Abstract

The paper describes the development of an equation of state (EoS) for two reservoirs of the Dukhan field, the Arab C and the Arab D reservoirs. The paper also describes the development of compositional maps for both the reservoirs. In absence of pre-production samples, an iterative approach was used to estimate composition at a known depth for the Arab C reservoir. Equilibrium contact mixing (ECM) method along with an iterative approach was used to estimate oil composition at the gasoil contact depth for the Arab D reservoir. By examining the heptanes-plus (C7+) properties of the samples collected from the depleted Arab C reservoir, existence of compositional gradient in the reservoir at discovery conditions was confirmed. It was shown that the existence of near constant C7+ properties in the oil-column of the depleted Arab D reservoir does not preclude the existence of compositional gradient at discovery conditions. By incorporating the gravity segregation and the thermal diffusion effects, oil composition maps for the Arab C and the Arab D (excluding Diyab sector) were developed. It was shown that the inclusion of thermal diffusion effect resulted in very small changes in component concentration for the Arab C reservoir. The large saturation pressure difference between three sectors of the Arab D and the fourth sector, Diyab was then modeled using molecular diffusion. For the Arab D gas-cap, the gas composition at the gas-oil contact depth was calculated by utilizing the tuned EoS and the oil composition at the gas-oil contact. The gravity segregation and the thermal diffusion significantly changed pentanes-plus (C5+) and C7+ distribution in the gas-cap gas with 23% and 30% reduction in mole% from the gas-oil contact to the crest of the structure.

Introduction

The large, mature Dukhan Field is located onshore Qatar, approximately 80km west of Doha. It is a North-South plunging anticline approximately 70km long by 8km wide (Figure 1)1. The field was discovered in 1939 and first production occurred in 1949. The field has more than 750 well penetrations. The major oil reservoirs are the Upper Jurassic Arab C and Arab D. There are lesser amounts of oil and gas in the Middle Jurassic and non-associated gas in the Permo-Triassic Khuff formation.

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