Nine distinct mud systems are defined here. The first seven are water-based, while the eighth is oil-based. The ninth category is a specialized one in which air or gas is the continuous fluid. The 9 categories are:
- Non dispersed. These may consist of spud muds, natural muds and other lightly treated systems generally used for shallow wells or top-hole drilling.
- Dispersed. At greater depths or where hole-conditions may be problematic, muds are often dispersed, typically by means of lignosulphonates or other deflocculants. These and similar products are also effective filtrate reducers.
- Calcium treated. Divalent cations such as calcium and magnesium, when added to a mud, inhibit the swelling of formation clays and shale, and are therefore added to control sloughing shale, hole enlargement and to prevent formation damage. Hydrated lime, gypsum (calcium sulphate) and calcium chloride are principal ingredients of calcium systems. Gyp systems (note:
Gyp = gypsum) usually have a pH of 9.5 to 10.5 and an excess gyp concentration of 2 to 4 lb/ bbl; Lime systems have an excess lime concentration of 1 to 15 lb/bbl and a pH of 11.5 to 12.0.
- Polymer. Muds incorporating long-chain, high-molecular-weight chemicals are effective in increasing viscosity, flocculating muds, reducing filtrate loss and stabilizing the formation. Various types of polymers are available for this purpose, including Bentonite extenders. Bio polymers and cross-linked polymers are also used and have good shear-thinning properties at low concentrations.
- Low solids. This includes systems in which the amount and type of solids are controlled. Total solids should not range higher than about 6% to 10% by volume (and clay < 3% by volume). One primary advantage of low-solids systems is that they significantly improve the rate of penetration.
- Saturated salt. Include several groups: Saturated salt systems have a chloride ion concentration of 189 000 ppm. Saltwater systems have a chloride content from 6 000 to 189 000 ppm, and at its lower level are usually referred to as brackish or seawater systems.
- Workover. Completion and workover fluids are specialized systems designed to minimize formation damage, and be compatible with acidizing and fracturing operations (acid soluble) and capable of inhibiting swelling clays that reduce formation permeability. Density is obtained through dissolved salt to avoid long term settling.
- Oil/synthetic. Oil-based fluids are used for high temperature wells, deviated holes and wells where pipe sticking and hole stabilization is a problem. They consist of two types of systems:
- Invert emulsion muds are water-in-oil fluids and have water as the dispersed phase and oil as the continuous phase. They may contain up to 50% water in the liquid phase. Emulsifier (commonly fatty acids amine derivatives, high-molecular-weight soaps), and water concentrations are varied to control rheological and electrical stability;
- Synthetic fluids are designed to duplicate the performance of oil-based muds, without the environmental hazards. Primary types of synthetic fluids are esters, poly alpha olefins and food grade paraffin. They are environmentally friendly, can be discharged offshore and are non-sheening and biodegradable.
- Air, mist, foam and gas. Four basic operations are included in this specialized category according to the IADC. These include:
- Dry air drilling, which involves injecting dry air or gas into the wellbore at rates capable of achieving annular velocities that will remove cuttings;
- Mist drilling involves injecting a foaming agent into the air stream, which mixes with produced water and lifts drill cuttings;
- Stable foam uses chemical detergents and polymers and a foam generator to carry cuttings in fast-moving air stream;
- Aerated fluids rely on mud with injected air (which reduces the hydrostatic head) to remove drilled solids from the wellbore.