Maintain well bore stability even under a forceful mud environment.
Chemical composition and mud properties must combine to provide a stable well bore. Weight of the mud must be within the necessary range to balance the mechanical forces.
Well bore instability = sloughing formations, which can cause tight hole conditions, bridges and fill on trips (same symptoms indicate hole cleaning problems).Well bore stability = hole maintains size and cylindrical shape.
If the hole is enlarged, it becomes weak and difficult to stabilize, resulting in problems such as low annular velocities, poor hole cleaning, solids loading and poor formation evaluation in sand and sandstones formations, hole enlargement can be accomplished by mechanical actions (hydraulic forces & nozzles velocities). Formation damage is reduced by conservative hydraulics system.
A good quality filter cake containing bentonite is known to limit bore hole enlargement. In shales, mud weight is usually sufficient to balance formation stress, as these wells are usually stable. With water base mud, chemical differences can cause interactions between mud & shale that lead to softening of the native rock. Highly fractured, dry, brittle shales can be extremely unstable (leading to mechanical problems).Various chemical inhibitors can control mud and shale interactions (calcium, potassium, salt, polymers, asphalt, glycols and oil – best for water sensitive formations)Oil (and synthetic oil) based drilling fluids are used to drill most water sensitive Shales in areas with difficult drilling conditions.
To add inhibition, emulsified brine phase (calcium chloride) drilling fluids are used to reduce water activity and creates osmotic forces to prevent adsorption of water by Shales.
Drilling muds are traditionally based on water, either fresh water, seawater, naturally occurring brines, or prepared brines. Many muds are oil-based, using direct products of petroleum refining such as diesel oil or mineral oil as the fluid matrix. In addition, various so-called synthetic-based muds are prepared using highly refined fluid compounds that are made to more-exacting property specifications than traditional petroleum-based oils.
In general, water-based muds are satisfactory for the less-demanding drilling of conventional vertical wells at medium depths, whereas oil-based muds are better for greater depths or in directional or horizontal drilling, which place greater stress on the drilling apparatus.
Synthetic-based muds were developed in response to environmental concerns over oil-based fluids, though all drilling muds are highly regulated in their composition, and in some cases specific combinations are banned from use in certain environments.
A typical water-based drilling mud contains a clay, usually bentonite, to give it enough viscosity to carry cutting chips to the surface, as well as a mineral such as barite (barium sulfate) to increase the weight of the column enough to stabilize the borehole.
Smaller quantities of hundreds of other ingredients might be added, such as caustic soda (sodium hydroxide) to increase alkalinity and decrease corrosion, salts such as potassium chloride to reduce infiltration of water from the drilling fluid into the rock formation, and various petroleum-derived drilling lubricants. Oil- and synthetic-based muds contain water (usually a brine), bentonite and barite for viscosity and weight, and various emulsifiers and detergents for lubricity.
Drilling mud is pumped down the hollow drill pipe to the drill bit, where it exits the pipe and then is flushed back up the borehole to the surface. For economic and environmental reasons, oil- and synthetic-based muds are usually cleaned and recirculated (though some muds, particularly water-based muds, can be discharged into the surrounding environment in a regulated manner).
Larger drill cuttings are removed by passing the returned mud through one or more vibrating screens, and sometimes fine cuttings are removed by passing the mud through centrifuges. Cleaned mud is blended with new mud for reuse down the borehole.
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