Abstract

This paper presents the results of a simulation study performed on a prolific reservoir in the Santa Barbara Field, located in the Northern part of Monagas State in Eastern Venezuela. This reservoir encompasses an area of 17,300 acres, with a STOIIP of 7.0 MMMBLS and 20500 MMMSCF of gas.

The initial reservoir pressure was 12,000 psi 15800 feet sub-sea. In order to develop and exploit this reservoir, a high pressure gas injection project is being considered both for partial pressure maintenance as well as for gas recycling purposes. With this in mind, a compositional, tridimensional simulation model was developed to optimize the production injection process.

Sub-saturated gas condensate is found in the upper part of the reservoir structure. The 36 API condensate is rather unusual, having a dew point pressure of 8,500 psi. A sub-saturated black oil is also found in the deepest part of the reservoir. Between these two regions there exists a transition zone with super-critical fluid behaviour. Due to the high compositional variation of the crude with depth, an equation of state (EOS) with 7 pseudo-components was used to characterize the complexity of the hydrocarbon column. The model uses a grid of 11,000 active cells (41x25x13) and was historically matched with pressure and production data, and the existence of lateral and vertical hydraulic continuity, was confirmed. Prediction runs indicate that the injection of 1,200 MMSCF of gas at 8,500 psi at the top of the structure, starting in 1999, will result in a total recovery of 2175 MMBLS (31% STOIIP) during the 30 year lifespan of the project.

Introduction

The Santa Barbara field was discovered in October of 1988 and it is located in the State of Monagas, 40 Kms to the west of its capital, Maturin, and 500 Kms to the east of Caracas (Fig. 1). The discovery well (SBC-1E) which penetrated the Oligocene Naricual formation and reached the Cretaceous formation at a depth of 16,000 feet sub-sea level, showed an initial production rate of 4,600 Stb/D through a completion into the middle part of the Naricual formation. The field has been producing an average of 94,000 Stb/D of oil, with a GOR of 5,500 Scf/Stb. As of December 1995, the field has accumulated 71 MMStb of oil (1 % STOIIP), through 55 completions. This study was performed using a 3D phase compositional simulator, in which hydraulic survey tables were incorporated into the simulation model in order to achieve a production forecast with a high degree of certainty. After the column of fluids was characterized and the isoproperty of the digital maps have been obtained, a thick grid simulation was built with thirteen (13) layers and two thousand (2,000) blocks, which was used for the balancing of the materials, the validation of the hydrocarbon volumes at the site and the equation of state which governs the behaviour of the fluids.

Based on the results of the thick grid and the incorporation of updated geologic information and the updating of reservoir maps, a fine grid simulation was built with eleven thousand (11,000) active blocks (42 × 25 × 13). A historic production and pressure match was made, as well as simulation forecasts, leading to the conclusion that the gas injection must commence in 1999 with the objective of maintaining the pressure of the deposit at 8,500 psi (dew point pressure), thus avoiding large losses of oil due to retrograde condensation effects and asphaltene deposits.

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