Changes

==Purpose of fluids==

==Purpose of fluids==

An essential element of drilling a well is the drilling fluid or mud. Drilling fluids serve a number of functions:

An essential element of [[drilling a well]] is the drilling fluid or mud. Drilling fluids serve a number of functions:

* Removal of cuttings from the bottom of the hole

* Removal of cuttings from the bottom of the hole

==Properties of fluids==

==Properties of fluids==

The large number of functions performed by the drilling fluid require that some minimum properties of the fluids be maintained. The measurement of these properties gives the mud engineer a “status report” of the fluid and how it is reacting with the formation and the subsurface environment. The most critical of the properties are density, viscosity, fluid loss control, and chemical composition.

The large number of functions performed by the drilling fluid require that some minimum properties of the fluids be maintained. The measurement of these properties gives the mud engineer a “status report” of the fluid and how it is reacting with the formation and the subsurface environment. The most critical of the properties are density, [[viscosity]], fluid loss control, and chemical composition.

===Density===

===Density===

* ppg = pounds per gallon (United States)

* ppg = pounds per gallon (United States)

* S.G. = specific gravity (dimensionless) (international)

* S.G. = specific [[gravity]] (dimensionless) (international)

* psi/ft = pounds per square inch per foot (uncommon)

* psi/ft = pounds per square inch per foot (uncommon)

* pcf = pounds per cubic foot (California)

* pcf = pounds per cubic foot (California)

===Fluid loss control===

===Fluid loss control===

The fluid loss gives a relative indication of how the mud is controlling loss of the base fluid into the formation. This becomes important when porous formations, particularly those containing oil or gas, are drilled. In porous formations, the drilling fluid may penetrate the rock and cause formation damage. (However, a low fluid loss does not always ensure minimal formation damage.) There are many types of fluid loss additives, such as bentonite, that can be used in the mud to help mitigate this problem.

The fluid loss gives a relative indication of how the mud is controlling loss of the base fluid into the formation. This becomes important when porous formations, particularly those containing oil or gas, are drilled. In porous formations, the drilling fluid may penetrate the rock and cause formation damage. (However, a low fluid loss does not always ensure minimal formation damage.) There are many types of fluid loss additives, such as [[bentonite]], that can be used in the mud to help mitigate this problem.

===Chemical composition===

===Chemical composition===

====Dispersed muds====

====Dispersed muds====

These muds have a chemical dispersant added to the system which is used to deflocculate mud solids. Most of the chemical dispersants in use (such as lignite and lignosulfonate) are acidic and require an alkaline environment in which to function properly. Of all the water-based muds, high pH muds are the most tolerant of solids and contamination. They are, without a doubt, the least difficult of the water muds to maintain. Clay (bentonite) is used as a viscosifier and fluid loss agent. Dispersants are use to permit enough clay into the system to control fluid losses. Caustic soda (NaOH) is used for pH control, and the density is adjusted with weight materials.

These muds have a chemical dispersant added to the system which is used to deflocculate mud solids. Most of the chemical dispersants in use (such as lignite and lignosulfonate) are acidic and require an alkaline environment in which to function properly. Of all the water-based muds, high pH muds are the most tolerant of solids and contamination. They are, without a doubt, the least difficult of the water muds to maintain. Clay ([[bentonite]]) is used as a viscosifier and fluid loss agent. Dispersants are use to permit enough clay into the system to control fluid losses. Caustic soda (NaOH) is used for pH control, and the density is adjusted with weight materials.

Dispersed muds can be broken into two smaller categories: ''calcium-based'' and ''seawater muds'',

Dispersed muds can be broken into two smaller categories: ''calcium-based'' and ''seawater muds'',

* ''Calcium-Based Mud''—Calcium-based mud systems maintain a desired amount of calcium in the water phase. The calcium concentration can be maintained by using gypsum (CaSO₄) or lime [Ca(OH)₂]. These muds are more inhibitive and can tolerate cement and anhydrite contamination better than a freshwater-dispersed fluid. However, their thermal limitation is somewhat reduced.

* ''Calcium-Based Mud''—Calcium-based mud systems maintain a desired amount of calcium in the water phase. The calcium concentration can be maintained by using [[gypsum]] (CaSO₄) or lime [Ca(OH)₂]. These muds are more inhibitive and can tolerate cement and [[anhydrite]] contamination better than a freshwater-dispersed fluid. However, their thermal limitation is somewhat reduced.

* ''Seawater Mud''—In seawater muds, the upper limit for conventional dispersed fluids to function efficiently is 20,000 mg/L chlorides (which is the salinity of seawater). The cost for this type of system is slightly higher than that of a freshwater system. However, in offshore environments, this cost is offset by allowing muds to be run using native seawater rather than transporting in freshwater.

* ''Seawater Mud''—In seawater muds, the upper limit for conventional dispersed fluids to function efficiently is 20,000 mg/L chlorides (which is the salinity of seawater). The cost for this type of system is slightly higher than that of a freshwater system. However, in [[offshore rig|offshore environments]], this cost is offset by allowing muds to be run using native seawater rather than transporting in freshwater.

====Nondispersed muds====

====Nondispersed muds====

* ''Reduction of torque and drag problems''—Since oil is the continuous phase, the borehole and the tubulars are wetted with a lubricating fluid. This is a distinct advantage in deviated wellbores.

* ''Reduction of torque and drag problems''—Since oil is the continuous phase, the borehole and the tubulars are wetted with a lubricating fluid. This is a distinct advantage in deviated wellbores.

* ''Thermal stability''—Oil-based muds have shown stability in wells, with BHTs of [[temperature::585°F]]

* ''Thermal stability''—Oil-based muds have shown stability in wells, with BHTs of [[temperature::585°F]]

* ''Resistance to chemical contamination''—Carbonate, evaporite, and salt formations do not adversely affect the properties of an oil mud. CO₂ and H₂S can easily be removed with the addition of lime (CaCO₃).

* ''Resistance to chemical contamination''—Carbonate, [[evaporite]], and salt formations do not adversely affect the properties of an oil mud. CO₂ and H₂S can easily be removed with the addition of lime (CaCO₃).

Disadvantages of oil-based mud systems include the following:

Disadvantages of oil-based mud systems include the following:

[[Category:Wellsite methods]]

[[Category:Wellsite methods]]