2 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.
3 Lesson 23 - Gas Cut Mud Effect of Drilling Rate Effect of Circulation RateMud/Gas Ratio at the bottom of the HoleMud/Gas Ratio at the SurfaceDensity of Gas Cut MudReduction of Bottom Hole Pressure due to Gas Cut MudSafe Drilling Practices
4 Read: Applied Drilling Engineering, Ch. 6 HW #12 – Csg. Design - due Nov. 1 HW #13 – dc - Exponent - due Nov. 6
5 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.
6 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.
7 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.
8 How Critical is Gas Cut Mud? cont’d (4) The following example problem gives an indication of the effect ofreduction of mud weight at the surfaceonthe reduction of hydrostatic head at the botton of the hole.
9 How Critical is Gas Cut Mud? Example ProblemWell 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
10 How Critical is Gas Cut Mud? Formation Properties
11 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.
12 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
14 Mud Density at the Surface: So the mud weight has been cut 2.84 ppg(from 15 to ) ppg
15 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:
20 Reduction in BHPThis means the gas reduced the hydrostatic head by only psi!The resulting bottom hole pressure will bep = 11,BHP = 11,669 psi
21 ConclusionIt 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.
24 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.
25 Summary of Gas-Cut Mud Problem At bottom:Gas expansion:
26 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.
27 Summary of Gas-Cut Mud Problem Density of mixDensity of Mud at surface = #/gal(-2.84 lb/gal)
28 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:
29 Note: It is very important in any drilling operation: To recognize the symptoms of increasing pore pressureTo be able to estimate the magnitude of the pore pressure
30 Note cont’d: To know the fracture gradients of the exposed formations To maintain the drilling practices within controllable limitsTo keep in mind that any one symptom of increasing pore pressure may not be sufficient to provide the basis for precise conclusionsLook at all the indicators...
32 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
33 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.
34 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.
35 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.
36 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
37 Drilling Techniques I. Balanced Drilling Balanced drilling by definition is when the hydrostatic head is equal to the pore pressure in the formation being drilledIn the Gulf Coast area, if the hydrostatic head is ppg over the actual pore pressure it is usually considered to be balanced drilling.
38 Drilling Techniques - Balanced Drilling Advantages to balanced drilling conditionsOptimizes the drilling rateLithology changes can be detected immediately from the ROP curveTransition zones can be detected sooner
39 Drilling Techniques - Balanced Drilling DisadvantagesThere is no room for errorThe wellbore must be carefully and continuously monitored for the first sign of formation pressure increase
40 Drilling Techniques - Balanced Drilling Application of balanced drillingBalanced drilling is generally used for wildcat or exploratory drillingIt is often used in hard rock formation drilling to optimize the rate of penetration
41 Drilling Techniques II. Overbalanced drilling Overbalanced drilling by definition is when the pressure exerted by the hydrostatic head exceeds the formation pore pressureIn the Gulf Coast region, if the mud weight is 0.4 ppg or more above the pore pressure, it is considered overbalanced drilling
42 Drilling Techniques - Overbalanced Advantages to drilling overbalancedReduces the chance of swabbing a well in or taking a kickDisadvantagesOverbalanced drilling reduces the rate of penetration substantially
43 Drilling Techniques - Overbalanced Disadvantages to drilling overbalanced cont’dDrilling too far overbalanced can disguise lithology changes and transition zonesDifferential sticking can be caused by the excessive pressure differential between the mud hydrostatic and the pore pressure
44 Drilling Technique - Overbalanced Application of overbalanced drillingThis 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
45 Drilling Techniques - Underbalanced III. Underbalanced drillingUnderbalanced drilling by definition is when the pressure exerted by the hydrostatic head of the mud is less than the pore pressure
46 Drilling Techniques - Underbalanced Advantages of underbalanced drillingIncreased rate of penetrationLess formation damage due to mud filtrate or whole mud loss
47 Drilling Techniques - Underbalanced Disadvantages of underbalanced drillingPossible kicksWells can be swabbed in more readilyWellbore formation cave-ins(wellbore stability)
48 Drilling Techniques - Underbalanced Application of underbalanced drillingUnderbalanced 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...
49 Drilling Techniques - Underbalanced Application of underbalanced drilling cont’dCompetent 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
50 Drilling Techniques - Controlled IV. Controlled DrillingControlled drilling, by definition, is when a constant rate of penetration is maintained by fluctuating the weight on bit
51 Drilling Techniques - Controlled Advantages of controlled drillingControl of gumbo problemsReduces cuttings generation rateReduces drilled gas problems
52 Drilling Techniques - Controlled Disadvantages of controlled drillingThis drilling technique disguises lithology changesFurthermore, and most importantly, it disguises transition zones and makes it almost impossible to detect these from the penetration rate curve.
53 Drilling Techniques - Controlled Application of controlled drillingThis should only be used when necessary and prudent, such as in troublesome gumbo sections where the pore pressures are well knownNote: This drilling technique should never be used when drilling in wildcat areas or areas where the pore pressures are not known.