Abstract

A state-of-the-art special core analysis (SCAL) program for the Dukhan Arab C and Arab D carbonate reservoirs was designed to provide reliable relative permeability and capillary pressure models for use in field-wide reservoir studies. The workflow process for the design and its implementation of such a program is described with a specific focus on four key requirements:

  1. measurements must be on representative rock samples (the right samples),

  2. measurements must be made under conditions representative of displacement processes in the reservoir (the right conditions),

  3. measurements must be made using precision equipment and techniques (the right equipment), and

  4. trained and experienced technologists are needed to conduct the measurements and model the data (the right people).

Results from the Arab D program are presented to demonstrate consistent saturation function models (capillary pressure and relative permeability) for simulation. The steady-state relative permeability methods, coupled with centrifuge, provide data over the range of saturation conditions encountered in the reservoir for both water-oil and gas-oil fluid pairings; this range of data coverage is significantly greater than is typically available in the industry and hence reduces uncertainty in the resultant relative permeability models. The water-oil relative permeability behaviors suggest a mixed-wet character with a preference to oil; however, one reservoir rock type (RRT) shows mixed wet character with a neutral preference for oil and water. The gas-oil relative permeability measurements are made on a unique apparatus capable of conducting the testing with reservoir fluids at reservoir conditions in the presence of irreducible water saturation. Centrifuge USBM wettability measurements were conducted on samples in the native, cleaned and restored state. Based on the measurements, it is concluded that the wettability restoration method used in this study was effective for a high-permeability Dukhan Arab D limestone core, but wettability restoration cannot be achieved for the moderately low-permeability limestone core. In general, restoration of carbonate cores to native state wettability is less successful than for siliciclastic cores. This is thought to be due to the complex pore structure of carbonates and the potential for change of pore structure through diagenesis after migration of petroleum.

Introduction

The large, mature Dukhan Field is located onshore Qatar, approximately 80km west of Doha. Hydrocarbons are contained in a North-South plunging anticline approximately 70km long by 8km wide. 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. The Arab C and Arab D reservoirs have gone through various phases of production including natural depletion, flank water injection, gas injection processes, vertical as well as horizontal well development and gas-lift implementation to maintain production and maximize hydrocarbon recovery.

Reservoir performance prediction is an integral component of an effective reservoir management program. The reliability of reservoir performance predictions is affected by the quality of the underlying relative permeability and capillary pressure models used in reservoir simulation. In order to develop reliable models, four key requirements must be employed:

  1. measurements must be on rock samples representative of the reservoir (the right samples),

  2. measurements must be made under conditions representative of displacement processes in the reservoir (the right conditions),

  3. measurements must be made using precision equipment and techniques (the right equipment), and

  4. trained and experienced technologists are needed to conduct the measurements and model the data (the right people).

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