pyResToolbox MCP Server

Calculate gas compressibility (Cg).

CRITICAL GAS PVT PROPERTY - Computes gas compressibility coefficient, which measures how much gas volume changes with pressure. Essential for material balance calculations, pressure transient analysis, and reserve estimation. Gas compressibility is much higher than oil compressibility (typically 100-1000 × 10⁻⁶ 1/psi vs 5-50 × 10⁻⁶).

Parameters:

• sg (float, required): Gas specific gravity (air=1.0). Valid: 0.55-3.0. Typical: 0.6-1.2. Example: 0.7.

• degf (float, required): Reservoir temperature in °F. Valid: -460 to 1000. Typical: 100-400°F. Example: 180.0.

• p (float or list, required): Pressure(s) in psia. Must be > 0. Can be scalar or array. Example: 3500.0 or [1000, 2000, 3000, 4000].

• h2s (float, optional, default=0.0): H2S mole fraction (0-1). Typical: 0-0.05. Example: 0.02.

• co2 (float, optional, default=0.0): CO2 mole fraction (0-1). Typical: 0-0.20. Example: 0.05.

• n2 (float, optional, default=0.0): N2 mole fraction (0-1). Typical: 0-0.10. Example: 0.01.

• zmethod (str, optional, default="DAK"): Z-factor method for compressibility. Options: "DAK", "HY", "WYW", "BUR". DAK recommended.

Compressibility Behavior:

• Decreases with increasing pressure (gas becomes less compressible)

• Typical range: 50-500 × 10⁻⁶ 1/psi at reservoir conditions

• At low pressure: Cg ≈ 1/P (ideal gas behavior)

• At high pressure: Cg decreases significantly

Formula: Cg = (1/Z) × (∂Z/∂P) - (1/P)

Where Z-factor and its pressure derivative are calculated using specified method.

Returns: Dictionary with:

• value (float or list): Compressibility in 1/psi (matches input p shape)

• method (str): Z-factor method used

• units (str): "1/psi"

• inputs (dict): Echo of input parameters

Common Mistakes:

• Using separator temperature instead of reservoir temperature

• Pressure in barg/psig instead of psia (must be absolute)

• Not accounting for non-hydrocarbon fractions

• Confusing gas compressibility (high, 100-1000 × 10⁻⁶) with oil compressibility (low, 5-50 × 10⁻⁶)

• Using ideal gas approximation (Cg = 1/P) instead of real gas

Example Usage:

Result: Cg decreases from ~1000 × 10⁻⁶ 1/psi at 1000 psia to ~250 × 10⁻⁶ 1/psi at 4000 psia.

Note: Gas compressibility is critical for material balance calculations. Always use reservoir conditions. Account for all non-hydrocarbon components. Cg values are small (micro-1/psi), so results are typically in scientific notation.