Drilling Fluid Classification Systems

Non-Dispersed System

This mud system consists of spud muds, “natural” muds, and other lightly treated systems. Generally used in the shallower portions of a well.

Dispersed Mud Systems

These mud systems are “dispersed” with deflocculants and filtrate reducers. Normally used on deeper wells or where problems with viscosity occur. The main dispersed mud is a “lignosulfonate” system, though other products are used. Lignite and other chemicals are added to maintain specific mud properties.

Calcium-Treated Mud Systems

This mud system uses calcium and magnesium to inhibit the hydration of formation clays/shales. Hydrated lime, gypsum and calcium chloride are the main components of this type of system.

Polymer Mud Systems

Polymers are long-chained, high molecular-weight compounds, which are used to increase the viscosity, flocculate clays, reduce filtrate and stabilize the borehole. Bio-polymers and cross-linked polymers, which have good shear-thinning properties, are also used.

Low Solids Mud System

This type of mud system controls the solids content and type. Total solids should not be higher than 6% to 10%. Clay content should not be greater than 3%. Drilled solids to bentonite ratio should be less than 2:1.

Saturated Salt Mud Systems

A saturated salt system will have a chloride content of 189,000 ppm. In saltwater systems, the chloride content can range from 6,000 to 189,000 ppm. Those at the lower end are normally called “seawater” systems.These muds can be prepared with fresh or salt water, then sodium chloride
or other salts (potassium, etc.) are added. Attapulgite clay, CMC or starch is added to maintain viscosity.

Oil-Based Mud Systems

There are two types of systems: 1) invert emulsion, where water is the dispersed phase and oil the continuous phase (water-in-oil mud), and 2) emulsion muds, where oil is the dispersed phase and water is the continuous phase (oil-in-water mud). Emulsifiers are added to control the rheological properties (water increases viscosity, oil decreases viscosity).

Air, Mist, Foam-Based Mud Systems

These “lower than hydrostatic pressure” systems are of four types: 1) dry air or gas is injected into the borehole to remove cuttings and can be used until appreciable amounts of water are encountered, 2) mist drilling is then used, which involves injecting a foaming agent into the air stream, 3) foam drilling is used when large amounts of water is encountered, which uses chemical detergents and polymers to form the foam, and 4) aerated fluids is a mud system injected with air to reduce the hydrostatic pressure.

Workover Mud Systems

Also called completion fluids, these are specialized systems designed to 1) minimize formation damage, 2) be compatible with acidizing and fracturing fluids, and 3) reduce clay/shale hydration. They are usually highly treated brines and blended salt fluids.