Reservoir Engineering
Reservoir Engineering tests how static rock and fluid data become volumetrics, recoverable reserves, and inflow forecasts. Expect material balance, Darcy flow, relative permeability, drive mechanisms, and recovery-factor screening.
Core rules
- Volumetric OOIP in field units: OOIP = 7758 A h phi (1-Sw)/Bo, with A in acres, h in ft, and 7758 bbl/acre-ft.
- Material balance links expansion, gas-cap, water influx, and withdrawal: F = N(Eo + m Eg + Efw) + We. Use it to check volumetrics and drive strength.
- Radial steady-state oil inflow: q = 0.00708 k h (Pr-Pwf)/(mu Bo [ln(re/rw)+s]) in STB/d with k in mD and pressures in psi.
- Relative permeability and fractional flow control water cut and gas breakthrough; recovery screening depends on drive type and heterogeneity.
Worked example
A reservoir has A = 1,000 acres, h = 30 ft, phi = 0.20, Sw = 0.25, and Bo = 1.2 rb/STB. The oil pore volume numerator is 7758 × 1,000 × 30 × 0.20 × 0.75 = 34,911,000. Divide by Bo: OOIP = 34,911,000/1.2 = 29,092,500 STB, or about 29.1 MMSTB. At a 30% recovery factor, recoverable oil is 8.73 MMSTB. The reasoning is to compute net oil-filled pore volume first, then shrink to stock-tank conditions.
Traps
Using gross thickness instead of net pay, entering porosity as 20 rather than 0.20, or forgetting the (1-Sw) term all inflate OOIP. A linear productivity index is invalid below bubble point; use Vogel or a composite IPR. Keep the radial-flow units consistent and do not ignore skin.
Key points
- OOIP = 7758 A h phi (1-Sw)/Bo in field units.
- Material balance is a volumetric and drive-strength check.
- Radial inflow constant is 0.00708 in STB/d field units.
- Use Vogel below bubble point; PI only for single-phase liquid.
- Relative permeability controls water cut and breakthrough.