Material balance · diagnostics

Cole and Campbell plots

Before you spin the aquifer slider, ask whether the tank even needs water influx. Cole (gas) and Campbell (oil) are the same idea on different axes: plot underground withdrawal divided by expansion versus a production measure. A flat plateau says depletion / volumetric; an upward horn says water drive. Both charts are live in the material balance calculator; this page is how to read them.

Oil → Campbell, gas → Cole

Campbell (oil) — Y = F/E_t vs X = F (diagnostic) or (F−W_e)/E_t (matched). Plateau height ≈ OOIP N.
Cole (gas) — Y = F/E_t vs X = G_p. Plateau height ≈ OGIP G. Pair with classical p/Z (upward bend = water drive).

Energy-plot cousin: Havlena–Odeh (F vs E_tot, slope ≈ N). Campbell/Cole answer “is there We?”; H–O answers “what N fits this energy plane?”

How to read the shape

Shape	Meaning	Next step
Flat plateau	Volumetric / weak aquifer	Match N or G; keep We ≈ 0
Upward horn / hump	Water influx grows with time	Turn aquifer on; use Matched mode
Scatter / no plateau	Bad PVT, bad rates, or multi-tank	Fix history / PVT before We

Diagnostic vs matched

Diagnostic ignores aquifer correction — if the field has We, the curve bends up and a naive slope underestimates N (or G). Matched plots (F−W_e)/E_t; when We is about right the series flattens near the true stock. Free ResvEngine uses manual sliders — diagnostics tell you which slider to move, not an autofit.

Y_{\mathrm{diag}} = \frac{F}{E_t}\,,\qquad Y_{\mathrm{matched}} = \frac{F - W_e}{E_t}

Do this before tuning We

Import pressure + production; check Metric/Field headers.
Open Campbell (oil) or Cole (gas) in Diagnostic with aquifer off.
Flat → set N/G to the plateau. Horn → enable Pot/Fetkovich and switch to Matched.
Only then refine H–O / p/Z and energy split.

Diagnosis ≠ uniqueness: N and W_e trade off. Free shows warnings; autofit and ambiguity bands are Pro.

References