due to edge–

to–edge and edge–to–face association of clay particles.

The flocculation is commonly caused by high active solids concentration, high electrolyte concentration, and high temperature. To reduce the flocculating tendency of the mud, chemical additives, also called deflocculants or thinners, are used. Thinners like phosphates, tannin, lignin, and lignosulfonate are used to lower the yield point and gel strength. When deflocculants are added, the pH is controlled by NaOH.

7.2.1.6 Salt–Saturated Muds

Several mud systems have been included in this classification. Saturated salt systems have a chloride concentration near 190,000 mg/l (saturated) and are

Curtin University of Technology Department of Petroleum Engineering

Master of Petroleum Well Engineering Drilling Engineering Fundamentals

used to drill salt formations. Saltwater systems have a chloride content of 10,000 to 190,000 mg/l. The lower levels are usually referred to as brackish or seawater systems.

Salt–saturated muds are used to drill through salt domes and salt sections.

These mud systems are saturated with sodium chloride (NaCl), which prevents severe hole enlargements due to washouts of the salt formations. Swelling of bentonitic shales is controlled by adding of polymer.

Various specialty products, such as attapulgite, CMC, starch, and others, are used to increase viscosity for hole–cleaning properties and to reduce fluid loss.

7.2.2 Oil–Base Muds

In oil–base mud systems, crude or diesel oil forms the continuous phase in the water–in–oil emulsion. In this way oil–base mud can have as little as 3% to 5%

or as much as 20% to 40% (invert emulsions) of water content. Oil–base mud systems are used when:

1. Drilling sensitive production zones or troublesome shales, 2. Directional drilling and slim hole drilling,

3. Drilling of depleted reservoirs,

4. Drilling salt sections and formations that contain hydrogen sulfide, 5. High risk of pipe sticking problems,

6. Drilling formations with higher temperatures.

Low–gravity solids content has to be monitored closely when drilling with oil–

base muds. The reason is because there is no hydration of solids (clays), which frequently causes the contents of low–gravity solids to exceed acceptable levels. This results in reduction of penetration rate, increase risk of formation damage, and increase risk of differential sticking.

Since oil–base muds contain substantially less colloidal particles, they ex-hibit an increased spurt fluid loss³. Due to the higher filtration rates, the mon-itoring of high–pressure/high–temperature filtration and the drilling conditions are important to ensure that excessive filtration or filter cake buildup does not lead to drilling problems.

3The instantaneous volume (spurt) of liquid that passes through a filter medium prior to deposition of a competent and controlling filter cake.

7.2.3 Synthetic Fluids

Synthetic fluids are designed to mirror oil–based mud performance without the environmental hazards. Primary synthetic fluids are esters, ethers, poly alpha–

olefin and isomerized alpha–olefin. They are environmentally friendly, can be discharged offshore, and are non–sheening and biodegradable.

7.2.4 Aerated Fluids

Four basic operations are included in this specialized category. These include:

1. Dry air drilling, which involves injecting dry air or gas into the wellbore at rates capable of achieving annular velocities that will remove cuttings;

2. Mist drilling, which involves injecting a foaming agent into the air stream that mixes with produced water and coats the cuttings to prevent mud rings, allowing drill solids to be removed;

3. Foam uses surfactants and possibly clays or polymers to form a high carrying-capacity foam;

4. Aerated fluids rely on mud with injected air (which reduces hydrostatic head) to remove drilled solids from the wellbore.

These fluids are discussed in details in advanced drilling courses (air drilling, underblanced drilling, etc).