pyResToolbox MCP Server

Calculate pressure from P/Z value.

MATERIAL BALANCE TOOL - Solves for pressure given a P/Z (pressure/Z-factor) value. Essential for gas material balance analysis where P/Z vs cumulative production is plotted. Uses iterative solution to find pressure that yields the specified P/Z value.

Parameters:

• pz (float or list, required): P/Z value(s) in psia. Must be > 0. Can be scalar or array. Example: 5000.0 or [4000, 5000, 6000].

• 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.

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

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

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

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

P/Z Method Applications:

• Volumetric Gas Reserves: P/Z vs Gp plot gives GIIP (Gas Initially In Place)

• Aquifer Influx Detection: Deviation from straight line indicates water drive

• Drive Mechanism Identification: Volumetric vs water drive vs gas cap

• Production Forecasting: Extrapolate P/Z to abandonment pressure

Material Balance Principle: For volumetric gas reservoirs: P/Z = (Pi/Zi) × (1 - Gp/G) Where Gp = cumulative production, G = GIIP

A straight line on P/Z vs Gp indicates volumetric depletion. Deviation suggests water influx, changing pore volume, or gas cap expansion.

Solution Method: Iterative Newton-Raphson method to solve: P/Z - pz_target = 0 Converges rapidly for well-posed problems.

Returns: Dictionary with:

• value (float or list): Pressure in psia (matches input pz shape)

• method (str): Iterative solution method with Z-factor method

• units (str): "psia"

• inputs (dict): Echo of input parameters

Common Mistakes:

• Using separator temperature instead of reservoir temperature

• Not accounting for non-hydrocarbon fractions

• Confusing P/Z (pressure/Z-factor) with pressure

• Using wrong Z-factor method (must match method used in material balance)

• Temperature in Celsius instead of Fahrenheit

Example Usage:

Result: Pressure ≈ 4500-5500 psia (depends on Z-factor at that pressure).

Note: P/Z method is fundamental to gas material balance. Always use the same Z-factor method throughout your analysis for consistency. Account for all non-hydrocarbon components as they affect Z-factor and thus P/Z values.