### Calculate Original Gas in Place with Excel

This Excel spreadsheet calculates the Original Gas in Place with a p/z plot.  Specifically, this spreadsheet fits the observed p/z data and cumulative gas product to the following curve

where
• (p/z) is the observed values of pressure over compressibility (collected during the life of a reservoir)
• Gp is corresponding cumulative gas production (again collected during the life of a reservoir)
• (p/z)i is the initial value of pressure over compressibility of the system
• G is the original gas in place
This equation is simply a material balance, which many engineers use to determine the Original Gas in Place in volumetric natural gas reservoirs. It assumes a constant pore volume and includes the effect of gas expansion.  Note that the equation predicts a linear relationship between p/z and G

This screen grab illustrates the contents of the spreadsheet.

The spreadsheet is easy to follow.  It simply minimizes the sum-square residuals (difference between predicted and actual values of p/z) by varying G.  You can also choose to vary (p/z)i in addition to G to minimize the sum-square residuals, but since this value is usually accurately measured, this is not recommended.

Fundamentally, this method is pretty simple to understand - we're simply locating the intercept on the x-axis (where p/z = 0) of a straight line fit of G against p/z, as illustrated by this picture.

This method of determining the Original Gas in Place is only suitable for normally pressurized gas reservoirs, where the gas expansion is the only significant factor contributing to gas production. This method, however, is not suitable for abnormally-pressurized reservoirs, where gas production is affected by formation compressibility (i.e.the expansion of sand, rock and water). Gas reservoirs are often abnormally-pressurized if they are in contact with aquifers