Nodal Analysis
Nodal analysis is a systems-analysis approach used to evaluate the performance of a producing well by dividing the production system into components (reservoir, completion, tubing, surface equipment) and analysing the pressure drops across each. The intersection of inflow and outflow curves determines the well's operating point - its deliverability.
The Production System
Reservoir → Sandface
Pressure drop from average reservoir pressure (Pr) to sandface pressure. Governed by the Inflow Performance Relationship (IPR) - described by Darcy's law for single-phase flow or Vogel's equation for two-phase flow.
Completion & Near-Wellbore
Pressure drop through perforations, gravel pack, or screens. Skin damage (positive skin) restricts flow; stimulation (negative skin) enhances it.
Tubing & Flowline
Pressure drop from the bottomhole to the wellhead due to gravity (hydrostatic head), friction, and acceleration. Described by the Vertical Lift Performance (VLP) or Tubing Performance Relationship (TPR).
Choke & Surface Facilities
Pressure drop through the surface choke and flowline to the separator. The separator pressure sets the downstream boundary condition.
IPR vs VLP: Finding the Operating Point
The operating point is where the IPR curve (what the reservoir can deliver) intersects the VLP curve (what the tubing can handle). This intersection gives the well's flowing rate and bottomhole pressure under current conditions.
Example: Well-D08 has an IPR showing a maximum rate (AOF) of 3,000 bbl/d. The VLP curve for the current 3.5" tubing intersects the IPR at 1,800 bbl/d and 2,100 psi Pwf. Analysis shows that installing 4.5" tubing would shift the VLP curve, moving the operating point to 2,300 bbl/d - a 28% production increase. This justifies a tubing replacement workover.
Sensitivity Analysis
Tubing Size
Larger tubing reduces friction but may cause liquid loading at low rates. Optimal tubing size depends on expected rate range.
Water Cut
Increasing water cut increases the hydrostatic head (heavier fluid column), shifting the VLP curve upward and reducing the deliverable rate.
Reservoir Pressure
As Pr declines, the IPR shifts left and down, reducing the deliverable rate. This is why waterflooding to maintain pressure is so critical.
Gas Lift Rate
Injecting gas lift lightens the fluid column, shifting the VLP curve downward - increasing the deliverable rate at the same reservoir pressure.