Estimation of OGIP in a Water-Drive Gas Reservoir Coupling Dynamic Material Balance and Fetkovich Aquifer Model

Zavaleta, Sebastian (Universidad Privada de Santa Cruz de la Sierra) | Adrian, Pedro Marcelo (YPFB Chaco S.A.) | Michel, Ricardo Marcelo (YPFB Chaco S.A.)



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. However, material balance requires availability of average reservoir pressure measurements, as these data is a critical input for its calculations. Alas, there are multiple scenarios where reservoir pressure cannot be measured as it requires for the well to be shut-in and this is subject to economic and operational restrictions.

In contrast, daily production data is commonly available and can be used to calculate the HCIIP by applying any production data analysis technique, such as the Dynamic Material Balance (DMB) method. It was widely demonstrated in recent years, how the application of such methods to volumetric gas reservoirs and naturally fractured reservoirs produced accurate and reliable estimations. Nevertheless, for the case of water drive gas reservoirs, were the water influx term should be introduced into the iterative process, research is scarce and field case applications are limited.

An extension to the DMB technique for water-drive gas reservoirs is presented in this paper. A methodology for simultaneous estimation of the Original Gas-in-place (OGIP) and the water influx term is derived and detailed. This 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.

This method was validated by means of numerical simulation on a synthetic model and a field study case. The OGIP, water influx volumes and average reservoir pressure calculated by the proposed coupled method were compared with simulator output where relative error was found to be negligible. Furthermore, application of the coupled method to the field study case yielded comparable results to those obtained by application of the volumetric method and conventional material balance.