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by Hans C. Juvkam-Wold Lesson 6

by Hans C. Juvkam-Wold Lesson 6. Dual Gradient Drilling Basic Technology. Gas Kick Behavior. Contents. 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

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by Hans C. Juvkam-Wold Lesson 6

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  1. by Hans C. Juvkam-WoldLesson 6 Dual Gradient DrillingBasic Technology Gas Kick Behavior Confidential to DGD JIP

  2. Contents • 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 Confidential to DGD JIP

  3. 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) Confidential to DGD JIP

  4. 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) Confidential to DGD JIP

  5. 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 = ? Confidential to DGD JIP

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

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

  8. 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 oR) Confidential to DGD JIP

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

  10. 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 oR and bottomhole temp. = 540 + 0.01 * 10,000 = 640 oR At the bottom of the hole, P = 6,000 psia, T = 640 oR, and V = 10 bbl At the surface, P = 1,000 psia, T = 540 oR, and V = ? Confidential to DGD JIP

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

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

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

  14. Gas Kick Behavior - cont’d Shallow Kick - Ideal Gas SOLUTION METHOD 2: Summary Temperature Kick Volume Gradient at Surface 0.00 deg F/ft 60.00 bbls 0.01 deg F/ft 50.63 bbls 0.02 deg F/ft 43.78 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%. Confidential to DGD JIP

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

  16. Gas Kick Behavior - cont’d Shallow Kick - Real Gas SOLUTION METHOD 3: PV = ZnRT When the temperature gradient = 0.02 deg F/ft Surface conditions: 540 oR and 1,000 psia Bottomhole conditions: 740 oR 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. Confidential to DGD JIP

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

  18. Gas Kick Behavior - cont’d Shallow Kick - Real Gas SOLUTION METHOD 3: PV = ZnRT So, the 60 bbl estimate is within a factor of 2 of the above value Confidential to DGD JIP

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

  20. 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. Confidential to DGD JIP

  21. 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. Confidential to DGD JIP

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

  23. Gas Kicks in Deep DGD Wells Assumed Pressure Profile in Annulus and Return Line 0 5,000 Mud Line 10,000 Vertical Depth, ft 15,000 20,000 25,000 30,000 0 5,000 10,000 15,000 20,000 25,000 Kick Pressure, psig Confidential to DGD JIP

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

  25. Gas Kicks in Deep DGD Wells PV = ZnRT PV = constant Confidential to DGD JIP

  26. Gas Kicks in Deep DGD Wells Mud Line Confidential to DGD JIP

  27. Gas Kicks in Deep DGD Wells Confidential to DGD JIP

  28. Gas Kick Behavior - Z-Factor Confidential to DGD JIP

  29. 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! Confidential to DGD JIP

  30. 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…(?) Confidential to DGD JIP

  31. 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! Confidential to DGD JIP

  32. 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. Confidential to DGD JIP

  33. 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! Confidential to DGD JIP

  34. 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! Confidential to DGD JIP

  35. 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... Confidential to DGD JIP

  36. 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. Confidential to DGD JIP

  37. 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! Confidential to DGD JIP

  38. 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: GGAS = 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? DP = 0.052 * (15 - 3) * 1,000 = 624 psi Confidential to DGD JIP

  39. Seawater Hydrostatic 624 psi BOP Annulus Mud Hydrostatic w/kick DEPTH DGD Mud Hydrostatic wo/kick Kick PRESSURE Static Pressures - DGD Confidential to DGD JIP

  40. 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? Confidential to DGD JIP

  41. 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? Confidential to DGD JIP

  42. 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 Confidential to DGD JIP

  43. Summary • 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 Confidential to DGD JIP

  44. Summary - cont’d • 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 Confidential to DGD JIP

  45. Summary - cont’d • 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 Confidential to DGD JIP

  46. by Hans C. Juvkam-WoldNovember 2000The End Dual Gradient DrillingBasic Technology 6. Gas Kick Behavior Confidential to DGD JIP

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