MaBal — Gas Material Balance

OGIP (Linear P/Z)

—Bcf

x-intercept of P/Z fit

Fit Quality R²

—

linear regression all points

Fitted Pi/Zi

—psia

should match measured

Current Recovery

—%

Gp / OGIP latest

Drive Mechanism

Enter at least three data points to identify the drive mechanism.

Drive Index Breakdown · Pirson awaiting data

Run analysis to compute DDI · WDI · EDI

DDI — depletion WDI — water influx EDI — rock + water

Survey History — Pressure / Production

Static bottomhole pressure at multiple time points. Surface pressures don't work — wellbore hydrostatics corrupt the signal. Drop suspect points rather than include them.

Time (yr)	P (psi)	Z	P/Z	Bg	Gp (Bcf)	Wp (MMbbl)

FIG. P/Z VS Gp · x-intercept gives OGIP. Click points to exclude · drag line center to translate · use Slope ± to rotate · drag dashed green line for Pab.

Recoverable Reserves Forecast

Z @ Pab	—
Pab/Zab	— psia
Bg @ Pab	— rcf/scf

Recoverable Gp,ab

—Bcf

Gp at Pab/Zab on fitted line

Recovery Factor

—%

Gp,ab / OGIP

Remaining

—Bcf

Gp,ab − current Gp

RF Bound Check

—

expected range for drive

Advanced Diagnostics

When P/Z curves upward, the Cole plot quantifies the aquifer. We model We with Fetkovich finite-aquifer formulation. Iterate J and Wei until F/Eg vs We/Eg is linear with slope ≈ 1.

Cole OGIP

—Bcf

y-intercept of F/Eg line

Cole Slope

—

target ≈ 1.0

Cole R²

—

linearity F/Eg vs We/Eg

COLE PLOT · F/Eg vs We/Eg · y-intercept = OGIP, slope = 1 confirms aquifer

For abnormally pressured reservoirs (initial gradient > 0.6 psi/ft) rock and connate-water expansion contribute materially to reservoir voidage. The naive P/Z plot overstates OGIP. Roach correction (P/Z)·[1 − ce·ΔP] with ce = (cf + cw·Swi)/(1 − Swi) restores linearity.

Initial Gradient

—psi/ft

Pi / TVD

Effective ce

—×10⁻⁶/psi

(cf + cw·Swi) / (1 − Swi)

Roach Uplift

—%

vs naive P/Z

Roach OGIP

—Bcf

corrected for rock + water

Roach R²

—

linearity (P/Z)* vs Gp

ROACH PLOT · (P/Z)·[1 − ce·ΔP] vs Gp — corrected line projects to true OGIP

Havlena-Odeh linearization: F = G·Eg + We. Plot F (= Gp·Bg + Wp·Bw) versus Eg (= Bg − Bgi). Volumetric: line through origin with slope = G. Curvature → water influx or compartmentalization.

HAVLENA-ODEH · F vs Eg · slope through origin = OGIP for volumetric

Compute OGIP from each individual pressure point (one-point material balance), plot vs time. Stable horizontal → correctly characterized reservoir. Upward drift → unmodeled aquifer / connected reservoir. Downward drift → missing production / compartmentalization.

SINGLE-POINT OGIP · G(i) = Gp(i) / (1 − (P/Z)(i)/(Pi/Zi)) — trend as diagnostic