Abstract
For the thick sandstone reservoir, due to gravity differentiation, water drive mainly uses the bottom oil of the reservoir, while gas drive mainly drives the oil along the top of the reservoir. Water-alternating gas (WAG) injection can effectively combine the advantages of water flooding and gas injection, so that gas and water can work in synergy, thus further expanding swept volume and enhancing oil recovery. Over the past half century, the technology has been successfully applied to more than 60 oilfields worldwide, but its mechanism remains to be further studied. In this paper, a total of three tests were conducted through conventional water-flooding (WF), water flooding followed by gas flooding and WAG in sand-packed 2d-model. These experimental processes are recorded by video with time so that the saturation distribution of each phase in porous media can be observed，migration law of gas and water is studied, and the mechanism of WAG displacement is analyzed. The results show that gravity and capillary force have great influence on the process of gas water alternation, thus increasing the overall swept volume of water and gas; The water/gas alternating injection has the highest oil recovery factor (RF) of 75.45% in test 3, in comparison with water flooding followed by gas flooding (70.85% in test 2) and water-flooding (66.7%in test 1); Increase in cycles of WAG tends to reduce residual oil saturation.
Highlights
Alternate gas-water injection, consisting of two traditional water flooding and gas flooding enhanced oil recovery (EOR) techniques, is a promising method for secondary and tertiary recovery.1-5 Water flooding, the volumetric sweep efficiency is relatively high but the microscopic displacement efficiency is low; gas flooding, the microscopic displacement efficiency is relatively high but the volumetric sweep efficiency is low
(3) Water flooding, water flooding followed by gas flooding and water / gas alternate flooding were carried out respectively, and the distribution of oil, gas and water was recorded in real time by camera
Three different flooding schemes were applied to study the mechanism of Water-alternating gas (WAG) displacement in the sandstone formation, i.e., water flooding, Water flooding followed by gas flooding, and water/gas alternating displacement
Summary
Alternate gas-water injection, consisting of two traditional water flooding and gas flooding enhanced oil recovery (EOR) techniques, is a promising method for secondary and tertiary recovery. Water flooding, the volumetric sweep efficiency is relatively high but the microscopic displacement efficiency is low; gas flooding, the microscopic displacement efficiency is relatively high but the volumetric sweep efficiency is low. Due to the interference of gas and water phases, the relative permeability of gas and water phases is reduced, and the oil recovery is improved.6-13 In this way, the advantages of water flooding and gas flooding can be combined by sweeping oil-bearing channels with different pore sizes by gas and water flooding, which can increase the macro sweep volume, and improve the micro displacement efficiency.. The advantages of water flooding and gas flooding can be combined by sweeping oil-bearing channels with different pore sizes by gas and water flooding, which can increase the macro sweep volume, and improve the micro displacement efficiency.14-17 It can well control the fluidity of the leading edge, inhibit viscous fingering, and make the leading edge displacement stable, so as to improve sweep efficiency. It is of great significance for the development of thick sandstone reservoirs
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