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PETE 625 Well Control

2. Contents. Density of real gases Equivalent Mud Weight (EMW) Wellbore pressure before and after kick Gas migration rate - first order approx. Gas migration rate

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PETE 625 Well Control

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    1. PETE 625 Well Control Lesson 3 Kicks and Gas Migration

    2. 2 Contents Density of real gases Equivalent Mud Weight (EMW) Wellbore pressure before and after kick Gas migration rate - first order approx. Gas migration rate – with temperature, mud compressibility and Z-factor considerations

    3. Assignments Homework #2: Ch 1, Problems 1.11-1.21 Read: All of Chapter 1

    4. 4 Density of Real Gases M = molecular weight m = mass n = no. of moles gg = S.G. of gas

    5. 5 Density of Real Gases What is the density of a 0.6 gravity gas at 10,000 psig and 200 oF? From Lesson 2, Fig. 1 ppr = p/ppc = 10,015/671 = 14.93 Tpr = (200+460)/358 = 1.84 Z = 1.413

    6. 6

    7. 7 Density of Real Gases rg = 2.33 ppg

    8. 8 Equivalent Mud Weight, EMW The pressure, p (psig) in a wellbore, at a depth of x (ft) can always be expressed in terms of an equivalent mud density or weight. EMW = p / (0.052 * x) in ppg

    9. 9 EMW EMW is the density of the mud that, in a column of height, x (ft) will generate the pressure, p (psig) at the bottom, if the pressure at top = 0 psig or, at TD: p = 0.052 * EMW * TVD

    10. 10

    11. 11

    12. 12 Gas Migration Gas generally has a much lower density than the drilling mud in the well, causing the gas to rise when the well is shut in. Since the gas, cannot expand in a closed wellbore, it will maintain its pressure as it rises (ignoring temp, fluid loss to formation, compressibility of gas, mud, and formation) This causes pressures everywhere in the wellbore to increase.

    13. 13

    14. 14 Gas Migration Example 1.7: A 0.7 gravity gas bubble enters the bottom of a 9,000 ft vertical well when the drill collars are being pulled through the rotary table. Flow is noted and the well is shut in with an initial recorded casing pressure of 50 psig. Influx height is 350 ft. Mud weight = 9.6 ppg.

    15. 15 Gas Migration Assume surface temperature of 70 oF. Temp gradient = 1.1 oF/100 ft. Surface pressure = 14 psia Determine the final casing pressure if the gas bubble is allowed to reach the surface without expanding Determine the pressure and equivalent density at total depth under these final conditions

    16. 16 Gas Properties at Bottom First assumption: BHP is brought to the surface Pressure at the top of the bubble P8,650 = 14 + 50 + 0.052 * 9.6 * (9,000-350) = 4,378 psia T9,000 = 70 + (1.1/100) * 9,000 + 460 = 629 oR

    17. 17 Gas Properties at Bottom ppc = 666 psia Tpc = 389 deg R ppr = 4,378/666 = 6.57 Tpr = 629/389 = 1.62 Z = 0.925

    18. 18 Bottomhole Pressure rg = 29*0.7*4,378 / (0.925 * 80.28 * 629) = 1.90 ppg DpKICK = 0.052 * 1.9 * 350 = 35 psi BHP = 4,378 + 35 BHP = 4,413 psia (~surface press.?

    19. 19 Pressure at Surface Assume, at first, that Zf = 1.0 (at the surface) Then,

    20. 20 Solution with Z-factor Corr. At surface: ppr = 3,988 / 666 = 6.00 Tpr = 530 / 389 = 1.36 Zf = 0.817 p0 = 3,258 psia

    21. 21 Solution with Z-factor A few more iterative steps result in Z0 = 0.705 and p0 = 2,812 psia At the surface rf = 29*0.7*2,812 / (0.705*80.28*530) = 1.9 ppg

