Mud/Gas Separator Sizing and Evaluation G.R. MacDougall, SPE, Chevron Canada Resources Ltd.

Summary. Recent wellsite disasters have led to an increased emphasis on properly sized mud/gas separators. This paper reviews and analyzes existing mud/gas separator technology and recommends separator configuration, components, design considerations, and a sizing procedure. A simple method of evaluating mud/gas separation within the separator vessel has been developed as a basis for the sizing procedure. A mud/gas separator sizing worksheet will assist drilling personnel with the sizing calculations. The worksheet provides a quick and easy evaluation of most mud/gas separators for a specific well application. A brief discussion of other mud/gas separator considerations is provided, including separator components, testing, materials, and oil-based-mud considerations.

Introduction The mud/gas separator is designed to provide effective separation of the mud and gas circulated from the well by venting the gas and returning the mud to the mud pits. Small amounts of entrained gas can then be handled by a vacuum-type degasser located in the mud pits. The mud/gas separator controls gas cutting during kick situa­tions, during drilling with significant drilled gas in the mud returns, or when trip gas is circulated up.

This paper discusses design considerations for mud/gas separa­tors. The purpose of this paper is to allow drilling rig supervisors to evaluate mud/gas separators properly and to upgrade (if required) the separator economically to meet the design criteria outlined in this paper, and to provide office drilling personnel with guidelines for designing mud/gas separators before delivery at the drillsite.

Principle of Operation The operating principle of a mud/gas separator is relatively sim­ple. The device is essentially a vertical steel cylindrical body with openings on the top, bottom, and side, as shown in Fig.!. The mud and gas mixture is fed into the separator inlet and directed at a flat steel plate perpendicular to the flow. This impingement plate minimizes the erosional wear on the separator's internal walls and assists with mud/gas separation. Separation is further assisted as the mud/gas mixture falls over a series of baffles designed to increase the turbulence within the upper section of the vessel. The free gas is then vented through the gas vent line, and mud is returned to the mud tanks.

Operating pressure within the separator is equal to the friction pressure of the free gas venting through the vent line. Fluid is main­tained at a specific level (mud leg) within the separator at all times. If the friction pressure of the gas venting through the vent line ex­ceeds the mud-leg hydrostatic pressure within the separator, a blow­through condition will result sending a mud/gas mixture to the mud tanks. As one can readily see, the critical point for separator blow­through eXists when peak gas flow rates are experienced in the sepa­rator. Peak gas flow rates should theoretically be experienced when gas initially reaches the separator.

Types of Mud/Gas Separators Three types of mud/gas separators commonly are used today: closed bottom, open bottom, and float type. The principle of mud/gas sepa­ration within each type of vessel is identical. Differences can be found in the method of maintaining the mud leg, as discussed below.!

The closed-bottom separator, as the name implies, is closed at the vessel bottom with the mud return line directed back to the mud tanks, as shown in Fig. 1. Mud leg is maintained in the separator by installation of an inverted V-shaped bend in the mud return line. Fluid level can be adjusted by increasing/decreasing the length of the V-shaped bend.

Commonly called the poor boy,2,3 the open-bottom mud/gas separator is typically mounted on a mud tank or trip tank with the bottom of the separator body submerged in the mud, as shown in

Copyright 1991 Society of Petroleum Engineers

SPE Drilling Engineering, December 1991

Fig. 2. The fluid level (mud leg) in the separator is controlled by adjusting the fluid level in the mud tank or by moving the separa­tor up or down within the tank. Mud-tank height can restrict the maximum mud leg obtainable for open-bottom mud/gas separators.

Fluid level (mud leg) is maintained in a float-type mud/gas separator4 by a float/valve configuration, as shown in Fig. 3. The float opens and closes a valve on the mud return line to maintain the mud-leg level. Valves can be operated by a manual linkage sys­tem connected from the float to the valve, or the valve can be air­operated with rig air. Mud-leg height can be controlled by adjust­ing the float assembly.

There are some inherent problems in the use of float-type mud/gas separators. The manual linkage separator has experienced prob­lems with linkage failure resulting in improper opening or closing of the mud-return-line valve. Air-operated valves fail to function if rig air is lost, resulting in no control of fluid level within the separator. Mud-return-line valves are prone to plug with solids, preventing mud flowback to the mud pits.

