1. 6. Gas Kick Behavior Confidential to DGD JIP Slide 1 of 48 by Hans C. Juvkam-Wold Lesson 6 Gas Kick Behavior Dual Gradient Drilling Basic Technology

2. 6. Gas Kick Behavior Confidential to DGD JIP Slide 2 of 48 Gas Kicks in Shallow Wells The “PV = constant” Assumption - Is it valid? The Perfect Gas Law: “PV = nRT ” The Real Gas Law: “PV = ZnRT. “ Z-Factor Gas Kicks in Deepwater Wells Effect of Temp. and Pressure on Real Gases Gas Kick Volume and Density forrReal Gases Pumping Gas with the MLP Solubility of Gas in Oil or Synthetic Based Mud Contents

3. 6. Gas Kick Behavior Confidential to DGD JIP Slide 3 of 48 Gas Kicks in Shallow Wells What is the volume of a gas kick as it is being circulated out of the hole under the following assumptions: Initial Kick Size = 10 bbl Stabilized BHP = 6,000 psia (absolute) Well Depth = 10,000 ft Maximum Choke Pressure = 1,000 psia (when the kick arrives at the surface choke)

4. 6. Gas Kick Behavior Confidential to DGD JIP Slide 4 of 48 Gas Kick Behavior - cont’d Gas Kicks in Shallow Wells What is the volume of a gas kick as it is being circulated out of the hole under the above assumptions? SOLUTION METHOD 1: Assume PV = constant (i.e., assume perfect gas and ignore any changes in temperature)

5. 6. Gas Kick Behavior Confidential to DGD JIP Slide 5 of 48 Gas Kick Behavior - cont’d Gas Kicks in Shallow Wells SOLUTION METHOD 1: PV = constant At the bottom, P = 6,000 psia, V = 10 bbl At the surface, P = 1,000 psia, V = ?

6. 6. Gas Kick Behavior Confidential to DGD JIP Slide 6 of 48 Gas Kick Behavior - cont’d Gas Kicks in Shallow Wells SOLUTION METHOD 1: ASSUME “PV = constant” i.e., so, V SURFACE = 60 bbl Kick expands from 10 bbls to 60 bbls.

7. 6. Gas Kick Behavior Confidential to DGD JIP Slide 7 of 48 Gas Kick Behavior - cont’d Gas Kicks in Shallow Wells SOLUTION METHOD 1: PV = constant. Maximum Choke Pressure = 1,000 psia

8. 6. Gas Kick Behavior Confidential to DGD JIP Slide 8 of 48 Gas Kick Behavior - cont’d Shallow Kick - Ideal Gas What is the volume of a kick as it is being circulated out of the hole under the above assumptions? SOLUTION METHOD 2: Assume PV = nRT (i.e., assume perfect gas. Note that the temperature must be expressed in o R)

9. 6. Gas Kick Behavior Confidential to DGD JIP Slide 9 of 48 Gas Kick Behavior - cont’d Shallow Kick - Ideal Gas SOLUTION METHOD 2: PV = nRTalso, o F + 460 = o R Let us assume that the surface temperature is 80 o F. 80 + 460 = 540 o R so, surface temperature = 540 o R Let us consider three different temperature gradients: 0.00, 0.01 and 0.02 o F / ft 0.00 o F / ft is the same as assuming PV = const.

10. 6. Gas Kick Behavior Confidential to DGD JIP Slide 10 of 48 Gas Kick Behavior - cont’d Shallow Kick - Ideal Gas SOLUTION METHOD 2A: PV = nRT When temperature gradient = 0.01 deg F/ft then surface temperature = 540 o R and bottomhole temp. = 540 + 0.01 * 10,000 = 640 o R At the bottom of the hole, P = 6,000 psia, T = 640 o R, and V = 10 bbl At the surface, P = 1,000 psia, T = 540 o R, and V = ?

11. 6. Gas Kick Behavior Confidential to DGD JIP Slide 11 of 48 Gas Kick Behavior - cont’d Shallow Kick - Ideal Gas ALTERNATE SOLUTION METHOD 2A: PV = nRT 0.01 deg F/ft V SURFACE = 50.63 bbl

12. 6. Gas Kick Behavior Confidential to DGD JIP Slide 12 of 48 Gas Kick Behavior - cont’d Shallow Kick - Ideal Gas SOLUTION METHOD 2B: When temperature gradient = 0.02 deg F/ft Surface temperature = 540 o R and bottomhole temp. = 540 + 0.02 * 10,000 = 740 o R Bottom: P = 6,000 psia, T = 740 o R, and V = 10 bbl Surface: P = 1,000 psia, T = 540 o R, and V = ?

