Imperforate bowl: centrifugal separators – Process
Patent
1996-12-13
1998-08-04
Cooley, Charles E.
Imperforate bowl: centrifugal separators
Process
494 2, 494 55, 494 56, B04B 1104, B04B 1108
Patent
active
057886210
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a method and apparatus for the separation of solids, and in particular rock cuttings, from drilling muds by centrifugal means.
BACKGROUND ART
In this specification the term "mud" is defined according to its usual meaning in the oil and gas drilling industries, namely to describe a drilling fluid used to transport rock cuttings from a wellbore. Such fluids or mud formulations are tailored to specific wellbore conditions and are costly to formulate. One objective in the process of drilling is, therefore, to conserve drilling muds.
In practice this objective of preserving drilling mud conflicts with a need to remove as much of the drill cuttings from the mud as possible. The accumulation of fine drill solids in muds can only be tolerated to a certain limit, beyond which the required properties of the mud decline.
One conventional form of separating cuttings from drilling muds is the use of shale shakers. These are vibrating screen devices through which the mud is passed. The finest possible screen sizes are required to maximise solids rejection. However, other factors such as mud circulation rate, drilling rate and type of formation being drilled must also be taken into consideration when selecting the screen size. Shale shakers are often operated near the limit of their capability. Thus, when drilling conditions change, whole mud loss occasionally occurs as equipment capabilities are exceeded. Shale shaker screens, especially in the fine mesh sizes, have a limited life expectancy and are costly to replace. Late replacement of torn screens also allows drill solids to accumulate in the mud system leading to higher chemical treatment costs.
In conventional systems shale shakers are often augmented with secondary solids removal devices, including hydrocyclones and partial flow decanting centrifuges. Hydrocyclones, due to inherent characteristics, discharge a relatively high proportion of mud along with the solids which they separate. They are often not maintained in efficient working condition due to blockages and wear on cone liners. Their use is restricted to low to medium density muds.
Conventional decanting centrifuges used for separating mud can be highly sophisticated. The sophistication of this equipment can also make it sensitive and vulnerable to breakdown. As a result such machinery can be unreliable, which is a significant problem where full flow implementation is required. Centrifuges are thus seldom used in full flow implementation and typically process only 10 to 15% of the circulating mud volume.
Shale shakers and hydrocyclones both introduce aeration points into the mud circulation system. The uptake of atmospheric oxygen increases the corrosion rate in the equipment, especially in drill pipes, and catastrophic failure can result from such corrosion.
To eliminate entrained gases from the mud system, conventionally equipped drilling rigs use a separate degasser. Some degasser types are not efficient in eliminating gas microbubbles from the mud system.
An additional complication in modern drilling occurs where sophisticated directional drilling equipment is used, in particular with downhole mud motors. Some types of mud motors are particularly sensitive to erosion by sand-sized drill cuttings in the drilling fluid. This is problematic mostly in tophole drilling where well deviation is initiated and where shale shaker capacity is often insufficient to allow for the use of fine mesh screens, thus resulting in the build up of undesirable concentrations of sand-sized particles in the drilling mud. Mud motor life is reduced under these conditions. If it were possible to maintain the drilling mud free of sand at all times, the life expectancy of existing mud motors would be improved and different, superior types of mud motors could be used.
A further shortcoming of conventional solids removal equipment is the inability to completely remove associated mud adhering to separated cuttings. This is particularly important with oil-based muds when used in environmen
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patent: 1062216 (1913-05-01), Berrigan
patent: 4155503 (1979-05-01), Sears
patent: 4950219 (1990-08-01), Luchetta
patent: 5037373 (1991-08-01), Mozley
patent: 5338285 (1994-08-01), Omori
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