Unconventional gas wells often show linear flow during their transient period, and this transient behavior can last for several years. Currently industry uses typecurve matching technique to analyze this linear flow. The typecurves in common use are based on the assumption that wells produce at constant rate. However, most of the time, the production rate is variable, and in fact is closer to constant pressure operation. The constant pressure typecurve is useful but not suitable when both rate and pressure vary. It is necessary to have an easy-to-use method for analyzing variable rate/pressure data in linear flow.
There are two objectives for this paper. First, it serves to review the formulation, typecurve, specialized plots and superposition time that are used to analyze transient linear flow. It helps readers gain a deeper understanding of the theory. Second, it serves to illustrate a practical and effective way for analyzing variable gas production data.
In this development, we studied the effect of skin on the typecurve and on the specialized plot. We converted the constant pressure solution to its constant rate equivalent by using material balance time and found it to be acceptable for all practical purposes. We converted real time to corrected pseudo-time to account for variable gas properties, and found the effect to be small in the analysis of actual production data. We investigated the effect of outliers on superposition time. In the end, we proposed an approach to analyze variable rate/pressure data during transient linear flow and confirmed the validity of our methodology using synthetic data that generated in numerical models.