Abstract: A wellbore is divided into a plurality of depth intervals. For each depth interval of the plurality of depth intervals, a specific gas gravity, pseudo-critical gas properties, a pseudo-reduced gas pressure and temperature, a gas deviation factor, and a gas gradient and a bottom node pressure are determined. The determined bottom node pressure is used as a static bottomhole pressure.

Abstract: An initial value is set for an apparent molecular weight. Iterating occurs on calculations to determine a converged value for the apparent molecular weight, including the following. A specific gas gravity is determined. Pseudo-critical gas properties are determined. A pseudo-reduced gas pressure and temperature are determined. A gas deviation factor is determined using the pseudo-reduced gas properties. An average pressure, an average temperate, and a pressure gradient are determined for each depth interval. A new apparent molecular weight is determined. The absolute value of a relative error between a current value of the apparent molecular weight and the new apparent molecular weight is determined. If the absolute value of the relative error has converged to a constant, the iterating is terminated.