Upload
george-moody
View
219
Download
1
Embed Size (px)
Citation preview
Confidential to DGD JIPConfidential to DGD JIP Slide 1 of 486. Gas Kick Behavior
by Hans C. Juvkam-Wold
Lesson 6
Gas Kick Behavior
Dual Gradient DrillingBasic Technology
Confidential to DGD JIPConfidential to DGD JIP Slide 2 of 486. Gas Kick Behavior
• 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
Confidential to DGD JIPConfidential to DGD JIP Slide 3 of 486. Gas Kick Behavior
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 JIPConfidential to DGD JIP Slide 4 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dGas 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 JIPConfidential to DGD JIP Slide 5 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dGas 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 JIPConfidential to DGD JIP Slide 6 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dGas Kicks in Shallow Wells
SOLUTION METHOD 1:
ASSUME “PV = constant”
i.e.,
so, VSURFACE = 60 bbl
Kick expands from 10 bbls to 60 bbls.
BOTTOMHOLESURFACE PVPV
BOTTOMHOLESURFACE 10*000,6V000,1
Confidential to DGD JIPConfidential to DGD JIP Slide 7 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dGas Kicks in Shallow WellsSOLUTION METHOD 1: PV = constant.
0
2,000
4,000
6,000
8,000
10,000
12,000
0 20 40 60 80
Kick Volume, bbl
De
pth
, f
t
Maximum Choke
Pressure= 1,000 psia
Confidential to DGD JIPConfidential to DGD JIP Slide 8 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dShallow 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 JIPConfidential to DGD JIP Slide 9 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dShallow Kick - Ideal Gas
SOLUTION METHOD 2:
PV = nRT also, oF + 460 = oR
Let us assume that the surface temperature is 80 oF. 80 + 460 = 540 oR
so, surface temperature = 540 oR
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 JIPConfidential to DGD JIP Slide 10 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dShallow 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 JIPConfidential to DGD JIP Slide 11 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dShallow Kick - Ideal Gas
ALTERNATE SOLUTION METHOD 2A:
PV = nRT
0.01 deg F/ft
VSURFACE = 50.63 bbl
BOTTOMHOLESURFACE nRT
PV
nRT
PV
BOTTOMHOLESURFACE 640
10*000,6
540
V000,1
BOTTOMHOLESURFACE T
PV
T
PV
Confidential to DGD JIPConfidential to DGD JIP Slide 12 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dShallow 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 JIPConfidential to DGD JIP Slide 13 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dShallow Kick - Ideal Gas
ALTERNATE SOLUTION METHOD 2B:
PV = nRT
0.02 deg F/ft
VSURFACE = 43.78 bbl
BOTTOMHOLESURFACE nRT
PV
nRT
PV
BOTTOMHOLESURFACE 740
10*000,6
540
V000,1
BOTTOMHOLESURFACE T
PV
T
PV
Confidential to DGD JIPConfidential to DGD JIP Slide 14 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dShallow Kick - Ideal Gas
SOLUTION METHOD 2: Summary
Temperature Kick Volume
Gradient at Surface0.00 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 JIPConfidential to DGD JIP Slide 15 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dShallow Kick - Ideal GasEffect of Temperature Gradient
0
2,000
4,000
6,000
8,000
10,000
12,000
0 10 20 30 40 50 60 70
Kick Volume, bbls
De
pth
, ft
0.00 deg F/ft
0.02 deg F/ft
0.01 deg F/ft
Confidential to DGD JIPConfidential to DGD JIP Slide 16 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dShallow Kick - Real Gas
SOLUTION METHOD 3: PV = ZnR T
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 JIPConfidential to DGD JIP Slide 17 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dZ-Factor - In Shallow Wells
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000
Pressure, psig
Ga
s C
om
pre
ss
ibili
ty F
ac
tor
(Z-F
ac
tor)
30 F
60 F
100 F
150 F
200 F
300 F
400 F
Confidential to DGD JIPConfidential to DGD JIP Slide 18 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dShallow Kick - Real Gas
SOLUTION METHOD 3: PV = ZnR T
BOTTOMHOLESURFACE ZnRT
PV
ZnRT
PV
BOTTOMHOLESURFACE740*100.1
10*000,6
540*852.0
V000,1
bbl91.33VSURFACE
So, the 60 bbl estimate is within a factor of 2 of the above value
Confidential to DGD JIPConfidential to DGD JIP Slide 19 of 486. Gas Kick Behavior
Gas Kick Behavior - cont’dShallow Gas Kick - Summary
Effect of Temperature and Z-Factor
0
2,000
4,000
6,000
8,000
10,000
12,000
0 10 20 30 40 50 60 70
Kick Volume, bbls
De
pth
, ft
Real Gas Ideal Gas PV = constant
0.02 deg F/ft
Confidential to DGD JIPConfidential to DGD JIP Slide 20 of 486. Gas Kick Behavior
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 JIPConfidential to DGD JIP Slide 21 of 486. Gas Kick Behavior
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% ).
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000
Pressure, psig
Ga
s C
om
pre
ss
ibili
ty F
ac
tor
(Z-F
ac
tor)
30 F
60 F
100 F
150 F
200 F
300 F
400 F
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.
