A sensitivity analysis of the Original-Gas-In-Place (OGIP) in the Manville group of the Western Canada Sedimentary Basin (WCSB) is carried out by using Star Plots in order to determine (1) what parameters have that largest impact on the gas volume estimation, (2) what is the error in the volume estimation, and (3) what is percent error in the choices of effective porosity and Archie’s exponents m and n on water saturation. The Manville group in the WCSB contains a very large resource of natural gas that was quantified to be in the order of 1500 tcf (Masters, 1984).
The gas in place calculations are based on the volumetric equation that takes into account area (A), net pay (h), effective porosity (PHI), true resistivity of the formation (Rt), water resistivity (Rw), porosity exponents "m" and "n," and initial pressure (Pi).
The sensitivity analysis is carried out by choosing possible errors around each input parameter. This permits to concentrate evaluation efforts on the tools and data that have the largest effect on the calculated values of OGIP.
It is concluded that intuition does not necessarily distinguish the data with the largest impact on the calculations. For example, the water resistivity (Rw) is the parameter with the smallest impact on OGIP estimations presented in this study. Based on results, three new polynomial correlations are developed to represent the effect of porosity (PHI) and the m and n exponents on water saturation estimations for the Manville Group. These correlations will improve significantly the way in which uncertainty and risk associated with reserves estimation are quantified.