Abstract

El Gassi is a volatile oil field located in the Algerian desert. The field has produced about 62 MMSTB oil since 1959, through primary depletion. Due to the heterogenity of producing formation, the total recovery during 45 years is only about 11%. The implementation of water-flood project was avoided due to low achievable water injection rate. A miscible gas injection project is examined in this study. The purpose of gas injection is to maintain reservoir pressure and improve the microscopic efficiency to increase the overall recovery in the field. A laboratory study revealed that the miscibility could be achieved with a pressure of about 241.4 bars (i.e. 3500 psia), which is below the reservoir average pressure in El Gassi field. Typical reservoir heterogeneities encountered in El Gassi field were incorporated into the simulation model. The study includes history matching of past reservoir performance, investigation of different injection scenarios, optimization of injection well number, optimization of gas injection and oil production rates, and the benefit of drilling infill wells. Edge line gas injection appeared to be promising.

Introduction

El Gassi field was discovered in 1959 and is situated in the Algerian desert, approximately 700 kilometers to the south of Algiers. It is about 100 km southwest of Hassi Messaoud Field. Its location is shown in Figure 1 and 2. The reservoir is producing under primary depletion. Drive mechanism is mainly solution-gas drive. The peak production rate of approximately 10360 STB was achieved in 1994 with 5 wells. The field has produced about 62 MMSTB oil since 1959 with a recovery of approximately 11% of original oil in place.

The reservoir was originally highly undersaturated. Original reservoir pressure was 447 bars with bubble point of 145.2 bars. The reservoir had an initial solution gas-oil ratio (GOR) of 169.5 cubic meters per cubic meter. Average reservoir pressure is currently at approximately 270 bars. The average permeability of reservoir is 1.7 mD with porosity of 7.5%. Due to low maximum achievable water injection rate, water flooding is not feasible.

The implementation of miscible gas injection project in El Gassi field is examined in this study. The purpose of gas injection is to maintain reservoir pressure and improve the microscopic efficiency, hence, increase the overall recovery in the field. The dynamic miscible process is examed with a continuous gas injection. A laboratory study revealed that the miscibility could be achieved with a pressure of about 241.4 bars (i.e. 3500 psia), which is below the reservoir average pressure in El Gassi field.

In this study, the applicability of the miscible gas injection process is examined. Different schemes of gas injection are tested, several relevant parameters are optimized and infill wells are proposed. Reservoir future performance is predicted and compared to the natural depletion and immiscible gas injection project.

Miscible displacement

Due to poor petro-physical properties of El Gassi field, injection of required quantities of water to reach economic level is not possible. These limitations to oil recovery may be overcome by the application of miscible displacement in which the injected fluid forms a single phase with the reservoir fluid at the reservoir conditions.1,2 The interfacial tension and capillary pressure are eliminated. Various gases and liquids are suitable for use as miscible displacement agents in either the first contact miscible or dynamic miscible processes. Miscible displacement processes 3, 4, 5 have enjoyed technical and economical success particularly in the last decade.

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