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Well Control Gas Solubility. 2 Contents  Solubility of Hydrocarbon Gases in Oil  Solubility of Non-Hydrocarbon Gases in Oil  Solubility in Water 

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Presentation on theme: "Well Control Gas Solubility. 2 Contents  Solubility of Hydrocarbon Gases in Oil  Solubility of Non-Hydrocarbon Gases in Oil  Solubility in Water "— Presentation transcript:

1 Well Control Gas Solubility

2 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 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 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 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 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 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 o F.

8 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 o F)  Therefore, 800 - 530 = 270 scf/bbl remain free  i.e. 2,700 scf of gas remain free (10*270)

9 9 Solubility of Methane in diesel (Fig. 1.17) T = 100 o F 530 Solubility, scf/bbl Pressure, psia

10 10 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

11 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 o F 300 o F Mentor 28 100 o F

12 12 Solubility of Gases in Mentor 28 (Fig. 1.21) Ethane Methane

13 13 Solubility in various fluids 13 ppg Oil base mud 18 ppg Oil base mud Mentor 28

14 14 Solubility of Methane in distilled water (Fig. 1.22) 10,000 psia 5,000 psia 1,000 psia Solubility Correction Factors (Fig. 1.23) 70 o F 250 o F Total Dissolved Solids, % Temperature, o F

15 15 Solubility of Gases in Diesel at 250 o F Solubility in 1,000 scf/bbl

16 16 Example 1.11  A 13.0 ppg 70:30 invert emulsion oil mud consists of (by volume) 54% diesel, 23% CaCl 2, 4% emulsifier, and 19% solids.  Estimate the natural gas solubility in the mud at 150 o F and 2,000 psia  Assume the gas is 95% hydrocarbon and 5% CO 2  Water salinity is 200,000 ppm TDS  Gas specific gravity is 0.65

17 17 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

18 18 Solution TABLE 1.3 – EQ. 1.45 CONSTANTS

19 19 CO 2 Solubility For CO 2, c = 1.0

20 20 Hydrocarbon Solubility in Oil  Next determine the hydrocarbon solubility in the oil and emulsifiers.  The constant c must first be calculated.  c oil = 0.3576 + 1.168  g + (0.0027 - 0.00492  g )T - (4.51*10 -6 - 8.198*10 -6  g )T 2

21 21 Hydrocarbon Solubility in Oil  c oil = 0.3576 + 1.168  + (0.0027 - 0.00492  )150 - (4.51*10 -6 - 8.198*10 -6  )150 2  c oil = 1.0605

22 22 Hydrocarbon Solubility in Emulsifier  c emul = 0.4 + 1.65  g - 1.01  g 2 = 0.4 + 1.65 * 0.65 - 1.01 * 0.65 2 = 1.0458  Thus,

23 23 Solution  Solubility of CO 2 in oil = 950 scf/bbl  Solubility of CO 2 in emulsifiers = 241 scf/bbl  Solubility of HC in oil = 408 scf/bbl  Solubility of HC in emulsifiers = 252 scf/bbl

24 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 o F and 2,000 psia,  HC solubility in fresh water = 12 scf/bbl 95% Hydrocarbons 5% CO 2

25 25 12 Temperature, o F Solubility of Methane in distilled water (Fig. 1.22) 2,000 psi 150 o F

26 26 Solubility Correction Factor for Salinity (Fig. 1.23) 150 o F 200,000 ppm

27 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 28 The solubility of CO 2 in fresh water is 145 scf/bbl (Fig. 1.25) 2,000 psi 150 o F 145

29 29 Salinity correction factor is 0.45 so solubility of CO 2 in salt water = 145 * 0.45 = 65 scf/bbl Fig. 1.26

30 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 oilwateremulsifier

31 31 Example 1.12  Mud:94% fresh water + 6% solids  Gas:0.92 mole fraction of Methane 0.06 mole fraction of CO 2 0.02 mole fraction of H 2 S  Estimate the natural gas solubility in the mud at 180 o F and 5,200 psia

32 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 CO 2 in fresh water = 182 scf/bbl

33 33 Solution  The H 2 S partial pressure = 0.02 * 5,200 = 104 psia  From Fig 1.27, the partial solubility of H 2 S is about 36 scf/bbl ~36 Solubility = 0.92*21+0.06*182+36 = 66 scf/bbl Methane CO 2 H 2 S Solubility in whole mud = 0.94 * 66 = 62 scf/bbl

34 34 Gas in solution Some free gas Domino effect

35 35 Drilled gas  Rock removal rate R ft d b in

36 36 Drilled gas  Entry rate of drilled gas

37 37 Drilled gas  If circ. Rate = q m bbl/min, then the ratio of gas to mud

38 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 o F Mud Density = 10.5, ppg Sand porosity = 25% Gas Saturation = 80% Circulation Rate = 8 bbl/min

39 39 Solution  What is the drilled gas concentration? All this gas goes into solution

40 40 Volume of Drilled Gas  Bubble point is reached at 70 psia and 90 o F. What is the volume of drilled gas?  The total downhole gas volume (from drilling through the 50 ft interval),

41 41 Find Depth of Bubblepoint  From the gas law This would happen at a depth of 100’ What happens to the mud above this point?

42 42 Why is this a problem for well control? V total = V 1 +V 2 V total < V 1 +V 2

43 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 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 ]

45 45 Diesel Oil Compressibility ~ 4*10 -6 per psi T = 100 o F

46 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 o F

47 47 Solution  From Fig. 1.31, for 400 scf/bbl at 5,000 psia and 200 o F, B og = 1.128 bbl/STB  From Fig 1.31, for gas free diesel at 5,000 psia and 200 o F, B ong = 1.012  Expansion = 1.128 - 1.012 = 0.116 bbl/STB

48 48

49 49 Solution  400 scf of gas under downhole conditions

50 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 51 Solution  From Fig. 1.31, for 600 scf/bbl at 200 o F and 5,000 psia, R so = 1.205 bbl/STB  1.205 – 1.012 = 0.193 bbl/STB  V 5,000 = 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 52 400 scf/bbl or 0.273 bbl/bbl under bottomhole conditions 4.2 bbl pit gain 10 bbl kick

53 53

54 54

55 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 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 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 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


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