NeqSim provides utilities for gas reservoir engineering calculations: the real gas pseudopressure integral and pseudocritical property correlations for natural gas mixtures.

Gas Pseudopressure

Class: neqsim.pvtsimulation.util.GasPseudoPressure

The real gas pseudopressure (also called real gas potential) is defined as:

\[m(P) = 2 \int_{P_{ref}}^{P} \frac{P'}{\mu(P') \cdot Z(P')} \, dP'\]

where $\mu$ is the gas viscosity and $Z$ is the gas compressibility factor. This function linearizes the gas flow equation by accounting for the pressure-dependent variations in viscosity and Z-factor.

Applications

Two Calculation Modes

Mode When to Use Accuracy
EOS-based Known gas composition available Highest — rigorous thermodynamic model
Correlation-based Only gas specific gravity and MW known Good — uses Hall-Yarborough Z + Lee-Gonzalez-Eakin viscosity

EOS-Based Calculation (Java)

Uses a NeqSim thermodynamic system (SRK, PR, GERG-2008, etc.) for rigorous Z and viscosity at each pressure step.

import neqsim.thermo.system.SystemSrkEos;
import neqsim.pvtsimulation.util.GasPseudoPressure;

// Define gas composition
SystemInterface gas = new SystemSrkEos(373.15, 200.0);
gas.addComponent("methane", 0.90);
gas.addComponent("ethane", 0.05);
gas.addComponent("propane", 0.03);
gas.addComponent("CO2", 0.02);
gas.setMixingRule("classic");

// Calculate pseudopressure
GasPseudoPressure calc = new GasPseudoPressure(gas);
calc.setNumberOfSteps(200); // Higher = more accurate

// m(200 bara) - m(10 bara)
double mP = calc.calculate(200.0, 10.0);

// With pressure units
double mPsia = calc.calculate(3000.0, 14.696, "psia");

// Single point (relative to atmospheric)
double mPat200 = calc.pseudoPressureAt(200.0); // bara

Pseudopressure Profile

Generate a table of pseudopressure values across a pressure range:

GasPseudoPressure calc = new GasPseudoPressure(gas);
double[][] profile = calc.pseudoPressureProfile(10.0, 300.0, 50);
// profile[0] = pressure array (bara)
// profile[1] = pseudopressure array

Correlation-Based Calculation (No EOS Required)

Suitable when only bulk gas properties are available:

// Parameters: P (bara absolute), Pref (bara absolute), T (K), gamma_g, MW (g/mol)
double mP = GasPseudoPressure.calculateFromCorrelation(
    206.84, 1.01325, 366.48, 0.65, 16.04);

// Delta pseudopressure (absolute value)
double deltamP = GasPseudoPressure.deltaPseudoPressure(
    206.84, 68.95, 366.48, 0.65, 16.04);

The correlation path uses the canonical Standing pseudocritical-property implementation and is available through bounded runCapability discovery and invocation. Inputs must be finite, both pressures and molecular weight must be positive, gas specific gravity must be from 0.55 through 1.80, reduced temperature must exceed 1.0, and the maximum reduced pressure must remain below 25. The signed method preserves pressure order; deltaPseudoPressure returns the absolute difference. This screening contract does not qualify sour-gas corrections, composition-based EOS behavior, well deliverability, or engineering approval.

Python (via neqsim-python)

from neqsim import jneqsim

SystemSrkEos = jneqsim.thermo.system.SystemSrkEos
GasPseudoPressure = jneqsim.pvtsimulation.util.GasPseudoPressure

# Define gas
gas = SystemSrkEos(373.15, 200.0)
gas.addComponent("methane", 0.90)
gas.addComponent("ethane", 0.05)
gas.addComponent("propane", 0.05)
gas.setMixingRule("classic")

# Calculate
calc = GasPseudoPressure(gas)
mP = calc.calculate(200.0, 10.0)
print(f"Pseudopressure: {mP:.2f} bara^2/cP")

# Profile
profile = calc.pseudoPressureProfile(10.0, 300.0, 20)
pressures = list(profile[0])
mP_values = list(profile[1])

Gas Pseudocritical Properties

Class: neqsim.pvtsimulation.util.GasPseudoCriticalProperties

Pseudocritical temperature ($T_{pc}$) and pressure ($P_{pc}$) are required inputs for corresponding-states correlations (Z-factor, gas viscosity, compressibility). These are estimated from gas specific gravity when detailed composition is unavailable.

