# PETE 411 Well Drilling Lesson 23 Gas Cut Mud.

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PETE 411 Well Drilling Lesson 23 Gas 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 #12 – Csg. Design - due Nov. 1 HW #13 – dc - Exponent - due Nov. 6

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

How Critical is Gas Cut Mud? cont’d
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.

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.

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

How Critical is Gas Cut Mud?
Formation Properties

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.

Ratio of surface volume of gas to bottom-hole volume of gas:
(PV = ZnRT) This shows there are 465 volumes of gas at the surface per volume of gas at the bottom of the hole

Mud/gas Volume Ratio at the Surface:

Mud Density at the Surface:
So the mud weight has been cut 2.84 ppg (from 15 to ) ppg

Mud Density at the Surface:
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:

Hydrostatic Pressure and C

Average T and Z

Reduction in BHP

Reduction in BHP This means the gas reduced the hydrostatic head by only psi! The resulting bottom hole pressure will be p = 11, BHP = 11,669 psi

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.

Drilled Cuttings Effect on Hydrostatic head:

Drilled Cuttings Effect on Hydrostatic head:
DBHP = 12 psi

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.

Summary of Gas-Cut Mud Problem
At bottom: Gas expansion:

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 parts of good mud.

Summary of Gas-Cut Mud Problem
Density of mix Density of Mud at surface = #/gal (-2.84 lb/gal)

Summary of Gas-Cut Mud Problem
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 DCl rMUD Dt d Gas Units rSH 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

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.

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.

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.

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

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 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

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

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

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 mud 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 of the mud is less than the pore pressure

Drilling Techniques - Underbalanced
Advantages of underbalanced drilling Increased rate of penetration Less formation damage due to mud filtrate or whole mud loss

Drilling Techniques - Underbalanced
Disadvantages of underbalanced drilling Possible kicks Wells can be swabbed in more readily Wellbore formation cave-ins (wellbore stability)

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...

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

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

Drilling Techniques - Controlled
Advantages of controlled drilling Control of gumbo problems Reduces cuttings generation rate Reduces drilled gas problems

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.

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.