Reservoir Simulation Gassim

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Reservoir Simulation - Gassim
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Gassim

• From textbook Gas Reservoir Engineering by Lee
  and Wattenbarger
• VBA program in Excel
• Intended for student usage
• Analytical, numerical, transformation methods

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Gassim

• 1D or 2D
• Cartesian or radial grid
• Single phase gas or liquid

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Data organization

• Single section
• Grid section
• Schedule section
• (each section ends with END)

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Single section

• Each data has one value (except CNST)
• Specifies
• Grid size
• Reservoir temperature, reference pressure
• Gas gravity, Sw, cf, cw
• Whether radial coordinates
• Whether liquid (with Bo, µo)
• Specifies certain run controls
• Matrix methods
• Newton iterations

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Example of Single Data

• CMNT
• CMNT Homogeneous Cylindrical Reservoir
• CMNT Radial Flow, Constant-rate production,
  Infinite-acting
• CMNT Slightly Compressible Fluid
• CMNT
• CMNT
• CMNT Single Value Input Data
• IMAX 20
• JMAX 1
• RWEL 0.5
  (radial coordinates)
• CROC 0.000015
• PREF 3000
• NEWT 1
• BETA 0
• CMNT Bo, rcf/scf mo, cp
• CNST 1.475 0.72 (liquid
  case)
• END
• CMNT Grid Input Data

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Grid Section

• Specifies data for each gridblock
• Specifies grid dimensions
• 2D (DELX, DELY and H)
• radial (RR, DELY)
• Grid data
• permeability
• porosity
• thickness
• initial pressure

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Example of cartesian grid
I
1 2 3 4 5
6 7 8 9
10

• IMAX 10
• JMAX 5
• RWEL 0.5
• .
• .
• .
• END
• DELX 110
• DELY 150
• H 30
• .
• .
• .
• END
• CMNT Schedule Data
• .
• .
• .

1 2 3 4
5
J
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Example of radial grid

• IMAX 10
• JMAX 2
• RWEL 0.5
• .
• .
• .
• END
• RR -1
• 0.77 1.19 1.84 2.84
  4.40
• 6.79 10.50 16.22 25.06 38.7
• DELY 150
• .
• .
• .
• END
• CMNT Schedule Data
• .
• .

-1 indicates that an array follows. Otherwise a
constant value is used
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Schedule Section

• Controls well specifications
• location (NAME)
• constant rate (QG)
• contant pwf (PWF)
• Controls time schedule output
• 1 means output at TIME output
• 2 means output each timestep
• Controls timesteps (DELT, ALPH, DTMX)

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Schedule example

• NAME 1 3 5 0
• QG 1 12000
• TIME 365
• END

Well 1 is located at I 3, j 5 and produces at
a constant rate of 12,000 scf/day for 365 days
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Schedule example

• NAME 2 6 8 0
• PWF 2 1500
• TIME 730
• END

Well 2 is located at I 6, j 8 and produces at
a constant BHP of 1500 psia for 730 days
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Schedule example

• NAME 1 3 5 0
• NAME 2 6 8 0
• QG 1 12000
• TIME 365
• PWF 2 1500
• TIME 730
• END

Well 1 produces at a constant rate of 12,000
scf/day for 730 days. Well 2 produces at constant
BHP after 365 days until 730 days.
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Schedule Section programming logic

• When a TIME data line is read, the simulator
  executes the timesteps required to reach that
  time
• then ---- it reads the data to the next TIME data
  line

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Timestep control

• DELT sets the first ?t for the time period
• ALPH sets ?t equal to ALPH time the previous
  timesteps ?t (after the first timestep in the
  period)
• DTMX sets a maximum value for ?t

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Timestep example
The timestep sequence is 1 1.5 2.25 3.375 5.06 7
.59 10 10 10 10

• DELT 1
• ALPH 1.5
• DTMX 10
• TIME 60

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Timestep control

• Well conditions (QO or PWF) change after a TIME
  data line, a small DELT should be included so the
  new rate/pressure conditions start with small
  timesteps

DELT 1 ALPH 1.5 NAME 1 3 5 0 NAME 2 6
8 0 QG 1 12000 TIME 365 DELT
1 PWF 2 1500 TIME 730 END