Many tight gas formations consist of numerous reservoir layers that are dispersed both vertically and laterally in a thick, complex sand dispersal system. A typical well will encounter layers of sandstone, siltstone, and shale. Depending upon the depositional and diagenetic history of the formation, these different layers of sandstone and siltstone can have significantly different values of permeability, porosity, and gas saturation. In many formations, like the Travis Peak in east Texas, the Wilcox in south Texas, the Frontier in Wyoming, or the Mesa Verde in Colorado, a well can encounter over one thousand feet of sandstone layers that contain gas. These complex reservoirs can be very difficult to produce because of low permeability and poor lateral continuity of the productive layers.

In these complex reservoir systems, one needs to be able to forecast both flow rates and ultimate gas recoveries to properly predict the economics of developing the formation. An engineer must evaluate existing wells to obtain a distribution of reservoir properties. This distribution of reservoir properties, if properly interpreted, can be used to predict well performance and compute the economics of drilling additional wells in a particular geographic area.

The knowledge of how reservoir properties are distributed is very important to the petroleum engineer. The most important properties are formation permeability, formation porosity, net pay thickness, and the areal size and distribution of the sandstone units. Once the distribution of these properties is known, one can determine the proper method of averaging these properties so that one can accurately predict the performance of additional wells drilled to this formation.

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