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) 2 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) 3 PE4103 – Reservoir Simulation 4 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 5 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 6 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 7 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 8 PE4103 – Reservoir Simulation 9 Example: Simulation Model Water Saturation (from .irf file) Depth (from .dat file) Pressure (from .irf file) PE4103 – Reservoir Simulation 10 Fluid in Place after Initialization ▪ OOIP is calculated by simulator after initialization is completed As printed in .out file PE4103 – Reservoir Simulation 11 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 12