Compressibility factor, z factor, values of natural gases is necessary in most petroleum gas engineering calculations. The most common sources of z factor values are experimental measurements, equations of state method and empirical correlations. If laboratory measured data were not available, empirical correlations maybe used to determine the compressibility factor. However, there might be discrepancies between laboratory-measured values with those obtained by the correlations which, in turn, might affect the outcome of a study. Sensitivity of analysis is a technique used to determine how different values of the compressibility factor would affect well testing and material balance equation calculations. Under a given set of data, this technique is used within specific boundaries that will depend on one or more methods, such as the effect that changes in reservoir's parameters; obtained from build-up test, and calculations of gas initial in place by using material balance equation. This study highlights how changes in z factor affect well testing results and volume of gas initially in reservoir calculated using material balance equation. Three wells producing under partial water drive mechanism were considered for this study. Dranchuk and Abu-Kassem’s method was found to have the lowest error.