Abstract

Today, there is a clear tendency towards structure and quality deterioration of reserves. One of the reasons lies in low initial oil saturation (about 0.45–0.55 for the regions of Western Siberia). Average residual oil saturation is equal to some 0.3 (in this case displacement efficiency - 0.4). With displacement efficiency being that low, final oil recovery factor will be just over 25% even if the reservoir sweep factor can be increased to about 70%.

In the mid-XX century the method of oil field development using gas injection which theoretically allows 100% displacement was developed.

The paper discusses two technologies for implementation of gas methods used in oil field development. - alternating gas water injection (AGWI) and water-gas stimulation (WGS) in which combined injection of displacement agents (gas and water) is used. It describes the principle of operation and features of special equipment developed on the basis of a multi-purpose booster compressor unit operating under the principle of a single-stage "compressor with hydraulic seal".

The paper presents procedures for realization of AGWI & WGS, as well as practical developments allowing to prevent typical problems which occur in the process of field equipment operation when pumping high pressure gas.

Actual results of WGS introduction in one of the JSC "RITEK" fields are presented and technological benefits of the process are assessed using 3D geologic-hydrodynamical simulation data.

Introduction

Today, there is a clear tendency towards structure and quality deterioration of reserves both in Russia and worldwide. With the recovery of reserves in the fields discovered and started to be developed last century, the share of reserves classified as hard-to-recover is increasing. Deterioration of reserves can be partially explained by the decrease in initial oil saturation of formations.

A great amount of Western Siberia oil reserves is concentrated in reservoirs with 0.45–0.55 saturation. Average residual oil saturation in these reservoirs according to numerous core analyses equals to 0.3. In this case displacement efficiency is 0.33- 0.45. But most of geological reserves still remain in reservoirs due to the only fact that oil and replacing water are mutually unsoluble and in the porous media at oil-water contact there occur capillary forces and significant capillary gradient which in the conditions of micro heterogeneity bury (close from any sides) remaining oil. With this low displacement efficiency, even if the reservoir sweep factor reaches about 70%, the final oil recovery factor will be just a little higher than 25%.

It is natural that the situation when only a quarter of all geological reserves can be recovered is unacceptable, nevertheless it is absolutely objective in the conditions of normal flooding for these reservoirs. The situation can be practically improved if the production technology of these fields can be fundamentally changed. Among these absolutely new approaches is gas flooding of oil bearing formations (RF Patent # 2142045) based on alternating gas-water injection.

Gas flooding differs in concept from water-gas stimulation, i.e. injection of water-gas mixture into oil-bearing formations. In gas flooding oil is replaced by frontal gas fringe being as broad as possible followed by water flood. The technology is based on the concept that gas flood ensures high displacement efficiency, while water flood ensures high sweep factor. After gas breakthrough into producing wells every injection well is transfered from gas injection to water injection. The pumped water buries gas but not oil, the gas fringe which separates oil and water becoming gradually short. Under favourable conditions when strata heterogeneity by permeability is relatively low the gas fringe will be available to the final production stage. Under less favourable conditions after broad water fringe the second broad gas fringe is required followed by another water flood. Thus, provided displacement efficiency is doubled and sweep factor remains the same, final oil recovery will be twice increased.

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