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PETE 411

Well Drilling

Lesson 23Gas Cut Mud

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What is Gas Cut Mud?

After drilling through a formation containing gas, this “drilled gas” will show up in the mud returns at the surface.

Gas cut mud is mud containing some gas - from any source.

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Lesson 23 - Gas Cut Mud

Effect of Drilling Rate Effect of Circulation Rate Mud/Gas Ratio at the bottom of the Hole Mud/Gas Ratio at the Surface Density of Gas Cut Mud Reduction of Bottom Hole Pressure due to Gas Cut Mud Safe Drilling Practices

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Read:

Applied Drilling Engineering, Ch. 6

HW #12 – Csg. Design - due Nov. 1

HW #13 – dc - Exponent - due Nov. 6

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How Critical is Gas Cut Mud?

(1) Most people tend to overreact when gas reaches the surface.

It is at this time one should be calm and determine where the gas units came from.

Monitor the gas units response before reacting.

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How Critical is Gas Cut Mud? cont’d

(2) It is true that gas at the surface will tend to cut the mud weight substantially. This cut can be as much as 5 to 7 PPG.

But, it should be further realized that these cuts occur mainly in the top 200 feet of the hole with the worst cuts occurring in the top 50 feet.

Therefore, the overall hydrostatic head is only reduced by a small margin.

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How Critical is Gas Cut Mud? cont’d

(3) Many times when large volumes of gas reach the surface the well will appear to be flowing.

This is not necessarily due to a formation flowing or a kick, but may represent the extreme expansion of the gas near and at the surface.

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How Critical is Gas Cut Mud? cont’d

(4) The following example problem gives an indication of the effect of

reduction of mud weight at the surface

on

the reduction of hydrostatic head at the botton of the hole.

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How Critical is Gas Cut Mud?

Example Problem

Well depth = 15,000 ft

Hole size = 7 7/8”

Drill pipe size = 4 1/2”

Mud weight = 15 ppg

Drilling Rate = 20 ft/hr

Circ. rate = 7.0 bbl/min

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How Critical is Gas Cut Mud?

Formation Properties

F100 T

F250 T

1.35 Z

1 Z

25% Porosity Sand

70% saturation gas Sand

S

B

B

S

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Bottom-Hole Ratio of Mud Volume to Gas Volume:

This indicates there are 1990 volumes of mud to 1 volume of gas at the bottom of the hole.

1990

hrbbl

0.2110

hrbbl

420

gs 0.7*porosity 25.0*cu.ft 61.5

bbl*

hrft 20

in/ft 12

in 87

7

4

hrmin 60

*minbbl 7

Gas

Mud2

Mud

Gas

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Ratio of surface volume of gas to bottom-hole volume of gas:

This shows there are 465 volumes of gas at the surface per volume of gas at the bottom of the hole

465 )R(710psi)(1.35) 7.14(

)R0psi)(1)(56 (11,700

law) (gas T

T

Z

Z

P

P

V

V

B

S

B

S

S

B

B

S

(PV = ZnRT)

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Mud/gas Volume Ratio at the Surface:

279.4465

1990

VolumeGas

VolumeMud:surface At

990,1VolumeGas

VolumeMud:BottomAt

465BottomatGas

SurfaceatGas:Expansion

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Mud Density at the Surface:

So the mud weight has been cut 2.84 ppg

(from 15 to 12.16) ppg

ppg 16.121279.4

ppg0)*1(ppg 15*(4.279)

Volume

Density) udsurface)(M @ vol vol/gas(

Total

Mudsurf

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It should be noted that in actual situations the mud cut would probably be less because we have assumed all gas stays in the mud-gas mixture. A certain amount of gas will break out.

The effects of gas cut mud on the hydrostatic head:

S

SB

SS

AASred.gas P

PPln

TC)Z(100

TZCPΔP

Mud Density at the Surface:

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R re, temperatuSurface - T

factorility compressib Surface - Z

R re, temperatuAverage - T

factorility compressib Average - Z

psi pressure, Surface - P

surface at the fluid totalof % Gas - C

wellof bottomat pressure cHydrostati - P

S

S

A

A

S

B

S

SB

SS

AASred.gas P

PPln

TC)Z(100

TZCPΔP

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18.94%4.2791

100%*1

mud of vol.gas of Vol.

