Drilling fluids are fluids that are used during the drilling of subterranean wells. They provide primary well control of subsurface pressures by a combination of density and any additional pressure acting on the fluid column (annular or surface imposed). They are most often circulated down the drill string, out the bit and back up the annulus to the surface so that drill cuttings are removed from the wellbore. Drilling fluids have a number of alternative names, acronyms and slang terms used within the industry.
The most widely used name is “mud” or “drilling mud”. Drilling fluid is a major factor in the success of the drilling program
The principal functions of drilling fluid are to:
Control subsurface pressures, maintaining well control
Remove drill cuttings from beneath the bit and circulate them to the surface
Maintain wellbore stability, mechanically and chemically
Transmit hydraulic energy to the drill bit and downhole tools
Cool and lubricate the drill string and bit
Allow adequate formation evaluation
Provide a completed wellbore that will produce hydrocarbons
Suspend or minimize the settling of drill cuttings or weight material when circulation is stopped, yet allow the removal of drill cuttings in the surface fluids processing system
Form a low permeability, thin and tough filter cake across permeable formations.
The properties of the drilling fluid should be adjusted to the hydraulics available for the drilling operation and the well design. Rate of penetration (ROP) and bit life can be improved by optimizing the hydraulic horsepower at the bit, especially for roller cone bits. The ROP and bit life for polycrystalline diamond compact (PDC) cutter bits is improved when an adequate flowrate is used with minimal overbalance. Drilling fluid properties and circulation rates determine the parasitic pressure losses in the drill string and the available pressure at the bit for optimized drilling performance. T
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