Petroleum PVT

Black-oil and screening-PVT skill for production and reservoir engineering. Use it when lab data are incomplete, when you need a quick consistency check, or when you need to translate a fluid report into inputs for nodal, PTA, or decline work.

Important: Correlations are for screening. If a compositional lab exists, honor the lab unless it clearly fails sanity checks. For rigorous EOS work, use pnge:thermo-eos.

Data To Request First

Reservoir pressure and temperature
API gravity, gas specific gravity, separator conditions
Bubble point or dewpoint if measured
Test oil, gas, and water rates with separator stages
PVT report type: differential liberation, CCE, CVD, separator test, swelling
Full composition if available

Workflow

Decide whether the fluid is best treated as black oil, volatile oil, gas condensate, or lean gas.
Prefer lab values when present; use correlations only to fill gaps or QC the report.
Carry units consistently in field units unless the user specifies SI.
State whether the result is lab-based, correlation-based, or a hybrid.

Module 1 - Core Black-Oil Correlations

def oil_specific_gravity(api):
    """Oil specific gravity from API gravity."""
    return 141.5 / (api + 131.5)

def standing_bubble_point_psia(rs_scf_stb, gas_sg, temp_f, api):
    """
    Standing bubble point correlation.
    """
    term = (rs_scf_stb / gas_sg) ** 0.83 * 10 ** (0.00091 * temp_f - 0.0125 * api)
    return 18.2 * term - 1.4

def standing_rs_scf_stb(p_psia, gas_sg, temp_f, api):
    """
    Standing solution GOR below bubble point.
    """
    term = (p_psia / 18.2 + 1.4) * 10 ** (0.0125 * api - 0.00091 * temp_f)
    return gas_sg * term ** (1.0 / 0.83)

def standing_bo_rb_stb(rs_scf_stb, gas_sg, temp_f, api):
    """
    Standing oil formation volume factor at or below bubble point.
    """
    gamma_o = oil_specific_gravity(api)
    term = rs_scf_stb * (gas_sg / gamma_o) ** 0.5 + 1.25 * temp_f
    return 0.9759 + 0.00012 * term ** 1.2

Use guidance

Property	Typical use
`Pb`	Determine if oil is saturated at reservoir pressure
`Rs`	Convert between oil and liberated gas behavior
`Bo`	Material balance, nodal, and reserve calculations

Module 2 - Gas Screening Properties

def sutton_pseudocritical(gas_sg):
    """
    Sutton pseudocritical properties for sweet natural gas.
    Returns Tpc in Rankine and Ppc in psia.
    """
    tpc_r = 169.2 + 349.5 * gas_sg - 74.0 * gas_sg**2
    ppc_psia = 756.8 - 131.0 * gas_sg - 3.6 * gas_sg**2
    return {"Tpc_R": tpc_r, "Ppc_psia": ppc_psia}

def pseudoreduced_props(p_psia, temp_f, gas_sg):
    """Ppr and Tpr for z-factor chart or correlation lookup."""
    pc = sutton_pseudocritical(gas_sg)
    t_r = temp_f + 459.67
    return {"Ppr": p_psia / pc["Ppc_psia"], "Tpr": t_r / pc["Tpc_R"]}

def gas_fvf_rb_scf(p_psia, temp_f, z):
    """
    Gas formation volume factor in reservoir bbl/scf.
    Bg = 0.00504 * z * T(R) / P(psia)
    """
    t_r = temp_f + 459.67
    return 0.00504 * z * t_r / p_psia

If gas composition is available or CO2 / H2S is significant, use a real EOS or corrected pseudocritical method instead of sweet-gas screening correlations.

Module 3 - Separator And Shrinkage Checks

def separator_shrinkage(stock_tank_stb, separator_liquid_bbl):
    """
    Stock tank barrels per separator barrel.
    """
    if separator_liquid_bbl <= 0:
        return None
    return stock_tank_stb / separator_liquid_bbl

def dewpoint_margin_psia(reservoir_pressure_psia, dewpoint_psia):
    """
    Positive margin means reservoir pressure is above dewpoint.
    Negative means condensate dropout is likely in the reservoir.
    """
    return reservoir_pressure_psia - dewpoint_psia

Interpretation

Result	Meaning
Shrinkage close to 1.0	Stable oil / little flash shrinkage
Large shrinkage loss	Volatile oil or condensate-rich liquid
Negative dewpoint margin	In-reservoir condensate dropout risk

Module 4 - Lab QC Checklist

Before trusting a lab report, check:

Are Rs, Bo, and viscosity trends monotonic where they should be?
Does reported bubble point or dewpoint match the reservoir fluid description?
Do separator test yields reconcile with field separator conditions?
Are gas specific gravity and flashed gas composition directionally consistent?
Do black-oil tables imply impossible densities or negative shrinkage?

If the lab fails these checks, state that explicitly and fall back to bounded correlation estimates.

Module 5 - Fluid-Type Decision Guide

Fluid type	Signs
Black oil	Moderate GOR, clear bubble point, small shrinkage
Volatile oil	High GOR, large shrinkage, strong separator sensitivity
Gas condensate	Dewpoint-controlled behavior, condensate dropout risk
Lean gas	Low condensate yield, gas properties dominate

Output Format

When using this skill, structure the answer as:

Fluid classification and data quality
Key PVT properties used or estimated
Lab-versus-correlation comparison
Operational implication for nodal, PTA, RTA, or facilities
Biggest missing PVT data item

Integration Points

Use pnge:nodal-analysis-multiphase for production system optimization.
Use pnge:well-test-analysis and pnge:rta-production when fluid properties drive interpretation.
Use pnge:surface-facilities for separator and compression implications.
Use pnge:thermo-eos for compositional EOS calculations and rigorous phase behavior.

petroleum-pvt