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Well Control Gas Solubility

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**Contents Solubility of Hydrocarbon Gases in Oil**

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

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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!

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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.

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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.

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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.

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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.

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**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, = 270 scf/bbl remain free i.e. 2,700 scf of gas remain free (10*270)

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**Solubility of Methane in diesel (Fig. 1.17)**

Solubility, scf/bbl 530 T = 100 oF Pressure, psia

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**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

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**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

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**Solubility of Gases in Mentor 28 (Fig. 1.21)**

Ethane Methane

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**Solubility in various fluids**

13 ppg Oil base mud Mentor 28 18 ppg Oil base mud

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**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

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**Solubility of Gases in Diesel at 250 oF**

Solubility in 1,000 scf/bbl Solubility of Gases in Diesel at 250 oF

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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

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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

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Solution TABLE 1.3 – EQ CONSTANTS

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CO2 Solubility For CO2, c = 1.0

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**Hydrocarbon Solubility in Oil**

Next determine the hydrocarbon solubility in the oil and emulsifiers. The constant c must first be calculated. coil = gg + ( gg)T - (4.51* *10-6 gg)T2

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**Hydrocarbon Solubility in Oil**

coil = (.65) + ( (.65))150 - (4.51* *10-6(.65))1502 coil =

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**Hydrocarbon Solubility in Emulsifier**

cemul = gg gg2 = * * = Thus,

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**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

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**Solution Mixture solubility in the oil**

95% Hydrocarbons 5% CO2 Mixture solubility in the oil = 0.95 * * 950 = 392 scf/bbl Mixture solubility in the emulsifier = 0.95 * * 241 = 251 scf/bbl From Fig. 1.22, at 150 oF and 2,000 psia, HC solubility in fresh water = 12 scf/bbl

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**Solubility of Methane in distilled water (Fig. 1.22)**

2,000 psi 12 150 oF Temperature, oF

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**Solubility Correction Factor for Salinity**

(Fig. 1.23) 150 oF 200,000 ppm

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**Solution From Fig 1.23 the salinity correction factor is 0.4**

Solubility of HC in salt water = 12 * 0.4 = 5 scf/bbl

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**The solubility of CO2 in fresh water is 145 scf/bbl (Fig. 1.25) 145**

2,000 psi 150 oF

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**Salinity correction factor is 0.45 so solubility of CO2 in salt water**

= 145 * 0.45 = 65 scf/bbl Fig. 1.26

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**Solution Mixture solubility in the salt water,**

= 0.95 * * 65 = 8 scf/bbl Finally, mixture solubility in whole mud = 0.54 * * * 251 = = 224 scf/bbl oil water emulsifier

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**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

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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

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**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 CO H2S Solubility = 0.92* * = 66 scf/bbl Solubility in whole mud = 0.94 * 66 = 62 scf/bbl

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Domino effect Some free gas Gas in solution

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Drilled gas Rock removal rate R ft db in

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Drilled gas Entry rate of drilled gas

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

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**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

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**Solution All this gas goes into solution**

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

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

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**Find Depth of Bubblepoint**

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

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**Why is this a problem for well control?**

Vtotal = V1+V2 Vtotal = V1+V2 Vtotal < V1+V2 Why is this a problem for well control?

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**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.

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**Solution volume factors**

821 scf/bbl of methane in diesel oil at 4,075 psia would have a volume factor of 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)*(5, )} = 23.2*10-6 per psi ]

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Diesel Oil Compressibility ~ 4*10-6 per psi T = 100 oF

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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 scf/bbl is dissolved in the diesel. At the bottom, circ. Pressure = 5,000 psia Circulating Temp = 200 oF

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

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Solution 400 scf of gas under downhole conditions

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**Solution i.e. Downhole solution GOR = 0.273 bbl/bbl**

Thus the pit gain is bbl for each bbl of free gas that has been dissolved in the diesel For the 10 bbl gas kick, Pit gain = bbl*(10 bbl/0.273 bbl) = 4.2 bbl expansion

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Solution From Fig. 1.31, for 600 scf/bbl at 200 oF and 5,000 psia, Rso = bbl/STB 1.205 – = bbl/STB V5,000 = 0.273*(600/400) = 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?

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4.2 bbl pit gain 10 bbl kick 400 scf/bbl or 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.

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**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)

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**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.

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**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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