100%*gas of Vol.C

psi 11,700ft 15000*ppg 15*0.052PB

Hydrostatic Pressure and C

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Average T and Z

175.12

35.11Z

6352

560710T

A

A

R

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Reduction in BHP

psi 30.57 ΔP

14.7

14.711,700ln

560)18.94)(1)((100

(635).7)(1.175)(18.94)(14ΔP

red.gas

red.gas

S

SB

SS

AASred.gas P

PPln

TC)Z(100

TZCPΔP

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The resulting bottom hole pressure will be

p = 11,700 - 30.57

BHP = 11,669 psi

This means the gas reduced the hydrostatic head by only 30.57 psi!

Reduction in BHP

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Conclusion

It can be seen that the surface gas cut of approx. 3 PPG resulted in a bottom hole pressure reduction of less than 31 psi.

There is one other factor that reduces the effect of gas cut mud even further and that is the effect of drilled solids in the mud. Drilled solids will tend to raise the overall density of the mud.

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Drilled Cuttings Effect on Hydrostatic head:

factors conversion*fraction solid*ROP*4

D π

unit timeper cut solids of Vol.2

gpm 0.632 generation solids drilled of Rate

in 144

ft 1*

ft

gal7.481*0.75*

min/hr 60

ft/hr 20*

4

π(7.875)

2

3

3

2

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Drilled Cuttings Effect on Hydrostatic head:

ppg 15.015

gpm 294 gpm 632.0

ppg 15*gpm 294 ppg 22.1*gpm 632.0

volumetotal

mud of weight solids ofweight wt.mud Average

AVG

P = 12 psi

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Drilled Cuttings Effect on Hydrostatic head:

In this problem, the cuttings had very little effect on the hydrostatic head.

But, if the rate of penetration were higher, the additional density added due to the drill solids could become significant.

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Summary of Gas-Cut Mud Problem

At bottom:

Gas expansion:

990,1rate generation gas

rate ncirculatio mud

465bottomat volume

surfaceat volume

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Summary of Gas-Cut Mud Problem

At surface:

i.e. At the surface, the mud mix contains one part of gas (by volume) for each 4.279 parts of good mud.

279.4465

990,1

rate ncirculatio gas

ncirculatio mud

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Summary of Gas-Cut Mud Problem

Density of mix

1279.4)0*1()15*279.4(

volume total

weighttotal

Density of Mud at surface = 12.16 #/gal

(-2.84 lb/gal)

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Summary of Gas-Cut Mud Problem

psi 31

P

PPln

TZ)C100(

TZCPp

S

SB

SS

AAS

A reduction in the mud density at the surface by 2.84 lb/gal resulted in a reduction in BHP of:

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Note:

It is very important in any drilling operation:

To recognize the symptoms of increasing pore pressure

To be able to estimate the magnitude of the pore pressure

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Note cont’d:

To know the fracture gradients of the exposed formations

To maintain the drilling practices within controllable limits

To keep in mind that any one symptom of increasing pore pressure may not be sufficient to provide the basis for precise conclusions

Look at all the indicators...

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ROP F.L.Temp Cl - MUD t

d Gas Units SH YP

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What should be done when gas cut mud is encountered?

(1) Establish if there is any fire hazard. If there is a fire hazard, divert flow

through mud-gas separation facilities.

(a) Notify any welder in area

(b) Notify all rig personnel of the pending danger

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What should be done when gas cut mud is encountered?

(2) Determine where the gas came from.

If the casing seat fracture gradient is being approached, and there is some concern about raising the mud weight:

Stop drilling and circulate, and observe the gas response. If source is drilled gas, the gas rate will decrease.

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What should be done when gas cut mud is encountered?

(a) If the gas units completely return to the original background gas, it would probably be safe to resume drilling.

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What should be done when gas cut mud is encountered?

(b) If there has been ample circulation time and the gas units do not drop back to the original background level, but stay at a higher value, this indicates that the mud weight is approaching the pore pressure and consideration should be given to increasing the mud weight.

