Abstract

Residual gas and oil saturations and relative permeabilities have been quantified in the Maui Field. Additional Special Core Analysis (SCAL) laboratory data was acquired using decane/brine and oil/brine centrifuge experiments. Such measurements are considered most representative of water influx into gas and oil reservoirs respectively. In the case of oil, ageing of the samples to restore wettability is shown to be essential. Relative permeability curves were obtained by history matching the raw experimental production data via numerical simulation. This process corrects for experimental artefacts and limitations, resulting in a significant reduction in residual hydrocarbon saturations compared to typical analytical interpretations. A correlation using a form originally proposed by Land has been used to relate the residual hydrocarbon saturations to the initial water saturations. In-situ field measurements of hydrocarbon saturations using pulsed neutron logs in water-flooded zones are shown to support the SCAL data.

The work described represents the state-of-the-art in quantification of residual hydrocarbons. In particular, the combination of the sampling methodology, the experiment design, the advanced numerical interpretation and the in-situ measurements is material previously unpublished in technical literature. The results are significant in that they show lower residual saturations than commonly expected, while adding to the limited published data on residual hydrocarbons. Use of lower residual hydrocarbon saturations together with the appropriate relative permeabilities in reservoir simulation has resulted in improved reservoir history matches and has had a positive influence on Maui Field reserves. Application of these state-of-the-art techniques to other water-drive fields is likely to have a similar impact.

Introduction

An understanding of the relative permeability of hydrocarbons and water is essential for reservoir simulation. The remaining hydrocarbon saturation after water flood and hence the ultimate recovery (UR) is strongly determined by the tail end shape of the imbibition relative permeability curve, close to the residual hydrocarbon saturation.

This paper reports the results of extensive SCAL studies to better quantify relative permeabilities, including residual hydrocarbon saturations, in most of the gas and oil reservoirs of the Maui Field.

The Maui gas and oil field, off the Taranaki coast, New Zealand, was discovered in 1969. Figure 1 gives an overview of the various gas/condensate and oil sands in the Maui-A and -B area. The Maui-A C and D gas sands were brought on stream in 1979 via a single production platform, MPA. In 1992 a second production platform, MPB, was installed to produce gas/condensate from the B area. In 1993 the Maui-B C and Upper D gas sands were brought on stream via this second platform tied back to the MPA platform. Also in 1993 an oil accumulation was discovered in the B F sands underlying the C and Upper D gas reservoirs and the Lower D oil reservoirs. The Maui-B Lower D and F oil sands have been producing since September 1996 via the FPSO Whakaaropai connected to MPB.

The residual gas and oil saturations previously used for Maui reservoir modelling are high compared with recent state-of-the-art SCAL studies carried out on sandstone reservoirs from various fields around the world1. In these recent studies, a systematic reduction in residual hydrocarbon saturations, in combination with higher Corey exponents, is observed.

There are two reasons that could explain the likely too-high estimates for the previous residual hydrocarbon saturation data.

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