Natural Gas Engineering
Natural Gas Engineering tests gas properties, z-factor behavior, gas-well deliverability, hydrates, compression, and decline. Expect the real-gas law, pseudopressure, non-Darcy flow, and gas-well test interpretation.
Core rules
- Real-gas law: PV = znRT. Density rho = PM/(zRT), with R = 10.73 psia-ft3/(lbmol-R).
- z-factor departs from 1 near critical conditions. Use Standing-Katz charts or an equation of state, and use absolute pressure and Rankine temperature.
- For high-rate gas wells, deliverability includes a non-Darcy term: pseudopressure or pressure-squared form has A q + B q^2. Absolute open flow is the extrapolated q at Pwf = 0.
- Hydrates form at low temperature and high pressure. Prevent with dehydration, methanol or glycol inhibition, or heating.
- Compression is limited by stage ratio and discharge temperature; intercooling improves efficiency.
Worked example
A gas has P = 2,000 psia, T = 600 R, M = 19.5 lb/lbmol, and z = 0.85. Density is rho = 2,000 × 19.5/(0.85 × 10.73 × 600) = 39,000/5,472 = 7.13 lb/ft3. If z were set to 1, density would be 6.06 lb/ft3, about 18% low. The main reasoning is to use absolute psia and Rankine and keep z in the denominator.
Traps
Using gauge pressure or Celsius, setting z = 1, or ignoring non-Darcy effects at high rate will misstate deliverability. Do not apply a low-pressure Darcy equation to a high-pressure gas well without pseudopressure. Liquid loading and hydrate formation can invalidate a pure gas-rate forecast.
Key points
- Real-gas density uses absolute P and T plus z.
- z-factor is essential near critical conditions.
- High-rate gas wells need a non-Darcy term.
- Hydrates are prevented by dehydration or inhibition.
- Compression stage ratio is limited by temperature.