Because of these problems, float-type mud/gas separators are not recommended and a closed-bottom separator is preferred. Open­bottom separators are acceptable; however, one should be aware that they are restricted to a maximum mud leg, somewhat lower than the mud-tank height. Although float-type mud/gas separators are strongly discouraged, these separators can be modified easily for disconnection of the float, removal of the valve, and installa­tion of a mud leg in the mud return line.

For the purpose of this paper, a closed-bottom mud/gas separa­tor will be considered for all separator designs.

Sizing the Mud/Gas Separator Table 1 shows a mud/gas separator worksheet to assist with the sizing calculation. The mud/gas separator illustrated in Fig. 4 will be evaluated for sufficient sizing in this paper.

Peak Gas Flow Rate. As discussed previously, the critical time for separator blow-through exists when peak gas flow rates are ex­perienced. Mud/gas separator blow~through is defined as inefficient separator operation resulting in a mud/gas mixture returning to the mud tanks through the mud return line.

Two situations can cause separator blow-through. I. Friction pressure of the gas venting through the vent line ex­

ceeds the mud-leg hydrostatic pressure, resulting in evacuation of fluid from the separator. Friction pressure of the mud through the mud return line is considered negligible because of its short length.

2. Vessel ID is too small, causing insufficient retention time for the gas to separate efficiently from the mud. This situation is com­monly called insufficient' separator cut.

To estimate a peak gas flow rate properly, we must consider a "typical" kick. The typical kick will depend on the well location, depth, type size, and component ratios of influx. Kick data should be based on previous offset well data and should be a realistic worst­case gas kick. The well and kick data in Fig. 5 will be used in this paper.

The volume and pressure of the gas upstream of the choke must first be calculated. Vsing the drilling applications module Dril-

279

f:::::::1 MUD & GAS MIXTURE

_ MUD

o GAS

IMPINGEM ENT PLATE

SEPARATOR INLET

Fig. 1- Closed-boHom mud/gas separator.

pro™,5 we concluded that Pc max = 1,750 psi and Vemax =75.9 bbl.

The driller's method was used for calculation purposes. Use of the wait-and-weight method would result in a lower peak gas flow rate. Driller's method calculations provide a worst-case well-control scenario for mud/gas separator sizing.

The following equation calculates the time necessary to vent gas:

t= Vemax/qk=75.9/3=25.3 minutes ................... (1)

With Boyle's gas law,2 calculate the volume of gas downstream of the choke, Ve' Assume an atmospheric pressure of 14.7 psi. 6

Neglect the effects of gas temperature and compressibility.

Pemax Vcmax =Pc Vc; .................................. (2)

therefore, Ve=(1,750X75.9)/(l4.7)=9,036 bbl.

Calculate the peak gas flow rate, qmax' as

qmax = Ve/t=9,036/25.3=357.2 bbl/min ............... (3)

Convert barrels per minute to cubic feet per day, 5

qmax =357.2x8,085.6=2,887,806 ft31D.

Vent-Line Friction Pressure. The formula used by this paper to calculate friction pressure of gas through a vent line is derived from the Atkinson-modified Darcy-Weisbach equation:?

hi =fsLq2/5.2A3.

If we assume an empirical friction factor for smooth, straight, steel pipe-lOxlO- lO Ibm-min2/ft4 and gas density =0.01 Ibm/ gal6-the following much simpler equation can be used:

PI =5.0x 10-12Leq'1max/df . ......................... (4)

Effective length,? Le, can be defined as the total vent-line length plus equivalent lengths for various bends, corners, etc. (Table 2), for the mud/gas separator shown in Fig. 4. The vent line consists of 200 ft of a 7-in.-ID circular steel line with three sharp right bends. Le can be calculated as

L e =L+Leq =2oo+(3x70)=41O ft .................... (5)

Vent-line friction pressure is

PI =(5.0x 10- 12 X41OX2,887,806)217.05 = 1.0 psi.

Note that effective vent-line lengths will be significantly affect­ed by the installation of flame arresters or some auto-igniters. 8 The effect of this additional backpressure should be included in the cal­culation of vent-line friction pressure.

280

~ I:.::.:.:::.::J -o

MUD & GAS MIXTURE

MUD

GAS

IMPINGEMENT PLATE

MUD TANK

Fig. 2-0pen-boHom mud/gas separator.

