This case study presents an analysis of production and pressure data from the Horst reservoir, an uplifted fault block with a thin oil rim and large gas cap in the greater Angostura field offshore Trinidad and Tobago. The study resulted in reinterpretation of the volumetrics and improved understanding of connectivity and compartmentalization in the field. This paper demonstrates how the application of fundamental Reservoir Engineering principles provides an effective way to understand reservoir flow dynamics and can guide the use of more sophisticated numerical techniques. The insights gained from this approach illustrate the importance of acquiring good quality data during the appraisal, start-up and production phases of an oil and gas field. Also, this study reinforces the benefits of using idle wells as observation wells for obtaining reservoir surveillance data.

The greater Angostura field consists of Oligocene age turbidite sandstone reservoirs with multiple compartments and complex geology. Some uncertainties remain in seismic interpretation with regards to mapping reservoir boundaries due to poor seismic imaging in the area. Consequently, analysis of production performance was critical to distinguish between different scenarios that were developed using seismic volume interpretation.

Formation pressures in the Horst reservoir were obtained after two years of production from offset reservoirs. Pressures were significantly different from the offset reservoirs indicating that Horst reservoir was potentially a separate compartment. The Horst commenced production after two years of offset production and was initially constrained due to high GOR. In the recent years, the Horst reservoir production has been considerably increased due to additional drilling and the commencement of gas sales from the greater Angostura field.

This paper presents the results of material balance and numerical simulation studies on the strong production performance seen in the Horst reservoir. The reservoir pressures for all blocks were estimated and the potential communication between reservoirs was investigated with material balance analysis. This led to an estimate of the Horst reservoir in-place volumes that were significantly higher than the originally mapped volumes. The insight from the material balance analysis was incorporated into a numerical simulation modeling effort. A robust simulation history match was obtained with a unique but practical approach to model the pressure depletion seen in the Horst reservoir during periods of no production. The simulation model was used to develop an optimized reservoir management strategy which has been implemented to increase production and recovery from the Angostura field. Furthermore, the improved understanding of reservoir performance has led to refinements of the geological and seismic interpretation of the Horst area.

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