Surface Facilities Overview
Introduction
Surface facility calculations address the engineering problems that arise after fluids leave the wellhead. Two primary areas dominate production engineering at the surface:
- Choke performance --- controlling flow rate, protecting downstream equipment, and measuring production
- Pipeline flow --- transporting fluids from wellhead to processing facilities with acceptable pressure losses
Petroleum Office provides a suite of functions covering both multiphase choke correlations and single-phase/two-phase pipeline equations.
When Chokes Are Used
A choke is a fixed or adjustable restriction placed at the wellhead (or downhole) to control flow. Chokes serve several purposes in production operations:
| Purpose | Description | Example |
|---|---|---|
| Rate control | Maintain constant production rate independent of reservoir pressure changes | Prorating wells to meet pipeline capacity |
| Well testing | Establish stable flow conditions for rate and pressure measurement | Multi-rate tests with different choke sizes |
| Sand control | Limit drawdown to prevent sand production | Critical drawdown management |
| Equipment protection | Prevent excessive pressure on surface equipment | Separator and flowline design limits |
| Reservoir management | Control GOR or water cut by limiting rate | Preventing gas/water coning |
Critical vs Subcritical Flow
Flow through a choke can be critical (sonic) or subcritical depending on the pressure ratio across the restriction:
Most production chokes operate in critical flow, which simplifies calculations because the downstream pressure does not affect the flow rate.
Pipeline Flow Types
Pipeline calculations determine the pressure drop (or flow capacity) for fluids moving through horizontal or near-horizontal surface lines.
Single-Phase Gas
Dry gas transmission lines where no liquid is present. Governed by the general energy equation with compressibility corrections.
| Equation | Best Application | Accuracy |
|---|---|---|
| Weymouth | Short, small-diameter lines (< 12 in.) | Conservative (underpredicts capacity) |
| Panhandle A | Medium-diameter lines, moderate Re | Good for turbulent flow |
| Panhandle B | Large-diameter, high-pressure trunk lines | Best for large systems |
Single-Phase Liquid
Oil or water pipelines where gas is absent or remains in solution. Governed by the Darcy-Weisbach equation with Moody friction factor.
Two-Phase (Gas-Liquid)
Gathering lines carrying both oil/gas or gas/condensate. Requires empirical correlations that account for:
- Liquid holdup --- fraction of pipe occupied by liquid phase
- Flow regime --- stratified, slug, annular, or dispersed
- Slippage --- gas and liquid travel at different velocities
Model Selection: Chokes
Decision Framework
Quick Reference
| Condition | Model | Notes |
|---|---|---|
| Multiphase, critical flow | Gilbert or Ros | Most common field scenario |
| Multiphase, subcritical | Sachdeva | Mechanistic, pressure-ratio dependent |
| Multiphase, subcritical | Ashford-Pierce | Semi-empirical alternative |
| Single-phase gas, sonic | Gas choke (sonic) | Upstream pressure only |
| Single-phase gas, subsonic | Gas choke (subsonic) | Both pressures needed |
| Single-phase liquid | Liquid choke | Bernoulli-based orifice equation |
Model Selection: Pipelines
Decision Framework
Quick Reference
| Condition | Model | Notes |
|---|---|---|
| Gas, short lines (< 20 mi) | Weymouth | Conservative, simple |
| Gas, medium lines | Panhandle A | Good for moderate Reynolds numbers |
| Gas, long trunk lines | Panhandle B | Large-diameter, high-pressure |
| Liquid, any size | Darcy-Weisbach | Requires friction factor |
| Two-phase gathering | Eaton or Dukler | Empirical holdup-based methods |
Available Calculation Categories
Choke Calculations
Functions for predicting flow rate through chokes, sizing chokes for target rates, and back-calculating wellhead pressure.
Full Documentation: Choke Models
Pipeline Calculations
Functions for gas pipeline capacity, liquid pipeline pressure drop, and two-phase flow correlations.
Full Documentation: Pipeline Flow
Practical Workflow
Wellhead-to-Separator Analysis
- Determine wellhead conditions from well performance (IPR + VFP)
- Size or evaluate choke using choke models to set desired rate
- Calculate pipeline pressure loss to verify separator arrival pressure
- Iterate if separator pressure constraint is not met
Related Documentation
Detailed Theory
- Choke Models --- Critical and subcritical choke correlations
- Pipeline Flow --- Gas, liquid, and two-phase pipeline equations
Well Performance
- Well Flow Overview --- IPR and productivity index
- Pipe Flow Overview --- Wellbore multiphase flow correlations
Fluid Properties
- PVT Overview --- Oil, gas, and water property correlations
- PVT Gas Properties --- Z-factor, gas viscosity, gas density
References
Golan, M. and Whitson, C.H. (1991). Well Performance, 2nd Edition. Prentice Hall. Chapter 7: Choke Performance.
Beggs, H.D. (1991). Production Optimization Using Nodal Analysis. OGCI Publications. Chapters 3-4.
Mokhatab, S., Poe, W.A., and Speight, J.G. (2006). Handbook of Natural Gas Transmission and Processing. Gulf Professional Publishing.
Brill, J.P. and Mukherjee, H. (1999). Multiphase Flow in Wells. SPE Monograph Vol. 17.
Kumar, S. (1987). Gas Production Engineering. Gulf Publishing Company. Chapters 8-9.