Overview
A conventional well model lumps every perforation onto a single bottomhole
pressure (BHP). That breaks down for long horizontal or deviated wells, where
friction and hydrostatic head make the flowing pressure vary along the wellbore —
the heel and the toe do not see the same drawdown. BirSim’s multi-segment well
(MSW) model resolves this by discretizing the wellbore into a tree of segments
(WELSEGS) and mapping each completion to its segment (COMPSEGS).
Each perforation is then driven by its own segment’s flowing pressure instead of one lumped BHP — reproducing the friction-choked toe of a long horizontal well.
The model
The wellbore is a tree of segments rooted at the top (BHP-datum) node. Given the operating BHP, BirSim solves the per-segment node pressures as a damped per-well fixed point:
- Each segment carries the reservoir-volume inflow of every perforation in its subtree.
- Its node pressure sits above its outlet by the hydrostatic head
ρ·g·Δtvdplus the Darcy–Weisbach frictionf·(L/D)·½ρv², with the friction factorffrom the Moody/Haaland correlation.
This is a semi-implicit coupling: the Newton Jacobian keeps the standard per-perforation linearization, while the well source uses the true segment pressures. The converged solution therefore satisfies the segment equations — the approximation only affects convergence rate, not the answer.
| Keyword | Role |
|---|---|
WELSEGS | Segment tree: top-node TVD/MD, INC/ABS mode, per-segment range / branch / outlet / length / TVD change / diameter / roughness |
COMPSEGS | Maps each completion (I,J,K) to its wellbore segment (explicit number, or inferred from branch + MD range) |
Validation
Setup. A horizontal producer deck run against OPM Flow’s multi-segment well model, swept across three tubing diameters — 0.05 m, 0.12 m, and 0.40 m — to exercise the friction term from strongly friction-choked to nearly frictionless.
| Diameter | Regime | BirSim vs OPM (steady state) |
|---|---|---|
| 0.05 m | Strongly friction-choked | FOPR / FPR / water-cut within ~1–3% |
| 0.12 m | Intermediate | FOPR / FPR / water-cut within ~1–3% |
| 0.40 m | Nearly frictionless | FOPR / FPR / water-cut within ~1–3% |
Beyond matching the aggregate rates, BirSim reproduces the friction-choked toe profile — the along-well pressure gradient that starves the toe of drawdown — and its response to tubing diameter: as the tubing widens, friction falls, the segment pressures flatten, and the inflow profile evens out. That diameter response is the discriminating test that a lumped-BHP model cannot pass.
Why it matters
Long horizontal and multilateral wells are standard in modern field development, and their productivity depends on the friction/hydrostatic balance along the wellbore. Matching OPM Flow’s MSW model across the diameter sweep — including the shape of the inflow profile, not just the total rate — shows BirSim resolves the in-wellbore hydraulics that a single-BHP completion model averages away.
Reference
Multi-segment well modeling follows the segmented-wellbore approach used in
Eclipse-style simulators (Holmes et al.). BirSim is validated against OPM Flow’s
WELSEGS/COMPSEGS implementation on the same decks; deck paths and command lines
are available on request.