A dry gas material balance with an infinite aquifer influence: A comparative study between the unsteady state model of van Everdingen-Hurst and analytical model

Aquifer water influx is an important natural mechanism for primary recovery. It affects the performance of all types of reservoirs, also natural gas reservoirs. Water influx provides pressure support during reservoir depletion, resulting in slower pressure decline. Consequently, gas reservoirs associated with large aquifers show a flattening, cubic behavior of the p/z vs. Gp curve, which allowed the development of the present analytical model. For modelling of water influx into a reservoir, classical models have been developed by many authors. Among the classical models, the unsteady state method of van Everdingen-Hurst was selected to be used in this work, as this is the best suited in terms of solving the diffusivity equation. In order to use the analytical model for comparative purposes, there was a need of calibrating the two unknown parameters, α and β, appearing in the water influx equation. In this work, two workflows were presented for computing water influx in a comparative manner between the unsteady state model of van Everdingen-Hurst and the analytical model. The results showed that the correlation between both models depends on the two unknown parameters, α and β. Key words: Infinite aquifer, dry gas material balance, cubic cumulative model, water influx.