Mud Leg. As previously discussed, mud-leg hydrostatic pressure must exceed vent-line friction pressure to prevent a separator blow­through condition. Minimum mud-leg hydrostatic pressure would occur if an oil/gas kick was taken and the mud leg was filled with 0.26 psi/ft oil. 8 This minimum condition mayor may not occur, depending on the well location. Offset well data should be evaluat­ed to establish a minimum mud-leg fluid gradient. For example, the 0.26-psi/ft mud-leg gradient would be considered extremely con­servative if dry gas were expected for the sample problem. A more realistic estimate would approach the gradient of whole mud for the dry-gas case. A realistic mud-leg gradient for a gas/water kick would be the gradient of native salt water.

In this paper, a worst-case scenario is considered with a mud­leg fluid gradient of 0.26 psi/ft. If we assume a 7-ft mud leg,

PmZ=hmzgmZ =7xO.26=1.8 psi, ...................... (6)

where PmZ>PI(1.8> 1.0 psi).

Therefore, a blow-through condition does not exist when vent-line friction pressure is calculated at peak gas flow rates.

Separator ID. A blow-through condition may exist because a small vessel ID results in insufficient separator cut. Several complicated models exist to describe gas movement within a liquid. 9 A sim­plified approach, taken in this paper, states that the gas migration rate upward within the separator must exceed the liquid velocity downward within the separator to give 100% separator cut and to prevent a separator blow-through condition. Gas migration rate is estimated at 500 ft/hr, or 8.4 ft/min,9 within the separator. This estimation is conservative and more realistic values would be higher; however, the slow gas migration rate serves as a worst-case scenario. Liquid flow rate through the separator can be estimated as 2xqk; for this paper 2x3=6 bbllmin. This factor of two was determined from gas volume at depth calculations (Boyle's law) using Drilpro ™ for various depths and kick sizes. Correlation of the data shows that the mud flow from the well approaches twice the mud flow into the well (kill rate) for various kick sizes, kill rateS, and wellbore geometries. A more accurate determination of mud flow from the well can be incorporated into the design procedure.

By calculating the liquid velocity downward within the separator

vL =2qkICsp, ..................................... (7)

where Csp =d;/1,029 bbllft. If we assume a 36-in. separator,

vL =[(2x3)/362]/1,029=4.8 ft/min.

SPE Drilling Engineering, December 1991

r:;::::::::I ~ -D

MUD & GAS MIXTURE

MUD

GAS

IMPINGEMENT PLATE

BAFFlES

Fig. 3-Float-type mud/gas separator.

We find that the gas migration rate is greater than the liquid veloc­ity in the separator, 8.4>4.8 ft/min. Therefore, a blow-through condition caused by insufficient separator cut does not exist.

Note that a separator cut < 100% frequently exists with mud/gas separators, and under some conditions, is not a major concern. As stated earlier, the mud/gas separator is designed to provide effec­tive separation of mud and gas with small amounts of entrained gas handled by a vacuum-type degasser located in the mud pits. Therefore, large active pit volumes may tolerate < 100% separa­tor cut.

Sizing Conclusion. Having evaluated sizing criteria for the mud/gas separator (Fig. 4), we may conclude that the separator is sized suffi­ciently to handle our worst-case kick properly.

OII·Based·Mud Considerations The effects of oil-based mud on the operation of the mud/gas sepa­ration can signifiantly affect sizing and design requirements. l

These concerns are currently being evaluated. However, some con­clusions can be made at this stage. 10

1-:-:-:-:1 MUD & GAS • - - - MIXTURE T _ MUD 4'

o GAS

27'

7 O' VENT LINE

Fig. 4-Mud/gas separator sizing.

SPE Drilling Engineering, December 1991

TABLE 1-MUD/GAS SEPARATOR SIZING WORKSHEET

Slow pump rate information, qslow Strokes per min psi bbl/stroke bbllmin

Mud/gas separator data Separator body 10, in. Gas vent-line 10, d j , in. Gas vent-line effective length,

La =L+Leq, Leq from Table 2, ft Kick data

Old mud weight, Ibm/gal Initial shut-in drillpipe pressure, psi Initial shut-in caSing pressure, psi Pit gain, bbl True vertical depth, ft

Peak gas-flow rate calculation Pcmax for driller's method, psi Volume of gas upstream of choke,

Vcmax , bbl Time to pump gas out of well,

t= Vcmax/qslow, minutes Volume of gas downstream of choke,

33 790

0.091 3.0

36 7.0

410

15.2 520 640 24

14,400

1,750

75.9

25.3

V c =Pcmax V cmax/Pc' bbl 9,036 Peak gas flow rate, qmax = Vc8085.61t, ft3/D 2,887,806

Vent-line friction-pressure calculation pf= (5.0 x 10 -12)(La)(qmax)2/d/, psi

Mud-leg calculation Minimum mud leg required, PI/gmt' ft

Separator 10 calculation Minimum separator 10, 15.56 X Jqslow (bbllmin), in.

