Surface network

#Overview

The Surface network module solves a gathering system at steady state: wells, flowlines, chokes, compressors, separators and delivery points. You draw the network on a canvas, set each element's properties, and the solver finds every node pressure and branch rate. It also allocates lift gas across wells and sets chokes to meet constraints. Use it to find deliverability, locate backpressure and bottlenecks, and test lift-gas and choke changes before you make them in the field.

#Inputs

The network solves on the project fluid, tabulated in pressure and temperature with the bubble point as a node. With no project fluid, the solver uses a typical black oil and the page warns you. See Project fluid.

#Nodes

Add nodes from the Build panel: Well, Source, Joint, Separator, Sink and Gas-lift manifold. A new network starts with one sink, the Delivery point.

#Wells

#Branches

Drag from one node to another to connect them. A branch from a gas-lift manifold becomes a Lift-gas line; every other branch starts as a Pipeline. Change the type in Properties.

#Methods

• Solver: Newton-Raphson on node pressures and branch mass rates, with line search and step caps. Flow direction is free, so loops and reverse flow work. Convergence requires mass balance and oil, gas and water component balance at every node, and the pressure equation on every branch.
• Multiphase pipe flow: Beggs & Brill (1973) with the Payne et al. (1979) corrections, integrated along the elevation profile. Temperature varies linearly between the ends.
• Wells: IPR by PI, Vogel (1968) with a straight line above the bubble point, Fetkovich (1973), or the gas back-pressure equation (Rawlins & Schellhardt 1936). The tubing is traversed with Beggs & Brill. Lift gas enters at the injection depth.
• Chokes: Gilbert-type critical-flow correlations (Gilbert, Ros, Baxendell, Achong).
• Compressors: fixed boost, fixed ratio, fixed power with polytropic compression, or boost to meet the sink pressure.
• Erosional velocity: API RP 14E with C = 100, reported as vm / Ve on each branch.
• Gas-lift allocation: equal slope (Kanu, Mach & Brown 1981) on the concave hull of each well's lift performance curve, re-solved in a network loop until total oil settles, and checked against an exhaustive grid search.
• Choke setting: opens every choke, then for each violated constraint bisects the bean of the worst offender, accepting only converged solves that meet the limit. Each binding constraint gets a shadow value: oil gained per unit of relaxation. This is a rule-based method, not a proven optimum.

#Workflow

1. Open Surface network and press Add a node. The network starts with the Delivery point sink.
2. In Build, add wells, joints, separators and any gas-lift manifold. Drag between nodes on the Canvas to connect them.
3. Select each node and branch and set its properties. Replace every value that shows a Typical value badge.
4. Leave Solve on Auto-solve, or set it to Paused and press Solve when you are ready.
5. Read the headline tiles, then the Nodes, Branches and Wells tabs. Use Pressure profile with Profile extent (Well to separator or Surface lines) to follow pressure along a path.
6. With a gas-lift manifold, choose the Lift-gas objective: Maximum oil, or Maximum net value with Oil netback ($/STB, default 60), Water handling cost ($/STB, default 2) and Lift-gas cost ($/Mscf, default 1). Press Allocate lift gas.
7. If the model has constraints, press Set chokes. Results appear on the Optimisation tab.

#Interpreting results

• Oil delivered shows how many wells are flowing. Sales gas is the headline gas rate. Its footer shows the gas balance: formation + lift = gas arriving at the separator = sales + recycled lift gas.
• Solver diagnostics lists mass balance, oil/gas/water balance at the worst node, pressure residual on the worst branch, Newton iterations and elapsed time. If a solve fails, the page names the node or branch and the physical reason when known.
• On Gas-lift allocation, Equal slope is the marginal oil per unit of lift gas at the optimum.
• On Choke settings and binding constraints, each constraint shows value, limit, slack, status (VIOLATED, binding or slack) and its shadow value. A large shadow value tells you which limit to relax first.
• A vm / Ve of 1 or more on a branch means it is at or over the erosional velocity.

#Common pitfalls

#Exports

• Nodes, Branches and Wells tabs each have a CSV download: network-nodes.csv, network-branches.csv and network-wells.csv.
• The page saves a version before each optimiser run.

#Related

• Well performance for detailed inflow, lift and nodal analysis on single wells
• Integrated forecast, which links to the project's surface network
• Black-oil PVT
• Units