PETE 625 Well Control

1
PETE 625Well Control

• Lesson 4
• Gas Solubility

2
Contents

• Solubility of Hydrocarbon Gases in Oil
• Solubility of Non-Hydrocarbon Gases in Oil
• Solubility in Water
• Solution Volume Factors
• Oil Mud Recommendations

3
Assignments

• Homework 2
• Ch 1, Problems 11-21
• Homework 3
• Ch 2, Problems 1-10
• Read Chapter 2 to p.45

4
Gas Solubility

• Gas will dissolve to some extent in any
  drilling fluid, but this can generally be
  ignored with a water base fluid.
• Gas dissolves readily in oil base muds.
• An operator drilling with a diesel or mineral
  oil must understand this!

5
Gas Solubility

• The solubility of a gas/liquid mixture may be
  expressed as the amount of free gas (scf/bbl)
  that can go into solution at a given temperature
  and pressure.
• In general, solubility will increase as the
  pressure increases, and as the temperature
  decreases.

6
Gas Solubility

• Solubility also increases as the molecular
  similarity between the gas and liquid
  composition increases.
• The bubble point pressure is the pressure at
  which the first bubble of free gas breaks out of
  solution with a given solution gas/liquid ratio
  at a given temperature.

7
Gas Solubility

• Free gas cannot coexist with the liquid at
  pressures in excess of the bubble point
• At pressures above the bubble point, gas
  solubility approaches infinity. Only liquid is
  present.

8
Example 1.10

• Using the data from Fig. 1.17, determine the
  amount of free gas remaining if
• 8,000 scf of methane are blended with
• 10 bbl of diesel.
• p 3,000 psia and
• T 100 oF.

9
Example 1.10

• The system gas/oil ratio,
• R 8,000 scf/10 bbl 800 scf/bbl
• From Fig. 1.17, at 3,000 psia, the gas
  solubility is 530 scf/bbl (at 100 oF)
• Therefore, 800 - 530 270 scf/bbl remain free
• i.e. 2,700 scf of gas remain free (10270)

10
Solubility of Methane in diesel (Fig. 1.17)
Solubility, scf/bbl
530
T 100 oF
Pressure, psia
11
Methane solubility in 2 diesel (Fig. 1.18)
Is anything wrong here?
At lower temperature, the solubility is higher
(p.14)!
Solubility, scf/bbl
Pressure, psia
12
Methane solubility (Figs. 1.19 1.20)
Mentor 28
Conoco LTV oil
100 oF
Mentor 28
Methane is most soluble in Conoco LTV oil,
least in Mentor 28
300 oF
Higher solubility at lower temperature
100 oF
13
Solubility of Gases in Mentor 28(Fig. 1.21)
Ethane
Methane
14
Solubility in various fluids
13 ppg Oil base mud
Mentor 28
18 ppg Oil base mud
15
Solubility Correction Factors (Fig. 1.23)
Solubility of Methane in distilled water (Fig.
1.22)
10,000 psia
250 oF
5,000 psia
70 oF
1,000 psia
Total Dissolved Solids,
Temperature, oF
16
Solubility in 1,000 scf/bbl
Solubility of Gases in Diesel at 250 oF
17
Example 1.11

• A 13.0 ppg 7030 invert emulsion oil mud
  consists of (by volume) 54 diesel, 23 CaCl2,
  4 emulsifier, and 19 solids.
• Estimate the natural gas solubility in the mud
  at 150 oF and 2,000 psia
• Assume the gas is 95 hydrocarbon and 5 CO2
• Water salinity is 200,000 ppm TDS
• Gas specific gravity is 0.65

18
Solution

• First determine the carbon dioxide solubility
  in the oil and emuslifiers
• In oil,

• a, b and c are constants listed in the next slide

19
Solution
TABLE 1.3 EQ. 1.45 CONSTANTS
20
CO2 Solubility
For CO2, c 1.0
21
Hydrocarbon Solubility in Oil

• Next determine the hydrocarbon solubility in
  the oil and emulsifiers.
• The constant c must first be calculated.
• coil 0.3576 1.168 gg
• (0.0027 - 0.00492 gg)T
• - (4.5110-6 - 8.19810-6 gg)T2

22
Hydrocarbon Solubility in Oil

• coil 0.3576 1.168(.65)
• (0.0027 - 0.00492(.65))150
• - (4.5110-6 - 8.19810-6(.65))1502
• coil 1.0605

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Hydrocarbon Solubility in Emulsifier

• cemul 0.4 1.65 gg - 1.01gg2
• 0.4 1.65 0.65 - 1.01 0.652
  1.0458
• Thus,

24
Solution

• Solubility of CO2 in oil 950 scf/bbl
• Solubility of CO2 in emulsifiers 241 scf/bbl
• Solubility of HC in oil 408 scf/bbl
• Solubility of HC in emulsifiers 252 scf/bbl

25
Solution
95 Hydrocarbons 5 CO2

• Mixture solubility in the oil
• 0.95 408 0.05 950 392 scf/bbl
• Mixture solubility in the emulsifier
• 0.95 252 0.05 241 251 scf/bbl
• From Fig. 1.22, at 150 oF and 2,000 psia,
• HC solubility in fresh water 12 scf/bbl

26
Solubility of Methane in distilled water (Fig.
1.22)
2,000 psi
12
150 oF
Temperature, oF
27
Solubility Correction Factor for Salinity
(Fig. 1.23)
150 oF
200,000 ppm
28
Solution

• From Fig 1.23 the salinity correction factor is
  0.4
• Solubility of HC in salt water
• 12 0.4 5 scf/bbl