1.0

3.8

27

If the mud/gas separator does not meet the sizing criteria, refer to the section on trouble-shooting for suggested modifications.

1. Gas kicks in oil-based mud can approach "possibly soluble" conditions while the kick is circulated from the well.

2. Gas kicks in oil-based mud that pass through the gas bub­blepoint while being circulated from the well can experience higher Pcmax and Vcmax values than were calculated for a kick of the same initial pit gain in a water-based mud. This results in higher peak gas flow rates through the separator and thus the requirement for a more stringent separator design.

3. Gas kicks in oil-based mud that do not pass through the gas bubblepoint until the gas is downstream of the choke will severely affect mud/gas separator sizing and design. Peak gas flow rates will be extremely high relative to those calculated for water-based mud

Well Data:

Straight Hole 14,400 It Casing 9%" x 8'12" at 12.200 It Shoe Test 16.6 Ibm/gal Mud WI. Equiv. BHA 310'-6'12" x 2";\." DC

465'-5" x 50.2 Ibm/It HWDP Drillpipe 5", 16.6 Ibm/It Mud Weight 15.2 Ibm/gal Pump 5'12" x 13" Triplex at

95% elf. (output - 0.091 bbllstroke) Pit Volume - 1,000 bbl

Kick Data :

Shul·ln Drlllpipe Pressure _ 520 psi Shul-in Casing Pressure - 640 psi Pit Gain - 24 bbl Slow Pump Rate

790 psi at 33 strokes/min (3 bblJmin)

Fig. 5-Well configuration.

281

TABLE 2-BEND/CORNER EQUIVALENT LENGTHS

SOURCE SKETCH EQUIV. LENGTH (FT)

BEND· ACUTE, rr=====:::::- 3 ROUND

BEND· ACUTE, fr=:::::::- 150 SHARP

BEND· RIGHT, (( 1 ROUND

BEND· RIGHT, II 70 SHARP

BEND· OBTUSE, ~ 1 ROUND

BEND - OBTUSE, ~ 15 SHARP

CONTRACTION, ~ - 1 GRADUAL --------CONTRACTION, ----- 10 -ABRUPT ----r-

EXPANSION, - 1 -GRADUAL ~

EXPANSION, --r--- 20 ABRUPT ------as outlined in this paper. Additional evaluation of the separator sizing should be completed if these well conditions exist.

Other Mud/Gas Separator Considerations 1 .4,8

Fig. 6 shows other separator components. A minimum 8-in.-ID mud return line is recommended for closed-bottom separators. Smaller lines may encounter problems with solids plugging the line. A larger­ID line would be considered beneficial. The impingement plate should be perpendicular to the separator inlet line and field replaceable.

Baffles within the separator should be located in the upper part of the separator and may continue into the lower part of the vessel. Typically, baffles consist of near-horizontal plates. The plates may be solid or have holes in them. The baffles should not impede the flow of liquid through the separator, which would cause fluid build­up above the baffles. Solids buildup in the baffles can also be a problem if the baffles are too restrictive.

An upper manway should be located on the upper part of the sepa­rator to permit visual inspection of the interior of the separator. The manway should be large enough to permit replacement of the impingement plate and equipped with a replaceable rubber seal to prevent leakage.

.. .. .. .. .. -~2' •• to t. 012*4,'7"

-Qu MignItoft Rite 500 ftlhr

..., 0.. ..., __ AMI 1000 ftJhr

.. 0. MIgtIIIOn RaIl 1100 M'Ir

-.. 0. ........ RMI 2000 M'lr

- <1M Migration RUe 2500 Mw

" OM _*kin RI" 3000 Mlr

Fig. 7-Effect of circulating kill rate on minimum separator 10.