13. 6. Gas Kick Behavior Confidential to DGD JIP Slide 13 of 48 Gas Kick Behavior - cont’d Shallow Kick - Ideal Gas ALTERNATE SOLUTION METHOD 2B: PV = nRT 0.02 deg F/ft V SURFACE = 43.78 bbl

14. 6. Gas Kick Behavior Confidential to DGD JIP Slide 14 of 48 Gas Kick Behavior - cont’d Shallow Kick - Ideal Gas SOLUTION METHOD 2: Summary Temperature Kick Volume Gradientat Surface 0.00 deg F/ft60.00 bbls 0.01 deg F/ft50.63 bbls 0.02 deg F/ft43.78 bbls Assuming a zero temperature gradient, when the actual temperature gradient was 0.02 deg F/ft resulted in overestimating the kick volume at the surface by 37%.

15. 6. Gas Kick Behavior Confidential to DGD JIP Slide 15 of 48 Gas Kick Behavior - cont’d Shallow Kick - Ideal Gas 0.00 deg F/ft 0.02 deg F/ft 0.01 deg F/ft

16. 6. Gas Kick Behavior Confidential to DGD JIP Slide 16 of 48 Gas Kick Behavior - cont’d Shallow Kick - Real Gas SOLUTION METHOD 3: PV = ZnR T When the temperature gradient = 0.02 deg F/ft Surface conditions: 540 o R and 1,000 psia Bottomhole conditions: 740 o R and 6,000 psia Under these conditions, assuming a gas of S.G. = 0.65): the Z-factor at the surface = 0.852 (density = 0.510 ppg) the Z-factor at the bottom = 1.100 (density = 1.731 ppg) These Z-factor values may be obtained by calculation, or, approximately, from the graph on the next page.

17. 6. Gas Kick Behavior Confidential to DGD JIP Slide 17 of 48 Gas Kick Behavior - cont’d Z-Factor - In Shallow Wells

18. 6. Gas Kick Behavior Confidential to DGD JIP Slide 18 of 48 Gas Kick Behavior - cont’d Shallow Kick - Real Gas SOLUTION METHOD 3: PV = ZnR T So, the 60 bbl estimate is within a factor of 2 of the above value

19. 6. Gas Kick Behavior Confidential to DGD JIP Slide 19 of 48 Gas Kick Behavior - cont’d Shallow Gas Kick - Summary Real Gas Ideal Gas PV = constant 0.02 deg F/ft

20. 6. Gas Kick Behavior Confidential to DGD JIP Slide 20 of 48 Gas Kick Behavior In the previous slides we have studied the behaviour of gas kicks in relatively shallow wells. We saw, in one case, when a temperature gradient of 0.02 deg F/ft was assumed, the predicted kick volume at the surface dropped from 60 bbs to 44 bbls. The initial kick volume was 10 bbls at a depth of 10,000 ft. When a correction for variation in Z-Factor was added, the more accurate prediction was 34 bbls at the surface. The predicted gas volumes varied by a factor of TWO or less in every case investigated.

21. 6. Gas Kick Behavior Confidential to DGD JIP Slide 21 of 48 Gas Kicks in Deep DGD Wells The main reason why the predicted results varied by no more than a factor of two in the cases studied is that the Z-factor was always close to 1 ( ± 20% ). In deep-water, very deep, high-pressure wells the Z- factor may vary from 0.7 to 2.5 or even more! This may yield unexpected results.

22. 6. Gas Kick Behavior Confidential to DGD JIP Slide 22 of 48 Gas Kicks in Deep DGD Wells Gas Density, lb/gal Z-Factor 0.65 S.G. and 400 0 F

23. 6. Gas Kick Behavior Confidential to DGD JIP Slide 23 of 48 Gas Kicks in Deep DGD Wells Assumed Pressure Profile in Annulus and Return Line 0 5,000 10,000 15,000 20,000 25,000 30,000 05,00010,00015,00020,00025,000 Kick Pressure, psig Vertical Depth, ft Mud Line

24. 6. Gas Kick Behavior Confidential to DGD JIP Slide 24 of 48 Gas Kicks in Deep DGD Wells As expected, most of the expansion occurs in the top 3,000 ft or so

25. 6. Gas Kick Behavior Confidential to DGD JIP Slide 25 of 48 Gas Kicks in Deep DGD Wells PV = constant PV = ZnRT

26. 6. Gas Kick Behavior Confidential to DGD JIP Slide 26 of 48 Gas Kicks in Deep DGD Wells Mud Line