    22. 22 New BHP & EMW New BHP = 2,812 + 0.052 * 1.9 * 350 + 0.052 * 9.6 * 8,650 New BHP = 7,165 psia EMW = (7,165 - 14)/(0.052 * 9,000) EMW = 15.3 ppg

    23. 23

    24. 24 Compression of Mud in Annulus vA = 0.1 bbl/ft) DV = compressibility * volume * Dp = -6 * 10-6 (1/psi) * 0.1(9,000-350)*2,626 DV = -13.63 bbls Initial kick volume = 0.1 * 350 = 35 bbls New kick volume = 35 + 13.63 = 48.63 bbl

    25. 25 Compression of Mud in Annulus From Boyle’s Law, pV = const p2 * 48.63 = 2,812 * 35 p2 = 2,024 psia p8650 poA poB poC Consider: V,p,Z const. p,Z change mud comp. 2nd iteration ? ……………. 3rd or, Is there a better way?

    26. 26 Gas Migration Rate A well is shut in after taking a 30 bbl kick. The SIDPP appears to stabilize at 1,000 psig. One hour later the pressure is 2,000 psig. Ann Cap = 0.1 bbl/ft MW = 14 ppg TVD = 10,000 ft

    27. 27 Gas Migration Rate How fast is the kick migrating? What assumptions do we need to make to analyze this question?

    28. 28

    29. 29 First Attempt If the kick rises x ft. in 1 hr and the pressure in the kick = constant, then the pressure increases everywhere, Dp = 0.052 * 14 * x x = (2,000 - 1,000) / (0.052 * 14) x = 1,374 ft Rise velocity = 1,374 ft/hr

    30. 30 Gas Migration Rate Field rule of thumb ~ 1,000 ft/hr Laboratory studies ~ 2,000 – 6,000 ft/hr Who is right? Field results? Is the previous calculation correct?

    31. 31 Second Attempt Consider mud compressibility Ann. capacity = 0.1 bbl/ft * 10,000 ft = 1,000 bbl of mud Volume change due to compressibility and increase in pressure of 1,000 psi, DV = 6*10-6 (1/psi) * 1,000 psi * 1,000 bbl = 6 bbl

    32. 32 Second Attempt i.e. gas could expand by 6 bbl, to 36 bbl Initial kick pressure =1,000 + 0.052 * 14 * 10,000 (approx.) = 8,280 psig = 8,295 psia

    33. 33 Second Attempt A 20% expansion would reduce the pressure in the kick to ~ 0.8*8,295 = 6,636 psia = 6,621 psig So, the kick must have migrated more than 1,374 ft!

    34. 34 Second Attempt How far did it migrate in 1 hour? The pressure reduction in kick fluid = 8,260 - 6,621=1,659 psi The kick must therefore have risen an additional x2 ft, given by: 1,659 = 0.052 * 14 * x2 x2 = 2,279 ft

    35. 35 Second Attempt 2nd estimate = 1,374 + 2,279 = 3,653 ft/hr What if the kick size is only 12 bbl? What about balooning of the wellbore? What about fluid loss to permeable formations? T? Z?...

    36. 36

    37. 37 Example 1.9 Kick occurs. After shut-in, initial csg. Press = 500 psig. 30 minutes later, p = 800 psig What is the slip velocity if the kick volume remains constant? MW = 10.0 ppg

    38. 38 Simple Solution

    39. 39 Gas slip velocity The bubble size, and the size of the gas void fraction, will influence bubble slip velocity. The “void fraction” is defined as the ratio (or percentage) of the gas cross-sectional area to the total flow area.

    40. 40 Gas slip velocity

    41. 41 Gas slip velocity Bubbles with a void fraction > 25% assume a bullet nose shape and migrate upwards along the high side of the wellbore concurrent with liquid backflow, on the opposite side of the wellbore

    42. 42 Gas slip velocity Large bubbles rise faster than small bubbles Other factors: Density differences Hole geometry Mud viscosity Circulation rate Hole inclination One lab study showed max. rate at 45o.

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