282

I':':':':j .... ~ .. MUD & GAS MIXTURE

MUD

Fig. 6-Mudlgas separator components.

Closed-bottom mud/gas separators should be designed with a minimum 1-ft sump at the bottom of the vessel. The sump will help prevent solids from settling and plugging the mud-retum-line outlet.

A lower manway should be located on the lower part of the sepa­rator to permit sump cleanout or unplugging of the mud return line. The manway should be equipped with a replaceable rubber seal to prevent leakage.

The mud/gas separator should be equipped with a valved inlet on the lower section of the vessel to permit mud to be pumped into the separator. Mud can be pumped into the lower section of the separator during operation to decrease the possibility of solids settling in the mud return line. The valved inlet also permits clean­ing solids from the lower portion of the separator, especially after separator use.

A siphon breaker or antisiphon tube may be required to prevent having to siphon mud from the separator into the mud tanks, espe­cially with configurations that require the mud return line to be ex­tended below the separator elevation to allow mud to return to the mud tanks. The siphon breaker is simply an upward-directed open­ended pipe attached to the highest point of the mud return line.

All separators must be built in compliance with the ASME Boil­er and Pressure Vessel Code, Sec. VIII, Div. I with all materials

. meeting requirements of NACE Standard MROJ-75-8412 (1980 Revision). All welding on the vessel must meet ASME requirements.

12

10

friction • -(pol)

•

2

o 7 10

Fig. 8-Effect of kill rate on vent·llne friction pressure.

SPE Drilling Engineering. December 1991

18

16

14

12

P 10

ml (pel) 8

6

4

2

0

0

- mud 16 Ibmlgal

mud 14 Ibmlgal

mud 12 Ibm/gal

- saft water B.6lbmlgal

light oil 5 Ibm/gal -~--,- . -' --

---.... -

2 4 6 8 10 12 14 16 18 20

Mud Leg Helght(lI)

Fig. 9-Effect of mud-leg height on mud-leg hydrostatic pressure.

New mud/gas separators should be hydrostatically tested to 188 psi to give a maximum working pressure of 150 psi, as recom­mended by ASME. 11 Periodic nondestructive testing should in­clude radiographic examination of wall thickness and ultrasound verification of weld continuity. 12 At each initial hookup, every separator should be circulated through with water at the maximum possible flow rate to check for possible leaks in the connections. Frequency of testing should depend on anticipated and historical use of the separator.

Bracing the mud/gas separator has always been a major prob­lem. When gas reaches the surface, separators tend to vibrate and, if not properly supported, can move, resulting in near-catastrophic problems. Thus, it is critical that all mud/gas separators be suffi­ciently anchored and properly braced to prevent movement of both the separator body and the lines.

Trouble·Shootlng an Insufficiently Sized Separator Frequently, the situation arises where a mud/gas separator is picked up with the rig contract, and the drilling rig supervisor and engi­neer must evaluate the suitability of the separator for the wellioca­tion. This evaluation typically should be conducted during the rig bid analysis process. If the separator is insufficient or marginal, it may be more economical to upgrade the existing separator to meet the sizing criteria as an alternative to renting or building a suitable one.

Small Vessel ID. We frequently do our calculations and determine that our vessel ID is too small. Reducing the kill rate will improve this situation; e.g., if the kill rate for the previously sized separa­tor were reduced from 3 to 1.5 bbl/min, then from Eq. 7:

vL = [(2 x 1.5)/362]/1,029 =2.4 ft/min.

Thus, reducing the kill rate also reduces the liquid velocity rate in the separator, which increases the mud/gas retention time and improves the efficiency of mud/gas separation.

Also note that a gas migration rate of 500 ft/hr (8.4 ft/min) is a worst-case scenario and values could be higher. Therefore, when vessel ID is considered, a marginal separator probably would be sufficient because of this built-in safety factor. Higher gas migra­tion rates may also be used in the sizing procedure, as previously discussed. Fig. 7 shows the effect of kill rate on the calculation of minimum separator ID for different gas migration rates.

Vent-Line Friction Pressure Exceeds Mud-Leg Hydrostatic Pres­sure. Another area of concern is vent-line friction pressure exceed­ing mud-leg hydrostatic pressure, Pj > Pml' Several options exist to help alleviate this problem.

