Abstract

The North Sumatra Offshore (NSO) field is a large gas field that supplies gas to the P. T. Arun Liquified Natural Gas (LNG) plant in Aceh, Indonesia. The field came on-stream in 1999 and produces about 450 MMscf/d currently. The field has produced about 34% of the Original Gas In Place (OGIP) as of 1/1/2007, and the ultimate recovery is expected to be around 75%. The producing carbonates have an average porosity of 23% with zones of higher porosity at the top of the reservoir. Geomechanical testing of Malacca limestone cores indicated that the rock has high compressibility and suffers elastic and significant plastic deformation when mechanical stresses are increased.

This paper presents results of the core analyses, the geologic, petrographic and stratigraphic description of the NSO carbonates, and the development of a reservoir simulation model that incorporates the rock mechanical properties, fluid flow properties and surface facilities models to history match NSO production and reservoir pressure behavior. The impact of porosity and permeability reduction by plastic deformation on field production is also examined. Rock compaction is shown to be an important mechanism for matching the NSO production history and it provides part of the support needed to explain the higher than predicted reservoir pressures. Aquifer influx is shown to be the other support mechanism that is essential to matching the field performance.

Introduction

The NSO field is located about 100 km offshore Northern Sumatra in 350 ft of water (Fig. 1). Mobil Exploration Indonesia Inc. operates the field as a Production Sharing Contractor to the Indonesian Oil and Gas Upstream Regulatory Body, BADAN PELAKSANA KEGIATAN USAHA HULU MINYAK DAN GAS BUMI (hereinafter called "BPMIGAS"). The NSO field is part of the gas field network that supplies gas for conversion to LNG at the P. T. Arun LNG plant. This LNG plant was constructed in the late 1970's to monetize the gas from the giant onshore Arun gas field.1 As the LNG market matured and additional international markets developed, other gas fields such as the onshore SLS and Pase fields and the offshore NSO field were also developed to meet LNG demand. With the passage of time, the onshore Arun field and its satellites have matured and declined, and the NSO field has taken on increasing importance as the major supplier of gas to the LNG plant. This paper shows how the application of state-of-the-art geoscientific and reservoir simulation technologies has helped in improving reservoir management and in elucidating the controlling reservoir mechanisms.

The NSO field was discovered in 1972 by the NSB-A1 well, which encountered a 415 ft gas-bearing reefal limestone. The Malacca limestone is Early to Middle Miocene age and stratigraphically equivalent to the Arun/Peutu limestone.2 A five-well appraisal program was conducted in 1981 to delineate the reservoir. Throughout the appraisal-drilling program, severe lost circulation hindered data recovery and adversely affected the quality and quantity of technical information collected.3 One additional appraisal well, the NSB-A7 was drilled in 1991 near the center of the field to obtain better geological and engineering data.

Commercial development of the block was approved in 1981, but field development was postponed until the Arun field productivity decreased and additional gas was needed for LNG supply. The NSO Plan of Development (POD) was developed in May 1995 and nine development wells (NSO-P1 to P9) were drilled and suspended between April and December 1997. The field was brought on-stream on July 10, 1999 with an average production rate of 335 MMscf/d.

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