27. 6. Gas Kick Behavior Confidential to DGD JIP Slide 27 of 48 Gas Kicks in Deep DGD Wells

28. 6. Gas Kick Behavior Confidential to DGD JIP Slide 28 of 48 Gas Kick Behavior - Z-Factor

29. 6. Gas Kick Behavior Confidential to DGD JIP Slide 29 of 48 Gas Kicks in Deep DGD Wells In the last few slides we have seen the behavior of gas kicks in deepwater, deep DGD wells. We saw that a 10-bbl gas kick at 30,000 ft was predicted, under the “PV = constant” assumption, to expand to 46 bbls by the time it reached the inlet to the MLP at the seafloor. When corrections for variations in Z-Factor and temperature were added, the more accurate prediction was 13 bbls at the MLP. The predicted gas expansion decreased from 360% to a mere 30% in the more accurate analysis!

30. 6. Gas Kick Behavior Confidential to DGD JIP Slide 30 of 48 Kicks Migration in Deep DGD Wells The predicted gas expansion decreased from 360% to a mere 30% in the more accurate analysis. Why is this significant? Well, it helps to know what to expect. For example, suppose this 10-bbl kick were to migrate up the hole under conditions where circulation was not possible. We would expect to bleed to allow for kick expansion to avoid excessive pressures in the wellbore. In this case we might expect to have to bleed 36 bbls when only 3 bbls are called for. Excessive bleeding could invite additional kicks. Maybe NO bleeding is really necessary in this case…(?)

31. 6. Gas Kick Behavior Confidential to DGD JIP Slide 31 of 48 Pumping of Gas with MLP In DGD gas kicks that are circulated out must pass through the MLP. Can this pump handle gas? How severe is the problem? What can we expect? Under the “PV = const.” assumption the 10-bbl gas kick would have to be compressed from 46 bbl to approximately 24 bbl. That can be done... The more accurate analysis says that the gas only needs to be compressed from 13 to 11 bbl! That is much less challenging!

32. 6. Gas Kick Behavior Confidential to DGD JIP Slide 32 of 48 Pumping of Gas with MLP What happens to pump efficiency as we try to pump gas? Should we expect “gas lockup”? In our example DGD well the pressure increase across the MLP is from 4,520 to 8,460 psi. If the pump is 100% efficient then there is no problem; when pumping gas the efficiency is still 100%. Let us consider a more modest pump efficiency of 90%. By that we mean that the “piston” sweeps 90% of the volume inside the pump. 10% remains in the pump.

33. 6. Gas Kick Behavior Confidential to DGD JIP Slide 33 of 48 Pumping of Gas with MLP Let us first consider the “PV = constant” case. In this case we ended up compressing the gas from 46 bbl to 24 bbl. During the first part of the stroke the gas is being compressed and nothing comes out. At the end of the stroke 10% of the pump volume still contains gas. At the beginning of the next stroke this 10% expands to 10 * 46/25) = 18.4% of the pump volume. 100 - 18.4 = 81.6 The resulting pump efficiency is therefore reduced from 90% to 81.6%. That would seem acceptable!

34. 6. Gas Kick Behavior Confidential to DGD JIP Slide 34 of 48 Pumping of Gas with MLP Let us now consider the “Real Gas” case. (PV = ZnRT) In this case we ended up compressing the gas from 13 bbl to 11 bbl. As before, at first gas is being compressed and nothing comes out. At the end of the stroke 10% of the pump volume still contains gas. At the beginning of the next stroke this 10% expands to 10 * 13/11) = 11.8% of the pump volume. 100 - 11.8 = 88.2 The resulting pump efficiency is therefore reduced from 90% to 88.2%. Hardly even noticable!

35. 6. Gas Kick Behavior Confidential to DGD JIP Slide 35 of 48 Pumping of Gas with MLP Two factors may further reduce this potential problem: 1. The actual MLP we’ll be using will probably have a volumetric efficiency in excess of 95%. In this case the remaining 5% expands to 5 * 13/11) = 5.9% of the pump volume. 100 - 5.9 = 94.1 The resulting pump efficiency is therefore reduced from 95% to 94.1%. LESS THAN 1% LOSS!! 2. The above calculations assumed that 100% pure gas would arrive at the pump. Dilution with mud will usually reduce this %age by a significant factor, further increasing efficiency...

36. 6. Gas Kick Behavior Confidential to DGD JIP Slide 36 of 48 Pumping of Gas with MLP Note that because of the high pump efficiency there is no significant reduction in the fluid circulation rate in the annulus! In extreme cases it may be necessary to speed up the pump very slightly in order to follow the drill pipe pressure decline schedule. There is a slight reduction in volumetric rate in the return line because of gas compression. There is no reduction in the average mass circulation rate in the return line! It remains the same as in the annulus.

