Birba Field PVT Variations Along the Hydrocarbon Column and Confirmatory Field Tests
Authors: Riemens W.G. , Schulte A.M. , de Jong L.N.J. 
Published: December 2007

This paper describes a practical application of thermodynamic modeling that indicated that the PVT properties of the reservoir fluid in the Birba field were strongly depth-dependent, explaining the presence of significantly undersaturated oil only 200 m [655 ft] below a gas cap. Field tests confirmed the thermodynamic model to be correct. Furthermore, this paper discusses the options for field development, including the paper discusses the options for field development, including the consideration of possible benefits of (developed) miscibility when gas is injected.

Introduction

The Birba field, located 22 km [14 miles] from the Marmul field in south Oman (Fig. 1), was discovered in 1978 by Well Birba (Well BB-1). The well encountered oil in a 50-m [165-ft] -thick carbonate stringer" embedded in Precambrian salt at a depth of 2600 m [8,530 ft] (Fig. 2). In 1976, a small oil accumulation had been discovered in Nasir, and after Birba several other exploration wells were drilled in the Greater Birba area, but with limited success.

The volume of stock-tank oil initially in place (STOIIP) in the Birba field is estimated at 26 × 10 m [164 × 106 bbl]. The reservoir is highly overpressured; the initial pressure is 53 200 kPa [7,720 psi], and the moderately light oil (0.866 g/cm3 [32API]) has a considerable amount of solution gas (260 m3/m3 [1,440' scf/STB]).

The Birba field has been producing since April 1982 from two wells at a relatively low rate, mainly to gather information on the fluid and reservoir properties. These data were required to decide whether pressure maintenance by gas injection would be feasible. The cumulative oil production reached 0.6x 106 m3 [3.8 × 10 bbl] by the end of Sept. 1984; cumulative gas production was 0.14 × 10 m3 [5.0 Bcf].

This paper describes the peculiar PVT properties of the Birba reservoir fluid, how these can be explained by thermodynamic models, and how this model has been confirmed by field tests. Second, it shows how the thermodynamic model combined with a reservoir model has been used to evaluate the attractiveness of pressure maintenance. pressure maintenance.

Reservoir Geology

Several carbonate stringers have been found near the base of the Precambrian Ara salt of the South Oman salt basin. In terms of Precambrian Ara salt of the South Oman salt basin. In terms of reservoir development, the uppermost stringer in the Birba field is the most important. The stringers are considered to mark episodes of basin freshening, when salt and sulfate deposition ceased and more favorable conditions for carbonate production existed. Three general depositional environments could be recognized from cores and related to reservoir type.Basinal includes claystones and laminated carbonate mudstones deposited below the wave base. They form generally poor reservoirs.Shallow marine are algal mats with abundant stromatolites deposited within the subtidal to intertidal zone. Locally, this includes higher-energy deposits, which make up the best reservoir in the Birba field.Very shallow marine to supratidal include carbonate mudstones frequently in association with anhydrite deposited in a low-energy lagoonal/tidal flat/sabkha environment. This is poor reservoir rock.

The Birba stringer represents two subsequent gradual shoaling-upward sequences (Fig. 3). This is broadly reflected in the porosity distribution within the stringer showing a low-porosity (0 to 5%) porosity distribution within the stringer showing a low-porosity (0 to 5%) unit sandwiched between two zones of higher porosity (10 to 20%).

PVT Properties PVT Properties Analysis of PVT samples obtained during a production test on Well BB-1 showed that the bubblepoint pressure is about 47 500 kPa [6,900 psi], 5700 kPa [825 psi] below the initial pressure, and the FVF is 1.65. The oil is further characterized by the presence of a C7+ fraction with a relatively high molecular weight (average of C40). Moreover, CO2 (3.8%) and H2S (2%) are present.

Similar oil was tested in Well BB-2, 43 m [141 ft] downdip of Well BB-1. In Well BB-3, 100 m [330 ft] downdip of Well BB-1, the stringer also contains oil [on the basis of repeat formation tests (RFT's)], but for mechanical reasons the well has never been production-tested. production-tested. A well 2.5 km [1.6 miles] south of Well BB-1, Well BBS-1, encountered the same stringer as Well BB-1. which is 212 m [696 ft] shallower and gas-bearing. PVT analysis showed the presence of a significant condensate volume (926 × 10 m /m [5.1 STB/Mcf]). Reservoir pressures indicated that this gas could have been in communication with the oil. This would be rather unconventional, however. because the oil is undersaturated.

It is known that under special conditions gravity forces can induce variations in composition along the hydrocarbon column. Thermodynamic calculations were therefore initiated to investigate whether the Birba and Birba South fluids could be related.

Thermodynamic Calculations

A prediction of the variation in reservoir fluid composition and properties in a reservoir can be made by assuming that the hydrocarbon properties in a reservoir can be made by assuming that the hydrocarbon fluids are in thermodynamic equilibrium in the field of gravity. This assumption implies that in addition to compositional and mechanical equilibrium, the temperature is taken as constant throughout the reservoir.

In general, and in Birba in particular, temperature differences in the reservoir are small with respect to the absolute reservoir temperature, and one may expect that an equilibrium model will give an adequate description.

The following equations govern the compositional grading.

JPT

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