Well Control

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

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!

4

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.

5

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.

6

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.

7

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.

8

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 (10*270)

9

Solubility of Methane in diesel (Fig. 1.17)

T = 100 oF

530

Sol

ubili

ty,

scf

/bbl

Pressure, psia

10

Methane solubility in # 2 diesel (Fig. 1.18)

Is anything wrong here?

At lower temperature, the solubility is higher (p.14)!

Sol

ubili

ty,

scf

/bbl

Pressure, psia

11

Methane solubility (Figs. 1.19 & 1.20)

Methane is most soluble in Conoco LTV oil, least in Mentor 28

Mentor 28

Higher solubility at lower temperature

Conoco LTV oil

100 oF

300 oF

Mentor 28

100 oF

12

Solubility of Gases

in Mentor 28(Fig. 1.21)

Ethane

Methane

13

Solubility in various fluids

13 ppgOil base mud

18 ppgOil base mud

Mentor 28

14

Solubility of Methane in distilled water (Fig. 1.22)

10,0

00 p

sia

5,000 psia

1,000 psia

Solubility Correction

Factors (Fig. 1.23)

70 oF

250 oF

Total Dissolved Solids, %

Temperature, oF

15Solubility of Gases in Diesel at 250 oF

Sol

ubili

ty in

1,0

00 s

cf/b

bl

16

Example 1.11 A 13.0 ppg 70:30 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

17

Solution

First determine the carbon dioxide solubility in the oil and emuslifiers

In oil,

c

bso aT

pR

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

18

Solution

TABLE 1.3 – EQ. 1.45 CONSTANTS

19

CO2 Solubility

bblscfR

oilinCO

so /950150*059.0

000,21

7134.0

2

c

bso aT

pR

bbl/scf241150*135.0

000,2R

EmulsifiertheinCO1

8217.0so

2

For CO2, c = 1.0

20

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 g

+ (0.0027 - 0.00492 g)T

- (4.51*10-6 - 8.198*10-6 g)T2

21

Hydrocarbon Solubility in Oil

coil = 0.3576 + 1.168

+ (0.0027 - 0.00492)150

- (4.51*10-6 - 8.198*10-6)1502

coil = 1.0605

bblscfRso /408150*922.1

000,20605.1

2552.0

c

bso aT

pR

22

Hydrocarbon Solubility in Emulsifier

cemul = 0.4 + 1.65 g - 1.01g2

= 0.4 + 1.65 * 0.65 - 1.01 * 0.652 = 1.0458

Thus,

bblscfRse /252150*162.4

000,20458.1

1770.0

c

bso aT

pR

23

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

24

Solution

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

95% Hydrocarbons 5% CO2

25

12

Temperature, oF

Solubility of Methane in distilled water (Fig. 1.22)

2,000 psi

150 oF

26

Solubility Correction Factor for Salinity(Fig. 1.23)

150 oF

200,000 ppm

27

Solution

From Fig 1.23 the salinity correction factor is 0.4

Solubility of HC in salt water

= 12 * 0.4 = 5 scf/bbl

28

The solubility

of CO2

in fresh water

is 145 scf/bbl

(Fig. 1.25)

2,000 psi

150 oF

145

29

Salinity correction factor

is 0.45 so solubility of CO2

in salt water

= 145 * 0.45

= 65 scf/bbl

Fig. 1.26

30

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

31

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

32

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

33

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

Solubility = 0.92*21+0.06*182+36 = 66 scf/bblMethane CO2 H2S

Solubility in whole mud = 0.94 * 66 = 62 scf/bbl

34

Gas in solution

Some free gas

Domino effect

35

Drilled gas

Rock removal rate

hr/ftRin144

ftind

4 2

22

b

hr

ft

3.183

Rd 32b

R ft

db in

36

Drilled gas

Entry rate of drilled gas

bb

bg2

b

TZ65.14*hrmin/60

520*0.1pS

3.183

Rd

bb

bgbgsc TZ

pSRdq

9.309

2

min

scf

37

Drilled gas

If circ. Rate = qm bbl/min,

then the ratio of gas to mud

bbl

scf

TZq

pSRdr

bbm

bgm 9.309

2

surfacetheat

38

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

39

Solution

What is the drilled gas concentration?

600*86.0*8*9.309

000,3*8.0*25.0*250*25.12r

2

m

All this gas goes into solution

bblscfrm /6.17

bbm

bgm TZq

pSRdr

9.309

2

40

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),

80.0*25.0702,9

12*50*25.12

4 322

in

bbl

ft

inftinVb

bblVb 5.1

41

Find Depth of Bubblepoint From the gas law

bottombubblept ZT

pV

ZT

pV

This would happen at a depth of 100’

bpb

bp 70

55099.0

600*86.0

000,35.1V

bblVbp 68

What happens to the mud above this point?

42

Why is this a problem for well control?

Vtotal = V1+V2

Vtotal = V1+V2

Vtotal < V1+V2

43

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.

44

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)*(5,070-4075)}

= 23.2*10-6 per psi ]

pVcV

45

Diesel Oil

Compressibility~ 4*10-6 per psi

T = 100 oF

46

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

47

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

48

49

Solution

400 scf of gas under downhole conditions

3

3000,5 6146.5

1

1

029.1

520

660

000,5

65.14400

ft

bbl

R

R

psia

psiaftV o

o

mudofbblpergasofbbl273.0V 000,5

50

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

51

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?

52

400 scf/bblor 0.273 bbl/bblunder bottomholeconditions

4.2 bbl pit gain

10 bbl kick

53

54

55

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.

56

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)

57

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.

58

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