Gas busters are a simple cylinder or baffle box at the flowline where
mixed drilling fluid and gas are roughly separated while flowing. The
drilling fluid goes to the shale shaker, and the gas is allowed to flow away or is sent to a flare line.
Drilling Mud Separators are holding tanks where mixed water, oil, and gas are allowed to separate by gravity. They have evolved in the last 50 years from simple open tanks to complex closed and pressurized tanks.
Separators can be informally divided into two groups: (1) atmospheric,
or unpressurized, and (2) pressurized, or closed
Degassers are somewhat different devices from the preceding two. The
degasser is a tank in which a vacuum and/or spray removes entrained
gas from the mud system. Degassers handle much smaller gas volumes
than do gas busters or separators but do a more complete job of
removing the gas.
The distinction between gas busters, separators, and degassers is not precise. There are unpressurized (atmospheric) degassers, and there are separators that use centrifugal force or an involute spiral to help bring about separation. However, the purpose of a gas buster or separator is toseparate mixed gas and water, or water and oil, while the purpose of adegasser is to remove entrained gas from the drilling fluid.
In all of these devices, the method of separation involves one or
several of the following processes.
-
Gravity Separation
Gravity separation depends on the difference in density of the materials,
the depth of the liquid column, the size of the gas bubbles, and internal
resistance to flow in the liquid. A simple mud, or fracturing (frac), tank is a gravity separator that holds liquid, solids, and gas until they separate
naturally. Gas rises and exits from the top of the tank. Oil separates from
the other fluids and floats to the top. After the oil overflows a weir or
plate inside the tank, it is pumped away. Drilling fluid or other liquids
such as saltwater are removed from near the bottom of the tank. Solids
settle to the bottom and can be left there or they can be stirred into the
liquid and removed with conventional solids-control equipment. Gravity
separation is the dominant method used in oil/water separator tanks and
in closed system pressurized separators.
2. Centrifugal Separation
The fluid is spun by sending it tangent to the inside of a round vessel,
or is spun in a rotating cylinder. The gas, oil, water, and solids are
separated by the artificial gravity caused by the centrifugal force in the
spinning liquid. This method is used in a number of the ‘‘atmospheric’’
or West Texas separators. An involute spiral is used in some of the
closed pressurized systems to cause centrifugal separation.
3. Impact, Baffle, or Spray Separation
In impact, baffle, or spray separation, the fluid is directed onto a
baffle at high velocity. The impact of the fluid containing gas starts
separation. The fluid velocity may be the result of flow from the flowline
or blooie line or it may be picked up by a pump and sent as a fluid stream
or spray.
4. Parallel-Plate and Thin-Film Separation
Parallel-plate or thin-film separation works with gas in a liquid. The fluid is spread out as a thin film over a plate, which allows the gas to escape more easily.
. In a parallel-plate separator, the fluid containing the gas is forced
between parallel plates, which distort the bubbles of gas and help
them break. This is common in commercial demisting or defoaming
operations.
. Thin-film separation is the process whereby the liquid is flowed in a
thin film over a plate that allows the gas bubbles to expand and break.
The thin-film process is part of most vacuum degasser operations.
5. Vacuum Separation
The vacuum degasser, which separates gas entrained in a liquid, uses
reduced pressure (a partial vacuum) that causes the gas bubbles to
expand and break. This method is used primarily in degassers to remove
entrained gas.
