Abstract
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.  
Highlights
Most hydrocarbon reservoirs are surrounded by aquifers
The general objective of this work is to analyse the correlation between the van Everdingen and Hurst model and cubic cumulative production model hereafter considered as analytical model
The results demonstrate clearly that the correlation between both methods depends on the calibration of the two unknown parameters α and β, appearing in the cubic cumulative model (Blansigame and Zonoozi, 2005)
Summary
Most hydrocarbon reservoirs are surrounded by aquifers. Aquifers may in some cases be significantly greater than the gas reservoir, ranging from infinite in size to less than insignificant, with corresponding large to negligible effect on the reservoir performance (Ahmed, 2005).In reservoirs adjoined by water aquifers, water drive may be the primary production mechanism. Most hydrocarbon reservoirs are surrounded by aquifers. Aquifers may in some cases be significantly greater than the gas reservoir, ranging from infinite in size to less than insignificant, with corresponding large to negligible effect on the reservoir performance (Ahmed, 2005). In reservoirs adjoined by water aquifers, water drive may be the primary production mechanism. In these reservoirs, the production of hydrocarbons causes a pressure drop in the hydrocarbon/water interface. The production of hydrocarbons causes a pressure drop in the hydrocarbon/water interface Due to this pressure drop, a pressure differential develops from the surrounding aquifer into the reservoir. The aquifer reacts by encroaching across the original hydrocarbon-water contact, filling the reservoir pore spaces (Feng et al, 2015)
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