Abstract

The purpose of this study was to investigate the relationship between horizontal and vertical permeability of the Arun Limestone reservoir in the Arun field. The study utilized core data from wells in which both horizontal and vertical permeability measurements had been made.

The Arun natural gas field, one of the world's largest gas fields, is located onshore in the North Sumatra Basin, Indonesia. This field has been producing gas and condensate since 1977. Gas production has increased since start up to the current average level of 3,300 million standard cubic feet per day (MMSCFD). Of this total daily volume 2,230 MMSCFD is transported to the LNG plant for liquefaction and export, and after NGL recovery 145 MMSCFD is supplied for the domestic market, 55 MMSCFD is used for fuel and 870 MMSCFD is reinjected into the reservoir to maximize condensate recovery.

The current geological model, which incorporates a 7-layer zonation based on porosity trends, is the basis for input to reservoir simulation studies. A good understanding of permeability distribution within the reservoir is critical to accurate forecasting of reservoir deliverability and injectivity performance.

The benefits from the results of the horizontal and vertical permeability relationship study are as follows:

  • An improved knowledge of vertical flow in the reservoir.

  • Improved reservoir modeling and compositional flow simulation.

  • Enhanced ability to select zones for well stimulation.

The study shows that there is a log-log linear relationship between the horizontal and vertical permeabilities in each zone.

Introduction

The Arun field is located in the northern part of Sumatra, Aceh Province, Indonesia (Figure 1). The field was discovered in 1971 by the Arun A-1 well and is operated by Mobil Oil Indonesia Inc. under a production sharing contract agreement with Pertamina, the Indonesian State Oil Company. The reservoir is a north-south trending reefal carbonate complex which was deposited in the North Sumatra Basin during early Miocene time. Average reservoir depth is about 9000 feet subsea in the north and 10,000 feet in the south, with the reservoir thickness locally exceeding 1,000 feet. The productive area is about 23,000 acres with an average net pay of about 500 feet. Typical reservoir parameters are 18% porosity, 10 millidarcies permeability and 10.7% water saturation.

The field started production in March 1977 and is currently producing at daily average rates of 3,300 MMSCFD of separator gas, 120,000 B/D of unstabilized condensate and 29,000 B/D of fresh water from condensation.

This study recognizes a general relationship between horizontal and vertical permeability in Arun field. The relationship was obtained by reviewing wells in which both horizontal and vertical permeabilities were available from core analysis.

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