Abstract

A laboratory technique has been successfully developed to perform gas gravity drainage experiments under reservoir perform gas gravity drainage experiments under reservoir conditions in three dimensions, that is on cylindrical cores having both lateral and horizontal faces open to flow.

As an application to a light oil reservoir, a series of experiments has been performed on outcrop cores of various permeabilities, in order to compare the efficiency of methane or nitrogen injection. In a first step, cores were saturated with connate water and volatile reservoir oil and subjected to gravity drainage by injection of a rich gas in thermodynamic equilibrium with the oil phase. In a second step. either methane or nitrogen was circulated around the cores, yielding substantial additional recovery of liquids.

The results obtained clearly indicate that: - During the first step, recovery was essentially the same as in onedimension experiments (lateral faces coated). - Both methane and nitrogen exhibit fast and complete recoveries of light and intermediate components and appreciable amounts of the heavy end fraction. Kinetics of hydrocarbon recovery isstrongly dependant on the nature and the injection rate of the gas injected. - An increase of pressure was investigated during gas Injection, which exhibited a favorable effect on the heavier componentsrecovery by methane. This influence was much less pronounced for nitrogen injection.

The laboratory experiments were interpreted with a fully compositional simulator. This confirmed the predominant role of vaporization and molecular diffusion.

Introduction

The rate of recovery and the ultimate recovery of oil from a fractured reservoir are functions of several variables. These include size and properties of the matrix block together with pressure and saturation history of the fracture system. Specific mechanisms controlling matrix fracture flow include water oil imbibition, oil imbibition, gas oil drainage, fluid expansion and rock expansion.

Gas gravity drainage occurs when gas in the fractures displaces oil in the matrix. The driving force in this case is governed by gravitational and capillary effects. The gravitational force is directly proportional to the density difference between the phases. The proportional to the density difference between the phases. The capillary forces are proportional to the I.F.T. between coexisting phases. when the conditions are right, gravity drainage may be phases. when the conditions are right, gravity drainage may be one of the most efficient ways of producing oil from a fractured reservoir. However natural depletion of fractured reservoirs in general leads to poor gas-oil gravity drainage.

When a lean gas such as nitrogen or methane is injected in a fractured reservoir, diffusion between lean gas in the fracture and fluids In the matrix occurs. Mechanisms such as oil swelling, stripping and molecular diffusion contribute to the oil production

If this gas is injected in a depleted reservoir, the oil in the matrix being at its bubble point, the swelling effect is very low but the diffusion process occurs, due to the concentration differences between lean gas in the fracture and the hydrocarbon phase (oil or gas) in the matrix.

This stripping/diffusion process could become the predominant factor in case of tight and highly fractured reservoirs, with a promising efficiency on the hydrocarbon recoveries. promising efficiency on the hydrocarbon recoveries. The purpose of this study was to put into evidence the efficiency of such mechanisms and to compare the relative performances of nitrogen and methane injections in case of a highly fractured reservoir.

EXPERIMENTAL PROCEDURE

Experimental set-up The objective of the laboratory experiments is to study the behaviour of a matrix block, simulated by an homogeneous cylindrical core sample, under gas injection in a surrounding fracture.

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