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Abstract

The Calcutta field in Suriname is comprised of shallow, low-pressure, heavy-oil sandstone reservoirs that are being produced by progressive cavity pumps. These reservoirs are difficult to characterize because of the complexity of the depositional environments encountered in the wetlands of Suriname.

The unconsolidated nature of these reservoirs makes it virtually impossible to recover any core successfully; hence, petrophysical parameters were originally derived from well-log analyses. Completions consist of gravel pack with screens to control the unconsolidated sands. Initial quality checks with surface liquid samples for PVT analysis routinely involve measurement of the bubble point pressure at ambient temperature. However, the emulsified water in the heavy oil made it difficult to conduct such PVT analysis.

The inability to obtain reliable core or fluid samples, made pressure-transient testing an essential tool for characterizing these reservoirs. After the initial operational and mechanical restrictions were resolved, a field-wide program was implemented in 2008. However, the quality of the recorded pressure data was adversely affected by frequent power failures that resulted in unplanned pressure buildup periods. The pressure transient analysis was also influenced by limitations in production data gathering during the flow periods. Additionally, uncertainty in fluid properties and problems with mechanical and pumping equipment caused undesirable pressure disturbances during the well testing periods.

Analyses of the tested wells showed better formation properties than suggested by the initial well-log-derived potentials, i.e., wells suspected to have formation damage proved to have negative skin, instead. Formation permeability was also found to be higher than the log-derived permeability values; this difference could have been caused by the gravel pack.

The geological interpretations were improved and uncertainties regarding completion efficiency have largely been eliminated. The results have contributed to a better understanding of reservoir performance and have led to increased production optimization efforts.

The field examples presented here illustrate the unique challenges experienced while applying pressure-transient testing in pumping wells to characterize the geologically complex reservoirs of the Calcutta field. This paper also describes the mechanical issues, well preparations, water production problems, and installation of permanent gauges in these wells.

Introduction

The Calcutta field is located in the marshy coastal plain of Suriname (Fig. 1) about 65 km west of Paramaribo, the capital of Suriname. The Calcutta field is a marginal swamp-operated oil field (20 km²). The full-scale development began in 2006 and currently, the field is producing 3,000 BOPD from a total of 178 wells. This field is being managed by an integrated Asset Team, consisting of a group from Geoscience, Operations, and Facilities.

A high-frequency stratigraphic analysis concluded that the Eocene- and Miocene-age reservoirs in the Calcutta field were deposited primarily in an estuarine environment that varied between tide-dominated and wave-dominated transgressive conditions. Fig. 2 shows the heterogeneous and compartmentalized nature of these reservoirs with high variations in sedimentary facies and a complex pattern in distribution and connectivity of reservoir sandstones similar to what has been interpreted in the Calcutta field. Attempts to recover cores in the unconsolidated sands have been largely unsuccessful. The most recent attempt resulted in poor recovery from some shaly sand intervals.

The heavy oil (17° API, 300 cp) is produced from several unconsolidated sandstone reservoirs at an average depth of 700 to 900 ft. Reservoir thickness ranges from 3 to 20 ft (1 to 6 m), reservoir temperatures are typically 95°F (35°C), and reservoir pressures are between 300 to 350 psia.