1. Reduce the circulating kill rate. As discussed previously, a reduction in the circulating kill rate may improve a separator's op­eration when vessel ID is considered and also when excessive vent­line friction pressures are considered. This reduction in kill rate may be the most economical solution to the sizing concern. For

SPE Drilling Engineering, December 1991

1.4

1.2

P 1 f

(pol) 0.8

0.6

0.4

0.2

~ ~ ~ ~ s _ ~ _ _ _

E_l.engtlt{n)

Fig. 10-Effect of effective length on vent-line friction pressure.

10

~ . (pol)

.. 5 6 7 10 11 12

Vont Une In_I Ol.me .... (In.)

Fig. 11-Effect of vent-line 10 on vent-line friction pressure.

example, if the kill rate for the previously sized separator were re­duced from 3 to 1.5 bbl/min, the peak gas flow rate would decrease. Combining Eqs. 1 and 3 and converting, we obtain

(=75.9/1.5=50.6 min

and qmax=9,036/50.6=1,443,903 ft31D.

This decrease in peak gas flow rate would significantly decrease the excessive vent-line friction pressure and improve the operation of the separator (Eq. 4).

Pj(5.0X 10-12 x410x 1,443,903)217.05 =0.25 psi.

Fig. 8 shows the effect of kill rate on the calculation of vent-line friction pressure for the previously sized separator.

2. Increase the mud leg. Another solution may be to increase the height of the mud leg. For example, if we increased the previously sized separator from a 7-ft mud leg to alOft mud leg, the mud-leg hydrostatic pressure should increase (Eq. 6).

Pml=lOxO.26=2.6 psi.

Thus, the mud-leg hydrostatic pressure increased from 1.8 to 2.6 psi, allowing the separator to operate more efficiently.

Fig. 9 shows the effect of mud-leg height on the calculation of mud-leg hydrostatic pressure for different mud-leg gradients. Note that the mud-leg height cannot exceed the separator height. The mud leg may also be restricted by bell-nipple elevation. If the mud leg is higher than the bell nipple, additional surface equipment may be required to permit the separator to operate when drilling with significant gas in the mud returns.

3. Adjust vent-line bends. As shown in Table 1, the type and number of bends in the vent line significantly affect the effective vent-line length, which in turn affects the calculation for vent-line friction pressure. If we were to replace the targeted T-bends on the previously sized separator with right-rounded bends, the cal-

283

Author

G.R. MacDougall Is a drilling engineer at Chevron Canada Resources Ltd. In calgary. Previously, he was an engineer at Chevron Services' Drilling Technolo­gy cantre. He holds a BS degree In min­Ing engineering from the Technical U. of Nova Scotia.

culations for the effective length (Eq. 5) and vent-line friction pres­sure (Eq. 4) would change:

Le =200+(3 X 1)=203 ft

and PI =(5.0x 10- 12 x203x2,887,806)217.05 =0.5 psi.

Hence, a vent-line friction-pressure decrease from 1.0 to 0.5 psi increases the efficiency of the separator for a given mud leg. In addition, the vent-line friction pressure increases proportionally to the effective length (Fig. 10).

4. Increase vent-line ID. Increasing the vent-line ID is generally the most expensive alternative but may be the only adjustment pos­sible to increase separator efficiency. Larger-ID vent lines will decrease the vent-line friction-pressure calculation. For the previ­ously sized separator, if an 8.0-in.-ID vent line were used, the cal­culation for vent-line friction pressure (Eq. 5) would change to

PI =(S.Ox 10- 12 X41OX2,887,806)217.05 =O.S psi.

Again, a vent-line friction-pressure decrease from 1.0 to 0.5 psi will increase separator efficiency for a given mud leg. Fig. 11 shows the effect of vent-line ID on the calculation of vent-line friction pres­sure for the previously sized separator.

Conclusions 1. The principle of mud/gas separation within most commonly

used mud/gas separators is identical. Differences can be found in the method of maintaining the mud leg.

2. A closed-bottom mud/gas separator is the preferred configu­ration. Open-bottom and float-type separators work well but are subject to limitations and prone to failure.

3. Sizing of a mud/ gas separator should be specific to individual well conditions.

4. Modeling of gas flow through a mud/gas separator can be ap­proximated by a simple procedure in a limited time.

S. A complete list of mud/gas separator components and con­siderations was compiled to assist with the design of mud/ gas sepa­rators.

