The Mango, Mahogany and Immortelle Field gas and condensate fields are located in the Columbus basin, 35-40 miles off the southeast coast of Trinidad in water depths of between 200 to 300 feet. All reservoirs are comprised of highly faulted stacked sandstone reservoir units of varying ages between Quaternary to Pleistocene. Most reservoir units are less than 0.5 tcf and are usually produced by one or two wells.

The permeability of these reservoirs varies from 10 – 2000 mD and due to the presence of both edge and bottom aquifer, water drive and subsequent water breakthrough prediction is key to understanding reservoir performance and reserves estimation. Some of the reservoirs in the Mango, Mahogany and Immortelle Field gas fields have tendencies toward pressure maintenance by natural water drive. The resultant effect of this phenomenon is not often readily apparent in the early stages of gas production.

This paper examines methods used in recognizing and evaluating this type of reservoir. The methodology here is to apply simple classical techniques before complex reservoir characterization and time consuming reservoir simulation models are built since it is not often necessary. The results of the comparison indicate the presence or absence of water drive. Results are illustrated using actual field data. It shows the "text book" Mango Field case compared to water drive anomalies from the Mahogany and Immortelle Fields; where the drive mechanism have historically been misunderstood. The earliest possible detection of water drive allows strategies to be formulated to achieve optimum conservation and economic benefits.

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