0.5
1.0
1.5
2.0
2.5
3.0
0 5,000 10,000 15,000 20,000 25,000
Pressure, psig
Co
mp
ress
ibili
ty (
Z-F
acto
r)
3060
100150200300
400
Temperature, oF
Confidential to DGD JIPConfidential to DGD JIP Slide 22 of 486. Gas Kick Behavior
Gas Density and Z-Factor
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
0 5,000 10,000 15,000 20,000 25,000 30,000
Pressure, psia
Gas D
en
sit
y,
lb
/gal
Gas Kicks in Deep DGD Wells
Gas Density, lb/gal
Z-Factor
0.65 S.G. and 400 0F
Confidential to DGD JIPConfidential to DGD JIP Slide 23 of 486. Gas Kick Behavior
Gas Kicks in Deep DGD Wells
Assumed Pressure Profilein Annulus and Return Line
0
5,000
10,000
15,000
20,000
25,000
30,000
0 5,000 10,000 15,000 20,000 25,000
Kick Pressure, psig
Ver
tica
l D
epth
, f
t
Mud Line
Confidential to DGD JIPConfidential to DGD JIP Slide 24 of 486. Gas Kick Behavior
Gas Kicks in Deep DGD Wells
Kick Volume vs. Depth0
5,000
10,000
15,000
20,000
25,000
30,000
0 5 10 15 20 25 30
Kick Volume, bbl
De
pth
, f
t
As expected, most of the expansion occurs
in the top 3,000 ft or so
Confidential to DGD JIPConfidential to DGD JIP Slide 25 of 486. Gas Kick Behavior
Kick Volume vs. Depth0
5,000
10,000
15,000
20,000
25,000
30,000
0 5 10 15 20 25 30 35 40 45 50
Kick Volume, bbl
De
pth
, f
tGas Kicks in Deep DGD Wells
PV = constant
PV = ZnRT
Confidential to DGD JIPConfidential to DGD JIP Slide 26 of 486. Gas Kick Behavior
Gas Kicks in Deep DGD Wells
Gas Density vs. Depth0
5,000
10,000
15,000
20,000
25,000
30,000
0.0 1.0 2.0 3.0 4.0
Gas Density, lb/gal
De
pth
, f
t
Mud Line
Confidential to DGD JIPConfidential to DGD JIP Slide 27 of 486. Gas Kick Behavior
Gas Kicks in Deep DGD Wells
Z-Factor vs. Depth0
5,000
10,000
15,000
20,000
25,000
30,000
0.0 0.5 1.0 1.5 2.0 2.5
Gas Compressibility Factor (Z-Factor)
Dep
th,
ft
Confidential to DGD JIPConfidential to DGD JIP Slide 28 of 486. Gas Kick Behavior
Gas Kick Behavior - Z-Factor
0.5
1.0
1.5
2.0
2.5
3.0
0 5,000 10,000 15,000 20,000 25,000
Pressure, psig
Co
mp
ressib
ility
(Z
-Facto
r)
3060
100150200300
400
Temperature, oF
Confidential to DGD JIPConfidential to DGD JIP Slide 29 of 486. Gas Kick Behavior
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 JIPConfidential to DGD JIP Slide 30 of 486. Gas Kick Behavior
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 JIPConfidential to DGD JIP Slide 31 of 486. Gas Kick Behavior
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...
Kick Volume vs. Depth0
5,000
10,000
15,000
20,000
25,000
30,000
0 5 10 15 20 25 30 35 40 45 50
Kick Volume, bblD
ep
th,
ft
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 JIPConfidential to DGD JIP Slide 32 of 486. Gas Kick Behavior
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 JIPConfidential to DGD JIP Slide 33 of 486. Gas Kick Behavior
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 JIPConfidential to DGD JIP Slide 34 of 486. Gas Kick Behavior
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 JIPConfidential to DGD JIP Slide 35 of 486. Gas Kick Behavior
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 JIPConfidential to DGD JIP Slide 36 of 486. Gas Kick Behavior
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 JIPConfidential to DGD JIP Slide 37 of 486. Gas Kick Behavior
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 JIPConfidential to DGD JIP Slide 38 of 486. Gas Kick Behavior
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?
P = 0.052 * (15 - 3) * 1,000 = 624 psi
Confidential to DGD JIPConfidential to DGD JIP Slide 39 of 486. Gas Kick Behavior
BOP
Static Pressures - DGD
PRESSURE
DE
PT
H
Seawater Hydrostatic
DGD MudHydrostatic
wo/kick
624 psi
Annulus MudHydrostatic
w/kick
Kick
Confidential to DGD JIPConfidential to DGD JIP Slide 40 of 486. Gas Kick Behavior
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 JIPConfidential to DGD JIP Slide 41 of 486. Gas Kick Behavior
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 JIPConfidential to DGD JIP Slide 42 of 486. Gas Kick Behavior
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 JIPConfidential to DGD JIP Slide 43 of 486. Gas Kick Behavior
• 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
Confidential to DGD JIPConfidential to DGD JIP Slide 44 of 486. Gas Kick Behavior
• 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
Confidential to DGD JIPConfidential to DGD JIP Slide 45 of 486. Gas Kick Behavior
• 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
Confidential to DGD JIPConfidential to DGD JIP Slide 46 of 486. Gas Kick Behavior
by Hans C. Juvkam-WoldNovember 2000
The End6. Gas Kick Behavior
Dual Gradient DrillingBasic Technology