Nodal Analysis And Multiphase Production Optimization

Production systems skill for matching reservoir inflow with wellbore and surface outflow. Use it for screening and sensitivity analysis; final design should still be checked with calibrated multiphase correlations or a field simulator.

Important: Nodal answers are only as good as the PVT, test separator, and pressure data behind them. If PVT is weak, couple this skill with pnge:petroleum-pvt.

Data To Request First

Reservoir pressure or latest static / build-up estimate
Test rate, flowing bottomhole pressure, and flowing wellhead pressure
Oil rate, gas rate, water rate, GOR, water cut, and fluid gravities
Tubing ID, setting depth, deviation profile, and choke size
Flowline and separator pressure constraints
Temperature profile or average flowing temperature
Artificial lift status, compression status, and separator conditions

Workflow

Select an inflow model consistent with fluid type and available data.
Build an outflow approximation from wellhead pressure back to perforations.
Solve for the intersection between IPR and VLP.
Run sensitivities on choke, tubing, lift gas, or compression.
State which lever moves the inflow curve and which lever moves the outflow curve.

Module 1 - Inflow Performance

def productivity_index(q_bpd, pr_psia, pwf_psia):
    """Linear PI for undersaturated oil or small drawdown cases."""
    dp = pr_psia - pwf_psia
    if dp <= 0:
        return None
    return q_bpd / dp

def vogel_qmax(q_test_bpd, pr_psia, pwf_test_psia):
    """
    Vogel inflow for solution-gas drive oil wells.
    q/ qmax = 1 - 0.2*(pwf/pr) - 0.8*(pwf/pr)^2
    """
    r = pwf_test_psia / pr_psia
    denom = 1.0 - 0.2 * r - 0.8 * r**2
    if denom <= 0:
        return None
    return q_test_bpd / denom

def vogel_rate(pr_psia, pwf_psia, qmax_bpd):
    """Oil rate from Vogel IPR."""
    r = pwf_psia / pr_psia
    return qmax_bpd * (1.0 - 0.2 * r - 0.8 * r**2)

def gas_deliverability_rate(pr_psia, pwf_psia, C, n):
    """
    Rawlins-Schellhardt backpressure equation in pressure-squared form.
    q = C * (pr^2 - pwf^2)^n
    """
    term = pr_psia**2 - pwf_psia**2
    if term <= 0:
        return 0.0
    return C * term**n

def gas_C_from_test(q_mscfd, pr_psia, pwf_psia, n):
    """Calibrate gas deliverability constant from one test point."""
    term = pr_psia**2 - pwf_psia**2
    if term <= 0:
        return None
    return q_mscfd / (term**n)

Model selection

Fluid system	Preferred screening IPR
Oil above bubble point	Linear PI
Solution-gas drive oil	Vogel
Dry gas / gas condensate	Backpressure deliverability
Tight gas with non-Darcy effects	Deliverability plus turbulence correction

Module 2 - Outflow / Vertical Lift Screening

def liquid_density_lbft3(oil_api=40.0, water_cut=0.0, water_density_lbft3=62.4):
    """No-slip bulk liquid density from oil and water fractions."""
    oil_density = 141.5 / (oil_api + 131.5) * 62.4
    return (1.0 - water_cut) * oil_density + water_cut * water_density_lbft3

def hydrostatic_gradient_psi_ft(mixture_density_lbft3):
    """Hydrostatic gradient from density."""
    return mixture_density_lbft3 / 144.0

def pwf_from_whp(whp_psia, tvd_ft, mixture_gradient_psi_ft,
                 friction_loss_psi=0.0):
    """
    Approximate flowing bottomhole pressure from wellhead conditions.
    """
    return whp_psia + mixture_gradient_psi_ft * tvd_ft + friction_loss_psi

def flowline_backpressure(separator_pressure_psi, line_loss_psi=0.0):
    """Backpressure seen by the tree from downstream constraints."""
    return separator_pressure_psi + line_loss_psi

Use with caution

Homogeneous mixture gradients are screening tools only.
Deviated wells, slug flow, and high GLR often need Beggs-Brill, Hagedorn-Brown, or field-calibrated lift tables.
If the answer changes materially with a small gradient change, report that sensitivity explicitly.

Module 3 - Choke And Surface Constraint Screening

def choke_dp(p_upstream_psia, p_downstream_psia):
    """Simple choke differential pressure."""
    return max(0.0, p_upstream_psia - p_downstream_psia)

def compression_ratio(p_suction_psia, p_discharge_psia):
    """Compressor pressure ratio."""
    if p_suction_psia <= 0:
        return None
    return p_discharge_psia / p_suction_psia

def rate_gain_pct(q_old, q_new):
    """Percent production gain from an optimization change."""
    if q_old == 0:
        return None
    return 100.0 * (q_new - q_old) / q_old

Operational levers

Lever	Moves mostly	Typical effect
Lower wellhead / separator pressure	Outflow curve down	Higher rate
Larger tubing ID	Outflow curve down	Less friction, more rate
Larger choke	Outflow restriction down	Higher rate until another bottleneck takes over
Gas lift / lower liquid holdup	Outflow curve down	More drawdown, less gradient
Stimulation / reduced skin	Inflow curve up	Higher rate at same pwf

Module 4 - Nodal Matching

def nodal_mismatch(q_ipr, q_vlp):
    """Positive value means inflow exceeds outflow at the tested point."""
    return q_ipr - q_vlp

def choose_best_candidate(candidates):
    """
    candidates: list of dicts with keys q_ipr, q_vlp, and any metadata.
    Returns the candidate with the smallest absolute mismatch.
    """
    if not candidates:
        return None
    return min(candidates, key=lambda c: abs(c["q_ipr"] - c["q_vlp"]))

In practice, scan a range of candidate rates and select the rate where inflow and outflow are closest. Then pressure-match that solution against measured flowing bottomhole or wellhead data.

Module 5 - Diagnostic Patterns

Observation	Likely bottleneck
Reservoir pressure healthy, high wellhead pressure sensitivity	Surface or flowline backpressure
Weak response to choke opening	Reservoir inflow limited or already critical flow
Rate rises strongly with larger tubing	Tubing friction dominated
Large rate gain from lower separator pressure	Compression / facility bottleneck
Gas lift only helps at high liquid loading	Gradient dominated outflow problem

Output Format

When using this skill, structure the answer as:

Inflow model chosen and why
Outflow assumptions and gradient sensitivity
Estimated operating point
Sensitivity ranking of choke, tubing, compression, or lift changes
Recommended next measurement or field action

Integration Points

Use pnge:petroleum-pvt for black-oil and gas property inputs.
Use pnge:artificial-lift for gas lift, rod pump, ESP, or plunger-lift optimization.
Use pnge:surface-facilities for separator and compression constraints.
Use pnge:rta-production or pnge:well-test-analysis when inflow uncertainty dominates.

nodal-analysis-multiphase