ch7 - CHAPTER7 CalculationsofWellstream...

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CHAPTER 7  Calculations of Wellstream Calculations of Wellstream     Composition from Surface Samples Composition from Surface Samples When reservoir fluid samples are taken at the  surface (i.e., at the separator and at the  stock  tank), it is necessary to calculate the wellstream  composition using the liquid and gas  compositions, producing gas oil ratios and other  measured properties of the surface samples. These recombination calculations are presented  in this chapter. Although all the examples  presented in Chapter 7 deal with wet gas  reservoirs, some of the recombination  calculations are valid for black oil, volatile oil,  retrograde condensate gas systems as well if they  are properly sampled at the surface. 7 - 1
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Therefore, the Chapter title of “Properties of Wet  Gases” used by McCain is somewhat misleading  for this chapter. 7 - 2
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Consider two stage separation with the second  stage being the stock tank. Case 1. Surface Compositions Known. Given: 1. First-stage separator gas composition (y SPj ). 2. First-stage separator producing gas-oil ratio  (R SP ). 3. Second-stage separator (stock tank) gas  composition (y STj ). 4. Second-stage separator (stock tank)  producing gas-oil ratio (R ST ). 5. Second-stage separator (stock tank) oil  composition (x STj ). 6. Second-stage separator (stock tank) oil  specific gravity (or API gravity). Required: 7 - 3
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To calculate the wellstream (first-stage separator  feed) composition (z STj ). 7 - 4
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Solution: 1. Calculate the apparent molecular weight of  the stock tank oil (2nd stage separator), M STO 2. Calculate the density of the stock tank oil,  ρ STO . 3. Convert the gas oil ratios from units of  scf/STB to units of lb mole gas/lb mole of  stock tank oil. The equation is GOR = R scf STB lb mole 379.4scf bbl 5.615 cu ft cu ft STO ρ STO  lbm STO M STO lbm STO lb mole STO (1) or GOR = RM STO 2130.33 ρ STO lb mole gas lb mole STO (2) The contribution of each gas component j to  the GOR is given by GOR j = RM STO y j 2130.33 ρ STO lb mole of component j in the gas lb mole STO (3) 4. Apply Eq. 3 to the 1st stage separator gas and  2nd stage (stock tank) separator gas to get GOR SPj = R SP M STO y SPj 2130.33 ρ STO lb mole j SP gas lb mole STO (4) 7 - 5
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GOR STj = R ST M STO y STj 2130.33 ρ STO lb mole j ST gas lb mole STO (5) 7 - 6
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5. For each component j, add the contributions  from Eqs. 4 (separator gas), 5 (stock tank gas)  and the stock tank liquid (x STj ) to obtain the lb  mole j of recombined reservoir fluid/lb mole  of STO. 6.
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ch7 - CHAPTER7 CalculationsofWellstream...

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