Although various novel techniques were developed in reservoir engineering for estimation of hydrocarbons initially in place (HCIIP), conventional material balance still remains as one of the most reliable. Average reservoir pressure is critical input data for material balance, which is usually obtained by well shut-in. Nevertheless, this operation might be restricted due to economic and operational restrictions such as water production in gas wells.

In contrast, daily production data is commonly available and can be used to calculate the HCIIP by applying any production data analysis techniques such as the Dynamic Material Balance (DMB) method. The application of such methods to volumetric gas reservoirs and naturally fractured reservoirs resulted in accurate and reliable estimations. However, for water drive gas reservoirs, where the water influx term should be introduced into the iterative process, research and field case applications are limited.

This paper presents an extension to the DMB technique to water-drive gas reservoirs. A simultaneous estimation of the Original Gas-in-place (OGIP) and the water influx term is achieved by coupling the DMB technique with the Fetkovich aquifer model. Average reservoir pressure estimation can also be attained as a result of the coupled method.

Results were validated by means of numerical simulation on a synthetic model and a field study case. Synthetic production data was generated by a commercial simulator and then analized with the coupled method. The calculated OGIP, water influx volumes and average reservoir pressure resulted comparable to simulator output as they presented a low relative error. Furthermore, application of the coupled method to the field study case yielded comparable results to those obtained by volumetric method.

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