Use of Compositional Simulation in the Management of Arun Gas Condensate Reservoir

Abstract

This paper describes the simulation of the Arun gas condensate reservoir using Mobil's fully compositional simulator, COSMOS (Compositional System Mobil Oil Simulator). The reservoir is a Miocene carbonate reef complex which occurs at a depth of approximately 10.000 feet, and is up to 1,000 feet thick in some areas. The Arun reservoir is a compositionally dynamic system. The purpose of this simulation study was to predict future reservoir performance under various demand scenarios and optimize gas and NGL recovery. The simulation mode utilizes the recovery. The simulation mode utilizes the Peng-Robinson equation of state to account for the compositionally dynamic behavior of the reservoir in predictions of future performance. The equation of state was modified by Mobil to incorporate special features for Arun such as water vaporization in the reservoir under high temperature conditions. A significant amount of time was spent on the geologic description of the field and initial data preparation, of the field and initial data preparation, which contributed to a good match of the historical data. The simulation model will serve as a reservoir management and planning tool to evaluate future operating strategies in the field. The technology presented in this paper is applicable to the management of other gas condensate reservoirs which exhibit physical phenomenal such as retrograde condensation, revaporization, and water vaporization. Introduction The Arun Field is located on the northern coast of Aceh Province in North Sumatra. The field was discovered in 1971 and is a giant gas condensate reservoir. Mobil Oil Indonesia operates the field as a Production Sharing Contractor for Pertamina. Gas is produced from the Arun Limestone at a depth of approximately 10,000 feet subsea. The gas pay is up to 1,000 thick in some areas. The initial reservoir pressure was 7,100 psig at the datum of 10,050 feet subsea, and is consequently overpressured. The temperature is 352 deg. F at this datum. At discovery, the reservoir was above the dew point, and had a stabilized condensate/gas ratio of about 48 STB/MMscf of water-free wellstream gas. The reservoir is underlain by an aquifer with a gas-water contact at about 10,600 feet subsea. Reservoir fluid properties and average rock properties and given in Table 1. The Arun Field began production in 1977. Field development was based on a cluster concept whereby only small areas of land are required for surface production and drilling facilities. This was done to minimize surface disruption to native farming and to the local community. There are four clusters in the field, each cluster designed to contain a maximum of sixteen wells. Producing wells are drilled directionally from these clusters. The gas is produced under a gas cycling scheme to maximize condensate recovery. Currently, there are ten gas injection wells located on the downdip perimeter of the field. The gas and condensate are delivered to the P.T. Arun LNG Plant for processing and liquefaction prior to export (Figure 1). The field production at present is about 2700 MMscf/D of separator gas, of which 800 MMscf/D are reinjected, 35 MMscf/D are used for fuel in the field, land 1865 MMscf/D are delivered to a 42-inch gas pipeline to supply two fertilizer plants and the P.T. Arun LNG Plant. About 135,000 bbl/d of unstabilized condensate are delivered to the P.T. Arun LNG Plant through a 20-inch pipeline. The Arun reservoir is a compositionally dynamic system. With pressure depletion, the water content of the reservoir gas increases significantly due to water vaporization under the high temperature conditions. Secondly, retrograde condensation and condensate revaporization effects impact compositional performance. Furthermore, injection of lean gas changes the fluid composition within the reservoir and reduces dew point pressure and hydrocarbon yields. Dissolution of hydrocarbons and carbon dioxide from both connate water and the aquifer also contribute to compositional changes. To properly fully compositional model was employed to simulate the field. P. 601^

The Arun Gas Field in Indonesia: Resource Management of a Mature Field

“…An extensive fluid collection and analysis program was conducted to characterize the reservoir fluids and compositional simulations were performed using state-of-the-art Equation of State (EOS) simulation models. 2,3 The importance of the water component was recognized early -both as a source of dissolved gas in the liquid connate water phase and the SPE 87042…”

“…3,4 The initial water fraction in the gas phase is 4.1 mole % and the gas contains about 14.1 mole % carbon dioxide and other non-hydrocarbons.…”