| Ultra low permeability is an important type in low permeability. The ultra low permeability oil and gas resource is more and more outstanding in the proportion of residual recoverable resources of low permeability oil and gas resources. Water injection is an important method for the development of oil reservoirs with low permeability or extra-low permeability. And it will be extended to the development of reservoirs with ultra low permeability. However, the pore throats in sandstone oil reservoirs with ultra-low permeability are more complex and subtle in dimension, leading to worse seepage ability and high injection pressure. The formation is seriously short of injection. Forced water injection at high pressure is very a challenge to the bearing capacity of the formation and injection equipment. Therefore, it is necessary to explore methods for depressurization and injection stimulation in sandstone oil reservoirs with ultra low permeability.In this paper, a comprehensive investigation of technologies used in the development of sandstone oil reservoirs with low permeability was conducted and the potential of application of these technologies in sandstone oil reservoirs with ultra low permeability was also analyzed. With the reference of reservoir's characteristics and injection requirements, characters during water injection and its influencial factors were investigated on the typical formation with ultra low permeability of Chang6 in Ordos Basin, China. Based on these analyses, paper pointed out the idea to improve water injection effect by weakening the resistances from interface in sandstone oil reservoirs with ultra low permeability. The effect and the mechanisms of depressurization by interfacial modification were analyzed. The effect of the depressurization on recovery and the way for further enhancement of oil recovery in sandstone reservoirs with ultra low permeability was finally proposed.The characteristics of water injection in sandstone reservoirs with ultra low permeability were systematically studied, and the factors affecting the effect of water injection were revealed. The experiments showed that the flow of oil or water single phase in the ultra low permeability reservoir was non Darcy flow and the two-phase flow restricted each other, which seriously affected the effective seepage capacity of oil and water, resulting in high injection resistance. The worse the physical properties were, the more difficult the core injection and the higher the pressure gradient for flow would be. The cores with ultra low permeability were injected with the quasi-initiation pressure gradient of 50-200MPa/m and the flooding recovery of 35.2%-57.7%. The injection pressure increased rapidly at initial, as well as the recovery. After reaching the peak injection pressure, the pressure began to slow down and tend to be flat, and the recovery rate increased gradually and slowly until definite. The high injection pressure resulted mainly from seepage resistance that from the worse physical properties and subtle pore throats, capillary effect and surface characteristics, and formation damage in reservoirs with ultra low permeability.Methods of depressurization and injection stimulation were explored in the sandstone oil reservoir with ultra low permeability and the effect of the interfacial modification was defined. Based on the mechanism of interfacial phenomenon and its effect on depressurization and injection stimulation in sandstone oil reservoirs with ultra low permeability, the basical requirements and the selection of the treatment agent for depressurization were put forward. A perfluorinated octyl quaternary ammonium fluoride to change the wettability of rock surface to hydrophobic or oleophobic and a Gemini surface active agent to reduce oil/water interfacial tension by 3-4 magnitude at low concentration was preferred chosed. The evaluation results showed that the pressure drop rate was 28.2% and the water phase flow ability increased by 38.7% under the effect of fluoride. The lower the core permeability was, the higher rate of pressure drop by fluoride would be. Under the effect of Gemini agent, the pressure drop rate was 17% and the water phase flow ability increased by 29.9%. These two kinds of agents made the recovery rate improved by 0-8.4%, with an average of 2.1%.A variety of micro analysis was comprehensively used to analyze the mechanisms of the depressurization and injection stimulation by interfacial wettability modification and oil/water interfacial tension reduction in the sandstone oil reservoirs with ultra low permeability. The adsorption of fluoride on rock surface was multilayer adsorption under the static electricity, hydrogen bonding and weak dispersion force at high temperature and displacement for long time. The adsorption was stable enough to make the change of the microstructure of rock surface. As a result, on the one hand, the rock surface transformed from the original oleophilic and hydrophilic to hydrophobic and oleophobic with the contact angles of oil and water on rock surface were all close to 90 degrees, thus greatly reduced the additional capillary pressure; on the other hand, interfacial modification made a decrease in surface energy and the solid-liquid interaction force, thereby the bondage of rock to water and the thickness of boundary layer was reduced and the seepage space expanded, as a result of which the flow resistance reduced. The comprehensive effect was depressurization and injection stimulation by fluoride solution in sandstone oil reservoir with ultra low permeability. Gemini surface active agent realized depressurization and injection stimulation through greatly reducing the additional capillary pressure by great reduction in the oil/water interfacial tension.The reason of injection stimulation but less further recovery during surfactant solution flooding after high water cut in the sandstone oil reservoir with ultra low permeability was analyzed. The standpoint that surfactant solution flooding which can effectively modify the surface wettability and reduce the oil/water interfacial tension should be carried out before the stage of high water cut, better early, for injection stimulation as well as further recovery was put forward. When the surfactant solution flooding was carried out after high water cut, the solution would flow along the early formed water channel and reduce the injection resistance in the dominant channel, which makes it easier to flow and can not expand the swept volume. As the oil displacement by water has been more thorough in dominant channels, further recovery would be difficult during surfactant solution flooding. The results showed that, compared with surfactant solution flooding after high water cut, the depressurization rate and the recovery rate improved by 6.4%?16.4% and 15%-33.6% respectively when surfactant solution flooding before dominant channel formed. The earlier the injection time was, the better the effect was. It can be achieved by surfactants with the effect of interfacial wettability modification or oil/water interfacial tension reduction. The complex solution of these two surfactants has both effects of good surface activity as well as interfacial wettability modification. With the two functions, better results would be achieved than single surfactant used and the concentration of single surfactant can be reduced.It was proposed and demonstrated that the surfactant with effect of surface modification or oil/water interfacial tension reduction can significantly enlarge the swept efficiency when flooding in sandstone oil reservoir with ultra low permeability. When surfactant solution flooding was conducted before high water cut, the oil/water interfacial tension reduction and wettability modification can greatly reduce the resistances from the interfacial effect. The displacement difference among capillaries will be weakened. The opportunity of seepage for narrow capillary that can not be displaced early will increase. Thereby the swept coefficient of flooding improved, leading to further recovery increased significantly.The results obtained in this paper provide new perspective for water injection as well as the exploration of methods to depressurization and enhanc oil recovery in the sandstone oil reservoirs with ultra low permeability. |
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