Initialization
PE4103 Reservoir Simulation
PE4103 – Reservoir Simulation
Initialization
Definition
▪ The process to calculate the initial condition (pressure, saturation, and solution gas ratio) of the
simulation model
▪ If necessary, the reservoir can be divided into separate “equilibrium regions”, in which hydrostatic
equilibrium exist independently of other regions
In this step the initial state of every cell in the model is defined
▪ Initial pressure and phase saturation
▪ Initial solution ratios
▪ Depth dependence of reservoir fluid properties (Pd vs depth; Pb vs depth)
▪ Oil and gas re-solution rates
▪ Initial analytical aquifer condition (i.e. energy in reservoir)
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PE4103 – Reservoir Simulation
Method for Initialization
▪ Equilibration
▪ To calculate pressure and saturation distribution based on the hydrostatic equilibrium
▪ This is the most common method used in flow simulation
▪ Enumeration
▪ To directly specify the pressure, saturation, solution gas ratio for each grid cell
▪ Restart
▪ Using previously simulated value (e.g. pressure, saturation, composition map from previous run)
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PE4103 – Reservoir Simulation
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Model Initialization
Pcgo
Pcgo = Pg − Po
Pcow
GOC
Pcow = Po − Pw
𝑃𝑜 = 𝑓(𝜌𝑜 )
OWC
Subsea elevation
Subsea elevation
𝑃𝑔 = 𝑓(𝜌𝑔 )
𝑃𝑤 = 𝑓(𝜌𝑤 )
Pressure
Pc
PE4103 – Reservoir Simulation
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Model Initialization
Pcgo
Pcgo
0
GOC
Sg
1
OWC
GOC
Subsea elevation
Subsea elevation
Pcow
OWC
Pcow
Pc
0
Sw
1
Sw
PE4103 – Reservoir Simulation
Fluid Distribution within Reservoirs
▪ Initial distribution of fluids in
an actual reservoir of
uniform permeability and in
static equilibrium
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PE4103 – Reservoir Simulation
Fluid Distribution within Reservoirs
▪ Water wet, coarse grained sand and oolitic and vuggy carbonates with large pores
have low capillary pressure
▪ Silty, fine grained sands have high capillary pressure and water saturation
▪ Water saturation reduces with increased height above the hydrocarbon water
contact
▪ Shallowest depth of 100% water saturation is termed as the water oil contact
▪ The non identifiable level, the free water level, is the level of zero capillary pressure
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PE4103 – Reservoir Simulation
Role of Rock Type in Initialization
▪ Each rock type can be represented by one Pc curve. Therefore, different Sw-Depth
relationship exists for each rock type
▪ Knowing rock type distribution in 3D space will determine which Pc curve to use for
initialization of each grid block
▪ The uncertainty of rock type during upscaling process contributes to the uncertainty
of the initial oil in place calculation in the dynamic model
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PE4103 – Reservoir Simulation
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Example: Simulation Model
Water Saturation
(from .irf file)
Depth
(from .dat file)
Pressure
(from .irf file)
PE4103 – Reservoir Simulation
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Fluid in Place after Initialization
▪ OOIP is calculated by simulator after initialization is completed
As printed in .out file
PE4103 – Reservoir Simulation
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Review Initialization
Review initialization process
(as calculated by the model)
include
▪ Compare original fluids in
place in the model to values
estimated using volumetric
(e.g. static model)
▪ Check pressure distribution
▪ Check saturation distribution
▪ Check contacts location
(gas-oil, oil-water, gaswater) against available data
(e.g. RFT/MDT, log, etc.)
As printed in .out file
PE4103 – Reservoir Simulation
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