23. gas cut mud

23. Gas Cut Mud PETE 411 Well Drilling Slide 1 of 53

PETE 411Well Drilling

Lesson 23Gas Cut Mud


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.


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


Read:Applied Drilling Engineering, Ch. 6

HW #11Slip Velocity due 10-24-01


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.


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 occuring in the top 50 feet. Therefore, the overall hydrostatic head is only reduced by a small margin.



(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 represents the extreme expansion of the gas near and at the surface.



(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.



Example Problem

Well depth = 15,000 ftHole size = 7 7/8”Drill pipe size = 4 1/2”Mud weight = 15 ppgDrilling Rate = 20 ft/hrCirc. rate = 7.0 bbl/min



Formation Properties

F100 T

F250 T

1.35 Z1 Z

25% Porosity Sand70% saturation gas Sand

S

B

B

S


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

hrbbl0.2110

hrbbl420

gs 0.7*porosity 25.0*cu.ft 61.5

bbl*hr

ft 20in/ft 12

in 877

4

hrmin 60*

minbbl 7

GasMud

2

MudGas


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 TT

ZZ

PP

VV

B

S

B

S

S

B

B

S

(PV = ZnRT)


Mud/gas Volume Ratio at the Surface:

279.4465

1990 VolumeGas VolumeMud:surface At

990,1VolumeGasVolumeMud:BottomAt

465BottomatGasSurfaceatGas:Expansion


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


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:


R re, temperatuSurface - T

factorility compressib Surface - ZR re, temperatuAverage - T

factorility compressib Average - Zpsi 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


18.94%4.2791100%*1

mud of vol.gas of Vol.

100%*gas of Vol.C

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

Hydrostatic Pressure and C


Average T and Z

175.12

35.11Z

6352

560710T

A

A

R


Reduction in BHP

psi 30.57 ΔP

14.714.711,700ln

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


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


Conclusion

It can be seen that the surface gas cut of approx. 3 PPG resulted in a bottom hole pressure reduction of only 30.57 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.


Drilled Cuttings Effect on Hydrostatic head:

factors conversion*fraction solid*ROP*4D π

unit timeper cut solids of Vol.2

gpm 0.632 generation solids drilled of Rate

in 144ft 1*

ftgal7.481*0.75*

min/hr 60ft/hr 20*

4π(7.875) 2

3

3

2



ppg 15.015

gpm 294 gpm 632.0ppg 15*gpm 294 ppg 22.1*gpm 632.0

volumetotalmud of weight solids ofweight wt.mud Average

AVG

P = 12 psi



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.


Summary of Gas-Cut Mud Problem

At bottom:

Gas expansion:

990,1rate generation gasraten circulatio mud

465bottomat volumesurfaceat volume



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.

279.4465990,1

raten circulatio gasncirculatio mud



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)



psi 31

PPPln

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:


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


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...


ROP F.L.Temp Cl - MUD t d Gas Units SH YP


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



(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.



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



(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.



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


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.


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



Disadvantages

There is no room for error

The wellbore must be continuously monitored for the first sign

of formation pressure increase



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


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


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


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 hydrostatic and the pore pressure


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


Drilling Techniques - Underbalanced

III. Underbalanced drilling

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



Advantages of underbalanced drilling

Increased rate of penetration

Less formation damage due to mud filtrate or whole mud

loss



Disadvantages of underbalanced drilling

Possible kicks

Wells can be swabbed in more readily

Wellbore formation cave-ins (wellbore stability)



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...



Application of underbalanced drilling cont’d

The 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


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



Advantages of controlled drilling

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



Disadvantages of controlled drilling

This drilling technique disguises lithology changes

Furthermore, and most important, it disguises transition zones

and makes it almost impossible to detect these from the

penetration rate curve.



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.

23. gas cut mud

Documents

ss bs sa

gas cut mud

cut mud problem

pore pressure

pore pressures

gas units

competent

pressure exerted