37. 6. Gas Kick Behavior Confidential to DGD JIP Slide 37 of 48 Pumping of Gas with MLP So, what ever happened to “gas lockup”? In DGD it is unlikely that we shall see a volumetric compression requirement much greater than a factor of two. Usually it will be much less. However, let us imagine a situation where the volumetric compression requirement is a factor of 10, and the pump volumetric efficiency is 90%: In this case the 10% that remains in the pump will expand to 10% * 10 =100%. In other words, the left-over gas will completely fill the pump at the next stroke. No gas is pumped. We would have achieved gas lockup!

38. 6. Gas Kick Behavior Confidential to DGD JIP Slide 38 of 48 Gas Gradients What is the pressure gradient in a gas at very high pressure? How does it affect wellbore pressures? At very high pressure the density may very well be as high as 3 lb/gal. This would correspond to a gradient of: G GAS = 0.052 * 3 = 0.156 psi/ft Consider a large gas kick that occupies 1,000 ft of the annulus, when drilling with 15 lb/gal mud. After pressures have stabilized, what is the increase in pressure at inlet to the MLP?  P = 0.052 * (15 - 3) * 1,000 = 624 psi

39. 6. Gas Kick Behavior Confidential to DGD JIP Slide 39 of 48 BOP Static Pressures - DGD PRESSURE DEPTH Seawater Hydrostatic DGD Mud Hydrostatic wo/kick 624 psi Annulus Mud Hydrostatic w/kick Kick

40. 6. Gas Kick Behavior Confidential to DGD JIP Slide 40 of 48 Solubility of Gas Kick in Oil or Synthetic Based Drilling Fluids We know from experience that, at relatively low pressures a gas kick may seem to disappear by dissolving into the mud? As the kick gets close to the surface some or even most of the gas may come out of solution and present some unpleasant surprises. If we are drilling in a deep DGD well with an oil or synthetic based drilling fluid, what should we expect?

41. 6. Gas Kick Behavior Confidential to DGD JIP Slide 41 of 48 Solubility of Gas Kick in Oil or Synthetic Based Drilling Fluids Will a gas kick disappear by dissolving into the mud in a deep DGD well? If we take a 10-bbl gas kick while drilling with a water-based drilling fluid we would expect to see a 10-bbl pit gain If we take a 10-bbl gas kick while drilling with an oil or synthetic based drilling fluid, would the pit gain be close to 10 bbl or closer to 1 bbl?

42. 6. Gas Kick Behavior Confidential to DGD JIP Slide 42 of 48 Solubility of Gas Kick in Oil or Synthetic Based Drilling Fluids If the kick takes place at high pressure in a deep DGD well the pit gain would probably be closer to 9 bbl! A 3 lb/gal gas kick behaves more like a liquid than a gas, and this fluid would mix with the drilling mud without substantial loss of volume The final mixture would have a density close to the weighted average of the two fluids

43. 6. Gas Kick Behavior Confidential to DGD JIP Slide 43 of 48 The “PV = constant” assumption appears to be more or less acceptable in evaluating shallow gas kicks. It could be off by a factor of two The Perfect Gas Law: “PV = nRT ” improves on our predictions by including the effect of temperature The Real Gas Law: “PV = ZnRT ” is required if we want to predict accurately the behavior of real gases in deep DGD wells Summary

44. 6. Gas Kick Behavior Confidential to DGD JIP Slide 44 of 48 The “ Z-Factor ” is a factor that distinguishes between real gases and ideal gases The Z-factor has a value near 1.0 under atmospheric conditions. It can vary from 0.7 to 2.5 or more Below 7,000 psi an increase in temperature increases the Z-factor Above 8,000 psi an increase in temperature decreases the Z-factor Summary - cont’d

45. 6. Gas Kick Behavior Confidential to DGD JIP Slide 45 of 48 Gas expansion in deep DGD wells is only a small fraction of what we might expect from shallow-well experience with gas kicks The density of a gas at 20,000 psi may be as high as 3 lb/gal or even higher! This gas behaves more like a liquid than a gas. At high pressures a Gas kick mixes with Oil or Synthetic Based Mud with little change in volume Summary - cont’d

46. 6. Gas Kick Behavior Confidential to DGD JIP Slide 46 of 48 by Hans C. Juvkam-Wold November 2000 The End 6. Gas Kick Behavior Dual Gradient Drilling Basic Technology