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8/13/2019 625 Lesson 2A
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AT M PETE 625 AT M
AT M A T M
Lesson 2A
Kicks & Gas Migration
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
Kicks & Gas Migration
Density of real gasses Equivalent Mud Weight (EMW) Wellbore pressure before and after kick Gas migration rate - first order approx.
Gas migration rate - w/mud compressibility
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
Kicks & Gas Migration
Read Ch 1 & 2 Watson Ch 8 Schubert Homework: 2.1-2.10
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
Density of Real Gasses
M = molecular weight m = mass
n = no. of moles gg = S.G. of gas
ZRT
p
M M
M
ZRT pM
V M
ZRT pV
ZRT pV
n
V nM
V m
g g
air g
g
g
g
g
29
29
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
Density of Real Gasses
What is the density of a 0.6 gravity gas at10,000 psig and 200 degF?
From Lesson 2, Fig. 1 p pr = p/p pc = 10,015/671 = 14.93 T
pr = (200+460)/358 = 1.84
Z = 1.413
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
Density of Real Gasses
g = (29*0.6*10,015)/(1.413*80.28*660)
g = 2.33 ppg
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
Equivalent Mud Weight, EMW
The pressure, p (psig) in a wellbore, at adepth of x (ft) can always be expressed interms of an equivalent mud density orweight.
EMW = p/(0.052*x) in ppg
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
Depth Pressureft psia
0 500 010 505.98 10
100 559.8 100150 589.7 150200 619.6 200300 679.4 300
400 739.2 400500 799 5001000 1098 10002000 1696 20003000 2294 30004000 2892 40005000 3490 50006000 4088 60007000 4686 70008000 5284 80009000 5882 9000
10000 6480 1000011000 7078 11000
11600 7436.8 11600
0
2000
4000
6000
8000
10000
12000
0 2000 4000 6000 8000
After Kick
Before Kick
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
Gas Migration
Gas generally has a much lower density than thedrilling 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 (ignoringtemp, fluid loss to formation, compressibility ofgas, mud, and formation)
This causes pressures everywhere in the wellboreto increase.
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
Gas Migration
Problem 2.6: A 0.7 gravity gas bubbleenters the bottom of a 9000 ft vertical well
when the drill collars are being pulledthrough the rotary table. Flow is noted andthe well is shut in with an initial recorded
casing pressure of 50 psig. Influx height is350 ft, Mud weight =9.6 ppg.
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
Gas Migration Assume surface temperature of 70 deg F.
Temp gradient = 1.1 deg F/100 ft. Surface pressure =14 psia
Determine the final casing pressure if thegas bubble is allowed to reach the surfacewithout expanding
Determine the pressure and equivalentdensity at total depth under these finalconditions
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
Solution
First assumption - is that BHP is brought tothe surface
Pressure at the top of the bubble =14 + 50+.052*9.6*(9000-350) = 4378 psia T9000 = 70 + (1.1/100)*(9000 - 350) = 629 deg R
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
Solution
p pc = 666 psia T pc = 389 deg R p pr = 4378/666 = 11.08 T pr = 629/389 = 1.62
Z = 0.925
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
Solution
Assume, at first, that Z f = 1.0 (at thesurface)
Then, 4378 * V = p f * V________0.925 * 629 1.0 * (70 + 460)
so, p f = 3988 psia
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AT M PETE 625 AT M
AT M A T MHarold Vance Department of
Petroleum Engineering
Solution
At surface: P pr = 3988 / 666 = 6.00
T pr = 530 / 389 = 1.36 Zf = 0.817
Pf = 3258 psia
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Solution
A few more iterative steps result in Zf = 0.705 and p f = 2812 psia
At the surface f = 0.7*2812/(2.77*0.705*530) = 1.9 ppg
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Solution
BHP =2812+0.052*1.9*350+.052*9.6*9650
BHP = 7160 psia
EMW = (7160 - 14)/(0.052 * 9000) EMW = 15.3 ppg
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Compression of Mud in Annulus
DV = compressibility * volume * D p = -6 * 10 -6 (1/psi) * 0.1(9000-350)*2626
DV = -13.63 bbls Initial kick volume = 0.1 * 350 = 35 bbls New kick volume = 35 + 13.63 = 48.63 bbl
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Compression of Mud in Annulus
From Boyles Law, p2 * 48.63 = 2815 * 35
p2 = 2024 psia p8650 poA PoB PoCConsider: V,p,Z const. P,Z change mud comp.
2nd iteration ? . 3rd or, Is there a better way?
TPETE 625T
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Gas Migration Rate
A well is shut in after taking a 30 bbl kick.The SIDPP appears to stabilize at 1000
psig. One hour later the pressure is 2000 psig.
Ann Cap = 0.1 bbl/ft
MW = 14 ppg TD = 10,000
TPETE 625T
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Gas Migration Rate
How fast is the kick migrating?
What assumptions do we need to make?
TPETE 625T
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
1 hr
TPETE 625T
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
First Attempt
If the kick rises x ft. in 1 hr and the pressurein the kick = constant, then the pressure
increases everywhere, D p = 0.052*14*x
x = (2000-1000)/(0.052*14) x = 1374 ft Rise velocity = 1374 ft/hr
TPETE 625T
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Gas Migration Rate Field rule of thumb ~ 1000 ft/hr Laboratory studies ~ 2000 - 6000 ft/hr
Who is right?
Field results?
Is the previous calculation correct?
TPETE 625T
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Second Attempt
Consider mud compressibility Ann cap = 0.1 bbl/ft * 10000 ft= 1000 bbl of mud Volume change due to compressibility and
increase in pressure of 1000 psi, DV = 6*10 -6 (1/psi) * 1000 psi * 1000 bbl
= 6 bbl
AT MPETE 625AT M
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Second Attempt
i.e. gas could expand by 6 bbl, to 36 bbl Initial kick pressure=1000 + 0.052 * 14 * 10000= 8280 psig= 8295 psia
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Second Attempt
How far did it migrate in 1 hour? The pressure reduction in kick fluid=8260-6621=1659 psi The kick must therefore have risen an
additional x 2 ft, biven by:1659 = 0.052 * 14 * x 2 x2 = 2279 ft
AT MPETE 625AT M
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Second Attempt
2nd estimate = 1374 + 22793653 ft/hr What if the kick size is only 12 bbl? What about balooning of the wellbore? What about fluid loss to permeable
formations?
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AT MPETE 625AT M
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Solution
hr f t v
v
hr t t f t
psi g
psi p pv
slip
slip
slip
/1154
5.00.10052.0
500800
12
12
Ignoringcompressibility andother effects
What factors affect gasslip velocity, ormigration rate?
AT MPETE 625AT M
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Gas slip velocity
The bubble size, and the size of the gas voidfraction, will influence bubble slip velocity.
The void fraction is defined as the ratio(or percentage) of the gas cross-sectionalarea to the total flow area.
AT MPETE 625AT M
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Gas slip velocity
AT MPETE 625AT M
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AT M PETE 625 AT M
AT M A T MHarold Vance Department ofPetroleum Engineering
Gas slip velocity
Bubbles with a voidfraction > 25% assumea bullet nose shapeand migrate upwardsalong the high side ofthe wellboreconcurrent with liquid
backflow, on theopposite side of thewellbore
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