Fluid low in porous media is described by the diffusional type of partial differential equation. In deriving the flow equation for the constant compressibility fluids, an important assumption is made regarding the magnitude of the pressure gradient is assumed that this gradient is small enough that the square of this term becomes negligible. In this study, an attempt is made to quantify the effect of this simplification. Data generated by numerical simulator and analytical solutions are compared in order to determine the conditions under which this simplification is justified. Except for extreme conditions (non-linear coefficient greater than 0.01), in is justifiable to ignore the square of the pressure gradient term in oil diffusional equation. The drawdown solution on gas wells contains gas properties, which are pressure dependent. As the system is being depleted. the average reservoir pressure declines. Although it seems logical to consider the gas properties at the prevailing reservoir pressure, the gas viscosity and compressibility are usually evaluated at the initial pressure level. Comparison of the analytical solutions with numerical solutions obtained for a variety or typical well rest conditions indicates the magnitude of inaccuracy incurred by using standard approach to data interpretation. Based on this investigation, several pressure levels for properties calculation are considered. In applying analytical equations for pseudo-steady state flow conditions, gas properties should be evaluated at an average value of the initial and current reservoir pressures.