Oil-based systems are used in various applications, where fluid stability and inhibition are necessary, such as high-temperature wells, deep holes, and where sticking and hole stabilization are problems. They consist of two types of systems:

  1. Invert emulsion muds are water-in-oil emulsions, typically with calcium chloride brine as the emulsified phase and oil as the continuous phase. They may contain as much as 50% brine in the liquid phase. Relaxed, invert emulsion muds are a “relaxed” emulsion, and have lower electrical stabilities and higher fluid-loss values. Concentration of additives and brine content/salinity are varied to control rheological, filtration and emulsion stability.
  2. Oil-based muds are formulated with only oil as the liquid phase and are often used as coring fluids. Although these systems pick up water from the formation, no additional water or brine is added. All oil systems require higher additional gelling agents for viscosity. Specialized oil-based mud additives include: emulsifiers and wetting agents (commonly fatty acids and amine derivatives) for viscosity; high-molecular-weight soaps; surfactants; amine treated organic materials; organo clays and lime for alkalinity.

Solids-free and typically formulated with aqueous salts, these fluids can achieve a wide density range by incorporating the appropriate salt (halides, bromides and formate brine) without using conventional weighting materials. They are usually designed to match specific reservoir criteria, taking into account critical factors like ESD, contamination risks and crystallization temperatures.

Specifically designed for high-pressure, high-temperature and hostile environments where conventional drilling fluids are impractical or uneconomical. They address safety and environmental compliance, lost circulation, decreased penetration rates, acid gases, and determination of downhole pressure, while allowing for more flexible tool and downhole-equipment selection.

These have the properties of a good drilling fluid and a completion fluid. Its primary attribute is the development of a filter cake which effectively prevents formation damage and is easily removed, with filtrate and filter cake being compatible with the completion process. They are usually composed of biopolymers and bridging materials and are designed to match specific reservoir criteria.

These are designed to mirror oil-based mud performance, without the environmental hazards. Primary synthetic fluids are esters, ethers, poly alpha olefins and isomerized alpha olefins. They can be discharged offshore, and are non-sheening and biodegradable.

These products are designed to remove mud and mud particles that are attached to the casing or become a part of the filter cake while drilling. They are usually part of aqueous and non-aqueous spacers and cleaners to ensure smooth displacement, optimal contact times and reduced disposal costs.

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

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