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What should be done when gas cut mud is encountered?

Establish Where did the gas come from?

(a) Drilled gas - no increase in mud weight is required

(b) Increasing pore pressure - (abnormal pore pressure) - May have to increase mud weight

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Drilling Techniques

I. Balanced Drilling

Balanced drilling by definition is when the hydrostatic head is equal to the pore pressure in the formation being drilled

In the Gulf Coast area, if the hydrostatic head is 0 - 0.4 ppg over the actual pore pressure it is usually considered to be balanced drilling.

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Drilling Techniques - Balanced Drilling

Advantages to balanced drilling conditions

Optimizes the drilling rate

Lithology changes can be detected immediately from the ROP

curve

Transition zones can be detected sooner

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Drilling Techniques - Balanced Drilling

Disadvantages

There is no room for error

The wellbore must be carefully and continuously monitored for

the first sign of formation pressure increase

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Drilling Techniques - Balanced Drilling

Application of balanced drilling

Balanced drilling is generally used for wildcat or exploratory

drilling

It is often used in hard rock formation drilling to optimize the rate of

penetration

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Drilling Techniques

II. Overbalanced drilling

Overbalanced drilling by definition is when the pressure exerted by the

hydrostatic head exceeds the formation pore pressure

In the Gulf Coast region, if the mud weight is 0.4 ppg or more above the

pore pressure, it is considered overbalanced drilling

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Drilling Techniques - Overbalanced

Advantages to drilling overbalanced

Reduces the chance of swabbing a well in or taking a kick

Disadvantages

Overbalanced drilling reduces the rate of penetration

substantially

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Drilling Techniques - Overbalanced

Disadvantages to drilling overbalanced -cont’d

Drilling too far overbalanced can disguise lithology changes and transition zones

Differential sticking can be caused by the excessive pressure differential between the mud hydrostatic and the pore pressure

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Drilling Technique - Overbalanced

Application of overbalanced drilling

This is most often used in areas of development drilling. In

such areas, the pore pressures are generally known and the mud weights are maintained high enough to ensure ~ never taking a kick or swabbing a well in. But, at the same time the mud weights are maintained low enough so as not to cause differential sticking

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Drilling Techniques - Underbalanced

III. Underbalanced drilling

Underbalanced drilling by definition is when the pressure exerted by the hydrostatic head of the mud is less than the pore pressure

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Drilling Techniques - Underbalanced

Advantages of underbalanced drilling

Increased rate of penetration

Less formation damage due to mud filtrate or whole mud

loss

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Drilling Techniques - Underbalanced

Disadvantages of underbalanced drilling

Possible kicks

Wells can be swabbed in more readily

Wellbore formation cave-ins

(wellbore stability)

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Drilling Techniques - Underbalanced

Application of underbalanced drilling

Underbalanced drilling is applied in areas that are very hard to drill such as some areas in West Texas. This is done to increase the rate of penetration. Note that this technique is used in areas that have very tight and competent formations. The tight formations reduce the chance of taking a kick...

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Drilling Techniques - Underbalanced

Application of underbalanced drilling cont’d

Competent formations have less tendency to slough or cave-in to the wellbore due to the absence of a sufficient hydrostatic head to hold it back.

Horizontal wells in the Austin Chalk

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Drilling Techniques - Controlled

IV. Controlled Drilling

Controlled drilling, by definition, is when a constant rate of penetration is maintained by fluctuating the weight on bit

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Drilling Techniques - Controlled

Advantages of controlled drilling

Control of gumbo problems Reduces cuttings generation rate Reduces drilled gas problems

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Drilling Techniques - Controlled

Disadvantages of controlled drilling

This drilling technique disguises lithology changes

Furthermore, and most importantly, it disguises transition zones

and makes it almost impossible to detect these from the

penetration rate curve.

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Drilling Techniques - Controlled

Application of controlled drilling

This should only be used when necessary and prudent, such as in troublesome gumbo sections where the pore pressures are well known

Note: This drilling technique should never be used when drilling in wildcat areas or areas where the pore pressures are not known.