Slide 1 of 48

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by Hans C. Juvkam-WoldLesson 6
Dual Gradient DrillingBasic Technology

• Gas Kick Behavior

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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

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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)

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Gas Kick Behavior - contd
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)

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Gas Kick Behavior - contd
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 ?
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Gas Kick Behavior - contd
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.
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Gas Kick Behavior - contd
Gas Kicks in Shallow Wells
SOLUTION METHOD 1 PV constant.
Maximum Choke Pressure 1,000 psia
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Gas Kick Behavior - contd
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)

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Gas Kick Behavior - contd
Shallow Kick - Ideal Gas
SOLUTION METHOD 2 PV nRT also, oF 460
oR Let us assume that the surface
temperature is 80 oF. 80 460 540 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.
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Gas Kick Behavior - contd
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 ?
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Gas Kick Behavior - contd
Shallow Kick - Ideal Gas
ALTERNATE SOLUTION METHOD 2A PV nRT 0.01 deg
F/ft VSURFACE 50.63 bbl
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Gas Kick Behavior - contd
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 ?
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Gas Kick Behavior - contd
Shallow Kick - Ideal Gas
ALTERNATE SOLUTION METHOD 2B PV nRT 0.02 deg
F/ft VSURFACE 43.78 bbl
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Gas Kick Behavior - contd
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.
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Gas Kick Behavior - contd
Shallow Kick - Ideal Gas
0.02 deg F/ft
0.00 deg F/ft
0.01 deg F/ft
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Gas Kick Behavior - contd
Shallow 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.
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Gas Kick Behavior - contd
Z-Factor - In Shallow Wells
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Gas Kick Behavior - contd
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
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Gas Kick Behavior - contd
Shallow Gas Kick - Summary
0.02 deg F/ft
Real Gas Ideal Gas PV
constant
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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.
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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.
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Gas Kicks in Deep DGD Wells
0.65 S.G. and 400 0F
Gas Density, lb/gal
Z-Factor
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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
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Gas Kicks in Deep DGD Wells
As expected, most of the expansion occurs in the
top 3,000 ft or so
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Gas Kicks in Deep DGD Wells
PV ZnRT
PV constant
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Gas Kicks in Deep DGD Wells
Mud Line
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Gas Kicks in Deep DGD Wells
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Gas Kick Behavior - Z-Factor
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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!
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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(?)
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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!
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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.
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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!
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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!
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Pumping of Gas with MLP
Two factors may further reduce this potential
problem
1. The actual MLP well 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...
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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.
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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!
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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
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Static Pressures - DGD
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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?

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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?

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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

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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

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Summary - contd

• 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

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Summary - contd

• 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

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by Hans C. Juvkam-WoldNovember 2000The End
Dual Gradient DrillingBasic Technology

• 6. Gas Kick Behavior