6. A trouble-shooting guide was developed to address economi­cal upgrading of an existing insufficiently sized separator to meet sizing guidelines as an alternative to building or renting a new separator.

Nomenclatu ...

284

A = cross-sectional area of gas vent line, ft2 CqJ = separator capacity, bbl/ft

d j = gas vent-line ID, in. ds = separator ID, in. t = empirical friction factor, Ibm-min2/ft4

gml = mud-leg fluid gradient, psi/ft hml = mud-leg height, ft

L = gas vent-line length, ft Le = gas 'vent-line effective length, ft

Leq = equivalent length of bends, ft Pc = pressure of gas downstream of choke=atmospheric

pressure, 14.7 psi Pcmax = pressure of gas upstream of choke, psi

PI = gas vent-line friction pressure, psi Pml = mud-leg hydrostatic pressure, psi

q = gas flow rate, bbllmin qk = kill rate, bbl/min

qmax = peak gas flow rate through mud/gas separator, bbl/min or ft3fD

qslow = slow pump rate, psi s = gas vent-line perimeter, ft t = time venting gas at surface, minutes

vL = liquid velocity in the mud/gas separator, ft/min Vc = volume of gas downstream of choke, bbl

Vcmax = volume of gas upstream of choke, bbl

Acknowledgments I thank Chevron Services Co., Chevron Canada Resources, and Chevron's Drilling Technology Centre for their assistance and per­mission to write and publish this paper.

References 1. Turner, E.B.: "Well Control When Drilling With Oil-Based Mud,"

Offshore Technology Report OTH86260, U.K. Operations & Safety, Dept. of Energy, London (Oct. 1986).

2. Butchko, D. et al.: "Design of Atmospheric Open-Bottom Mud/Gas Separators," paper SPE 13485 presented at the 1985 SPEIIADC Drilling Conference, New Orleans, March 5-8.

3. Grigg, P.C.: "The Poor Boy Degasser as a Well Control TooI,'~ paper presented at the 1980 IADC/CAODC Drilling Technology Conference, Dallas, March 17-20.

4. Swaco Mud-Gas Separator Operation and Service Manual. Report No. 0380-0250, Dresser Industries Inc. (April 1982).

5. Brewton, I., Rau, W.E., and Dearing, H.L.: "Development and Use of a Drilling Applications Module for a Programmable Hand-Held Cal­culator," paper SPE 16657 presented at the 1987 SPE Annual Techni­cal Conference and Exhibition, Dallas, Sept. 27-30.

6. Engineering Data Book, ninth edition, Gas Processors Suppliers Assn., Tulsa (1979) Chap. 16, 1-41.

7. Hartman, H.L.: Mine Ventilation and Air Conditioning, John Wiley & Sons Inc., New York City (1982) 131-61.

8. Spec. 121, Specificationfor Oil and Gas Separators, sixth edition, API, Dallas (June 1, 1988).

9. Rader, D.W., Bourgoyne, A.T., and Ward, R.H.: "Factors Affecting Bubble-Rise Velocity of Gas Kicks," JPT (May 1975) 571-84.

10. O'Bryan, P.L. and Bourgoyne, A.T.: "Methods for Handling Drilled Gas in Oil-Based Drilling Fluids," SPEDE (Sept. 1989) 237-46.

11. Boiler and Pressure Code, Section Vlll Div. 1, Pressure Vessels, ASME, Dallas (Dec. 1989) 101-36.

12. Standard MROl-75-84, Material Requirement, Sulfide Stress Cracking Resistant Metallic Materials for Oil Field Equipment, NACE, Houston (Jan. 1984).

51 Metric Conversion Factor. bbl x 1.589.873 E-Ol m3

ft x 3.048* E-Ol m ft3 x 2.831685 E-02 m3 gal x 3.785412 E-03 m3

in. x 2.54* E+OO cm Ibm x 4.535924 E-Ol kg psi x 6.894757 E+OO kPa

• Conversion factor is exact. SPEDE

Original SPE manuscript received for review Sept. 2, 1990. Paper accepted for publication Sept. 30, 1991. Revised manuscript received Sept. 12, 1991. Paper (SPE 20430) first presented at the 1990 SPE Annual Technical Conference and Exhibition held in New Orleans, Sept. 23-26.

SPE Drilling Engineering, December 1991