Mud Gas Separator Poor Boy DegasserTue, 15 Mar 2011 04:53:40 | Well Control

The height and diameter of an atmospheric separator are critical dimensions which affect

the volume of gas and fluid the separator can efficiently handle. As the mud and gas

mixture enters the separator, the operating pressure is atmospheric plus pressure due to

friction in the gas vent line. The vertical distance for the inlet to the static fluid level allows

time for additional gas break-out and provides an allowance for the fluid to rise somewhat

during the operation to overcome friction loss in the mud outlet lines. As shown in Figure

39, the gas-fluid inlet should be located approximately at the midpoint of the vertical

height. This provides the top half for a gas chamber and the bottom half for gas separation

and fluid retention. The 30 in. diameter and 16 ft minimum vessel height requirements

have proven adequate to handle the majority of gas kicks. The separator inlet should have

at least the same ID as the largest line from the choke manifold which is usually 4 in. Some

separators use tangential inlet, which creates a small centrifugal effect on the gas-fluid

mixture which causes faster gas break-out.

The baffle system causes the mud to flow in thin sheets which assists the separation

process. There are numerous arrangements and shapes of baffles used. It is important that

each plate be securely welded to the body of the separator with angle braces.

A 8 in. minimum ID gas outlet is usually recommended to allow a large volume of low

pressure gas to be released from the separator with minimum restriction. Care should be

taken to ensure minimum back pressure in the vent line,. On most offshore rigs, the vent

line is extended straight up and supported to a derrick leg. The ideal line would be

restricted to 30 ft in length and top of the line should be bent outward about 30 degrees to

direct gas flow away from the rig floor. If it is intended that the gas should be flared, flame

arresters should be installed at the discharge end of the vent line.

As stated previously, when the gas pressure in the separator exceeds the hydrostatic head

of the mud in the U-tube, the fluid seal in the bottom is lost and gas starts flowing into the

mud system. The mud outlet downstream of the U-tube should be designed to maintain a

minimum vessel fluid level of approximately 3 1/2 ft in a 16 ft high separator. Assuming a

9.8 ppg mud and total U-tube height of 10 ft the fluid seal would have a hydrostatic

pressure equal to 5.096 psi. This points out the importance for providing a large diameter

gas vent line with the fewest possible turns to minimise line frictional losses.

The mud outlet line must be designed to handle viscous, contaminated mud returns. As

shown in Figure 39 an 8 in. line is recommended to minimise frictional losses. This line is

recommended to minimise frictional losses. The line usually discharges into the mud ditch

in order that good mud can be directed over the shakers and untreatable mud routed to the

waste pit.

During well control operations, the main purpose of a mud gas separator is to vent the gas

and save the drilling fluid. This is important not only economic reasons, but also to

minimise the risk of circulating out a gas kick without having to shut down to mix

additional mud volume. In some situations the amount of mud lost can be critical when

surface volume is marginal and on-site mud supplies are limited. When a gas kick is

properly shut in and circulated out, the mud gas separator should be capable of saving

most of the mud.

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There are a number of design features which affect the volume of gas and fluid that the

separator can safely handle. For production operations, gas oil separators can be sized and

internally designed to efficiently separate gas from the fluid. This is possible because the

fluid and gas characteristics are known and design flow rates can be readily established. It

is apparent that 'gas busters' for drilling rigs cannot be designed on the same basis since

the properties of circulated fluids from gas are unpredictable and a wide range of mixing

conditions occur downhole. In addition, mud rheological properties vary widely and have a

strong effect on gas environment. For both practical and cost reasons, rig mud gas

separators are not designed for maximum possible gas release rates which might be

needed; however, they should not handle most kicks when recommended shut-in

procedures and well control practises are followed. When gas low rates exceed the

separator capacity, the flow must be bypassed around the separator directly to the flare

line. This will prevent the hazardous situation of blowing the liquid from the bottom of the

separator and discharging gas into the mud system.

Figure 39 illustrates the basic design features for atmospheric mud gas separators. Since

most drilling rigs have their own separator designs, the Drilling Supervisor must analyse

and compare the contractor's equipment with the recommended design to ensure the

essential requirements are met.

The atmospheric type separator operates on the gravity or hydrostatic pressure principle.

The essential design features are:

Height and diameter of separator Internal baffle arrangement to assist in additional gas break-out Diameter and length of gas outlet A target plate to minimise erosion where inlet mud gas mixture contacts the

internal wall of the separator, which provides a method of inspecting plate wear A U-tube arrangement properly sized to maintain fluid head in the separator.

Figure 39 - Mud Gas Separator

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