29
The solubility of CO2 in fresh water is 145
scf/bbl (Fig. 1.25)
145
2,000 psi
150 oF
30
Salinity correction factor is 0.45 so solubility
of CO2 in salt water 145 0.45 65 scf/bbl
Fig. 1.26
31
Solution

• Mixture solubility in the salt water,
• 0.95 5.0 0.05 65 8 scf/bbl
• Finally, mixture solubility in whole mud
• 0.54 395 0.23 8 0.04 251
• 213 1.8 10
• 224 scf/bbl

oil
water
emulsifier
32
Example 1.12

• Mud 94 fresh water 6 solids
• Gas 0.92 mole fraction of Methane
• 0.06 mole fraction of CO2
• 0.02 mole fraction of H2S
• Estimate the natural gas solubility in the mud
  at 180 oF and 5,200 psia

33
Solution

• The only component capable of dissolving any
  gas is the fresh water.
• From Fig. 1.22, the solubility of methane in
  fresh water 21 scf/bbl
• From Fig. 1.25, the solubility of CO2 in fresh
  water 182 scf/bbl

34
Solution

• The H2S partial pressure 0.02 5,200 104
  psia
• From Fig 1.27, the partial solubility of H2S is
  about 36 scf/bbl

36
Methane CO2 H2S
Solubility 0.92210.0618236 66 scf/bbl
Solubility in whole mud 0.94 66 62 scf/bbl
35
Domino effect
Some free gas
Gas in solution
36
Drilled gas

• Rock removal rate

R ft
db in
37
Drilled gas

• Entry rate of drilled gas

38
Drilled gas

• If circ. Rate qm bbl/min,
• then the ratio of gas to mud

39
Example 1.13

• Gas sand thickness 50 ft
• Bit diameter 12.25
• Drilling rate, R 250 ft/hr
• Depth 6,000
• BHP 3,000 psia
• BHT 140 oF
• Mud Density 10.5, ppg
• Sand porosity 25
• Gas Saturation 80
• Circulation Rate 8 bbl/min

40
Solution

• What is the drilled gas concentration?

All this gas goes into solution
41
Volume of Drilled Gas

• Bubble point is reached at 70 psia and 90 oF.
  What is the volume of drilled gas?
• The total downhole gas volume (from drilling
  through the 50 ft interval),

42
Find Depth of Bubblepoint

• From the gas law

This would happen at a depth of 100
What happens to the mud above this point?
43
Vtotal V1V2
Vtotal V1V2
Vtotal lt V1V2
Why is this a problem for well control?
44
Solution volume factor

• For solutions, the final volume is less than
  the sum of the component volumes.
• Kick sizes are determined by pit volume gain
• A large gas kick that dissolves in oil mud,
  will not result in as much pit gain as a
  similarly sized kick in water based mud.

45
Solution volume factors

• 821 scf/bbl of methane in diesel oil at 4,075
  psia would have a volume factor of 1.254 bbl/STB.
  
• An increase in pressure to 5,070 psia will
  reduce the volume factor to 1.225
• See Table 1.4 and Fig. 1.31
• Compressibility (1.254-1.225)/(1.254)(5,070-4
  075)
• 23.210-6 per psi

46
Diesel Oil
Compressibility 410-6 per psi
T 100 oF
47
Example 1.14

• 10 bbl of methane enters the wellbore. No. 2
  diesel oil is used as drilling mud.
• Determine the surface pit gain if 400 scf/bbl
  is dissolved in the diesel.
• At the bottom, circ. Pressure 5,000 psia
• Circulating Temp 200 oF

48
Solution

• From Fig. 1.31, for 400 scf/bbl at 5,000 psia
  and 200 oF, Bog 1.128 bbl/STB
• From Fig 1.31, for gas free diesel at 5,000
  psia and 200 oF, Bong 1.012
• Expansion 1.128 - 1.012 0.116 bbl/STB

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Solution

• 400 scf of gas under downhole conditions

51
Solution

• i.e. Downhole solution GOR 0.273 bbl/bbl
• Thus the pit gain is 0.116 bbl for each 0.273
  bbl of free gas that has been dissolved in the
  diesel
• For the 10 bbl gas kick,
• Pit gain 0.116 bbl(10 bbl/0.273 bbl)
  4.2 bbl

expansion
52
Solution

• From Fig. 1.31, for 600 scf/bbl at 200 oF and
  5,000 psia, Rso 1.205 bbl/STB
• 1.205 1.012 0.193 bbl/STB
• V5,000 0.273(600/400) 0.409 bbl/bbl
• So, a 10 bbl kick would result in a pit gain of
  0.193(10/0.409) 4.7 bbl

What happens in very deep wells, at very high
pressures?
53
4.2 bbl pit gain
10 bbl kick
400 scf/bbl or 0.273 bbl/bbl under
bottomhole conditions
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Oil mud recommendations

• Gas wells can be drilled safely with oil base
  drilling fluids.
• Certain precautions must be taken, like
  drilling with a rotating head, to direct
  evolved gas away from the rig floor.

57
Oil mud recommendations

• Set limits on quantities of drilled gas allowed
  in the annulus (by limiting the penetration
  rates, and the number of gas sands penetrated)
• Consider keeping annular back pressure above
  the bubble point, if possible (consider effect
  on penetration rate and fracture integrity)

58
Oil mud recommendations

• Mud-gas separator must be properly designed and
  sized for potential well control procedures.
• Remember that pit gain on the surface, for a
  given kick size, will be smaller than it would
  be for water based drilling fluids.

59
Oil mud recommendations

• Pit level alarms should be set at a lower level
• Educate crews on the differences between oil
  and water based muds, and on how kicks behave in
  the two systems.
• Be alert