Free Online Material Balance Tool: P/z Plot and Gas in Place

Geology
All tools
Report
Load the demo or paste pressure survey history. The fit runs entirely in your browser.
Each point is a shut-in pressure survey converted to P/z, plotted against cumulative production. A volumetric (depletion drive) gas reservoir plots as a straight line whose x-intercept is the original gas in place. Points curving up and off the early trend are the classic signature of aquifer support, and the tool warns when it sees it.

Material balance in your browser

The P/z plot is the oldest trick in reservoir engineering and still one of the most powerful: measure reservoir pressure a few times over a field life, divide each by the gas deviation factor z, plot against cumulative production, and a closed gas reservoir gives you a straight line whose x-intercept is the original gas in place. The tool above does the whole workflow: paste your pressure surveys as Gp, pressure, and z, and it fits the line by least squares, reports OGIP, initial P/z, recovery to date, and fit quality, and draws the extrapolation to the intercept. The demo field loads a textbook 100 Bcf case with realistic survey scatter.

P/z versus cumulative gas production plot with pressure survey points falling on a straight line extrapolated to the original gas in place at the x-intercept
The P/z plot: a closed gas reservoir declines on a straight line, and the x-intercept is the gas in place.

This opens the Reservoir Engineering Suite, linked in the bar above. Like every calculatgeek tool, the data stays in your browser, which matters because pressure histories are commercial information.

Why P/z is a straight line

A volumetric gas reservoir is a fixed tank obeying the real gas law. Remove gas and the remaining moles drop in proportion, so pressure over z falls linearly with cumulative production. No geology, no maps, no seismic: two good surveys define the line and a third tests it. That economy is the enduring appeal, and also the trap. The line is only straight if the tank is closed. When an aquifer supports pressure, late points sit above the depletion trend, the extrapolated intercept overstates the decline and understates future pressure, and a naive fit gives an OGIP that is too small while promising recovery that arrives too easily. The tool checks the residuals and warns when the late data starts lifting off the line.

Reading the numbers

  • OGIP is the x-intercept: the size of the tank the pressure history implies, independent of the volumetric estimate from maps. When the two disagree, something interesting is true.
  • Recovery to date is cumulative production over OGIP; volumetric gas fields commonly recover 80 to 90 percent, which is why the extrapolation is worth real money.
  • R squared near 1 with random scatter supports depletion drive. A systematic bow in the points is geology talking, not noise.
  • Garbage in warning: P/z is only as good as the shut-in pressures. Surveys taken before full buildup plot low and bend the line.

What this tool is for

Quick OGIP from a pressure history, auditing a booked figure against its own data, and teaching the method with numbers students can check by hand. It implements the classic dry gas P/z balance; oil material balance with gas caps and water influx terms, and p/z variants for overpressured reservoirs, are the natural next steps up and belong to full reservoir engineering packages. For the question how big is this gas tank, this page answers it in a minute.

Related Calculators and Reference