The * Volumetric Method* for OGIP is essentially the same for gas reservoirs. The method uses static geologic data to determine the volume of the pore space of the reservoir. Once the volume of the pore space is estimated, then the gas formation volume factor, ${B}_{g}$ can be used to estimate the OGIP.

By simply using the definition of reservoir volumes, the original-gas-in-place of the reservoir be determined by:

or, equivalently (after applying the Saturation Constant):

Where:

- $G$ is the original-gas-in-place of the reservoir, SCF
- ${V}_{grv}$ is the gross rock volume, ft
^{3} - $\left(\overline{{h}_{n}}/\overline{{h}_{g}}\right)$ is the net-to-gross thickness ratio, fraction
- $\overline{{h}_{n}}$ is the average net reservoir thickness, ft
- $\overline{{h}_{g}}$ is the average gross reservoir thickness, ft

- $\overline{{S}_{g\iota}}$ is the average initial gas saturation, fraction
- $\overline{{S}_{w\iota}}$ is the average initial water saturation, fraction
- $\overline{\varphi}$ is the average reservoir porosity, fraction
- $\overline{{B}_{g\iota}}$ is the average formation volume factor of the gas at initial conditions (p
_{ri}and T_{r}), ft^{3}/SCF

As with the oil volumetric equation (**Equation 4.02**), **Equation 5.01** is evaluated at initial pressure and saturation conditions because the original in-place volume is the desired result. Also, as with the volumetric oil equation, the net-to-gross ratio in the volumetric gas equation is simply the thickness fraction that converts the total reservoir thickness to the thickness that contributes to hydrocarbon storage and flow in the reservoir. We use thickness to apply the net-to-gross ratio because rock formations are formed as layers during deposition, and we assume that layers of poor-quality rock may be deposited and intermixed with layers of good quality reservoir rock. We can further define the gross rock volume as:

Where:

- ${V}_{grv}$ is the gross rock volume, ft
^{3} - $43,560$ is a unit conversion constant, ft
^{2}/acre - $A$ is the mapped area of the reservoir, acres
- $\overline{{h}_{g}}$ is the average gross reservoir thickness, ft

As with oil reservoirs, the volumetric method for estimating the in-place volumes is also considered to be less accurate than the material balance method. The reason again being the use of all of the averages in **Equation 5.01**. This can also be improved by the use of the iso-contour technique describe in Lesson 4.