Supported Correlations

Correlation Input Best For
Standing (1981) $\gamma_g$ only Sweet gases with low non-HC content
Sutton (1985) $\gamma_g$ only Wider range of gas compositions
Piper-McCain-Corredor (2012) $\gamma_g$ + $y_{H_2S}$, $y_{CO_2}$, $y_{N_2}$ Sour/acid gas systems

Acid Gas Correction

Correction Purpose
Wichert-Aziz (1972) Corrects $T_{pc}$ and $P_{pc}$ for H2S and CO2 content

Standing Correlation

Simple two-constant correlation for sweet natural gas:

\[T_{pc} = 168 + 325 \gamma_g - 12.5 \gamma_g^2 \quad (\text{Rankine})\] \[P_{pc} = 677 + 15 \gamma_g - 37.5 \gamma_g^2 \quad (\text{psia})\] 
double tpc = GasPseudoCriticalProperties.pseudoCriticalTemperatureStanding(0.75);
double ppc = GasPseudoCriticalProperties.pseudoCriticalPressureStanding(0.75);

Sutton Correlation

More accurate than Standing for a wider range of gas specific gravities (0.57 - 1.68):

\[T_{pc} = 169.2 + 349.5 \gamma_g - 74.0 \gamma_g^2 \quad (\text{Rankine})\] \[P_{pc} = 756.8 - 131.0 \gamma_g - 3.6 \gamma_g^2 \quad (\text{psia})\] 
double tpc = GasPseudoCriticalProperties.pseudoCriticalTemperatureSutton(0.75);
double ppc = GasPseudoCriticalProperties.pseudoCriticalPressureSutton(0.75);

// All public correlation results are already SI: K and bara.

Standing accepts finite gas specific gravity from 0.55 through 1.80; Sutton accepts 0.57 through 1.68. Values outside those documented ranges fail explicitly. These four sweet-gas methods are available through bounded runCapability discovery and invocation. They are screening correlations, not substitutes for composition-based EOS phase behavior or sour-gas correction.

Piper-McCain-Corredor Correlation

Uses non-hydrocarbon mole fractions for improved accuracy with sour gas:

double gammaG = 0.80;
double yH2S = 0.10;
double yCO2 = 0.05;
double yN2 = 0.02;

double tpc = GasPseudoCriticalProperties.pseudoCriticalTemperaturePiper(
    gammaG, yH2S, yCO2, yN2);
double ppc = GasPseudoCriticalProperties.pseudoCriticalPressurePiper(
    gammaG, yH2S, yCO2, yN2);

Wichert-Aziz Acid Gas Correction

Corrects pseudocritical properties from any base correlation for H2S and CO2:

\[\varepsilon = 120 (A^{0.9} - A^{1.6}) + 15 (B^{0.5} - B^4)\]

where $A = y_{H_2S} + y_{CO_2}$ and $B = y_{H_2S}$.

\[T'_{pc} = T_{pc} - \varepsilon, \quad P'_{pc} = \frac{P_{pc} \cdot T'_{pc}}{T_{pc} + B(1 - B) \varepsilon}\] 
// Step 1: Get base pseudocriticals (any correlation)
double tpc = GasPseudoCriticalProperties.pseudoCriticalTemperatureSutton(0.80);
double ppc = GasPseudoCriticalProperties.pseudoCriticalPressureSutton(0.80);

// Step 2: Apply Wichert-Aziz correction
double yH2S = 0.10;
double yCO2 = 0.05;
double[] corrected = GasPseudoCriticalProperties.wichertAzizCorrection(
    tpc, ppc, yH2S, yCO2);
double tpcCorrected = corrected[0]; // Rankine
double ppcCorrected = corrected[1]; // psia

// Step 3: Calculate reduced properties
double T = 660.0; // Rankine
double P = 2000.0; // psia
double tpr = GasPseudoCriticalProperties.pseudoReducedTemperature(T, tpcCorrected);
double ppr = GasPseudoCriticalProperties.pseudoReducedPressure(P, ppcCorrected);

Full Workflow: Sour Gas Z-Factor via Pseudocritical Properties

// Gas properties
double gammaG = 0.80;
double yH2S = 0.10;
double yCO2 = 0.05;

// 1. Pseudocritical properties (Sutton)
double tpc = GasPseudoCriticalProperties.pseudoCriticalTemperatureSutton(gammaG);
double ppc = GasPseudoCriticalProperties.pseudoCriticalPressureSutton(gammaG);

// 2. Acid gas correction (Wichert-Aziz)
double[] corrected = GasPseudoCriticalProperties.wichertAzizCorrection(
    tpc, ppc, yH2S, yCO2);

// 3. Reduced properties
double TF = 200.0; // °F
double P = 3000.0; // psia
double Tr = (TF + 459.67) / corrected[0];
double Pr = P / corrected[1];

// 4. Now use Tr, Pr in any Z-factor correlation
//    (e.g., Hall-Yarborough, Dranchuk-Abu-Kassem)