Many tight gas wells (permeability less than 0.1 md) exhibit linear flow through their transient period. This transient period may last for years insome cases. It has been learned that this behavior differs in many ways from radial flow behavior. This paper reports another import difference between linear flow and radial flow - rate sensitivity. It has been shown and accepted for years that real gas pseudo-pressure can be used to apply analytical solutions to transient radial flow. However, it has been noticed that analytical solutions can be in serious error when applied to transient linear flow. Specifically, the slope of the t departs from the analytical value as the flow rates or degree of drawdown become higher. This paper demonstrates therate/drawdown sensitivity of transient linear flow. Then a correction factor is presented which corrects the slope of the plot and improves the accuracy of kAc and OGIP, as calculated from production/pressure performance.
Many wells in tight gas reservoirs have long-term performance which exhibit only linear flow, not radial flow, during the transient period. Wells have been observed which stay in the transient linear flow regime for several years. Someof these wells have hydraulic fractures and some do not. It is usually not practical to analyze tight gas wells with build-up tests, but long term production and pressure data can be used for analysis. Previous papers have presented methods of analysis1–5. The analysis of these wells comes from plotting [m(pi) - m(pwf)]/qg vs. t and observing the slope,CPL m ~, and the end of the straight line, tesr (end of the transient linear flow period). From these values, c k A and OGIPcan be calculated. The equations for this analysis are given in Table1. The value of permeability can be calculated if the value of Accan be estimated. For example, Ac can be estimated for a hydraulicfracture of known fracture length.
