Air-foam Flooding Mechanism Of Chang 6 Oil Reservoir With Low Permeability Sandstone In Ganguyi Region,Ordos Basin

Reservoir Chang 6 of M well in Ganguyi oilfield(Ordos Basin),one of the main battlefield of exploration and development in Northern Shanxi,is a typical low porosity and low permeability reservoir.Multi-:methods including physical property analysis,cast thin section,scanning electron microscope,X diffraction of clay minerals,high pressure mercury,magnetic resonance,sensitivity analysis,wetability analysis,oil-water relative permeability and three-axial stress were carried out to study the reservoir basic geological and rock physical properties.In this study,we also studied the mechanism of air-foam and rock,evaluated the properties of air-foam on the aspects of foam agent performance,foam stability,foam viscosity,foam corrosion and compatibility,and analyzed the rock mechanism from the static and dynamic aspects.Meanwhile,through the tube displacement experiment and static and dynamic analysis of low temperature oxidation,we analyzed the characteristics of air-crude oil mixed phase as well as the minimum miscibility pressure,and further to studied the formation mechanism,migration characteristics and seepage characteristics of air-foam in porous medium.Furthermore,according to real sandstone micro displacement experiments,we analysed the air-foam flooding efficiency,clarified the flooding method and residual oil occurrence of water flooding and foam-gasflooding,and calculated the safe oxygen content.The flooding effect of air-foam was also evaluated by water flooding-gas flooding,water flooding-gas flooding-foam flooding and water flooding-foam flooding-gas flooding,respectively.The following salient conclusions can be drawn from this study:(1)The main rock type of the reservoir Chang 6 of M well is medium to fine grained feldspathic sandstone,with the average porosity of 10.57%and the average permeability of 0.81×10-3?m2.Therefore,it belongs to typical low porosity and low permeability reservoir.The main diagenesis in the study area includes compaction,cementation,dissolution and fracturing,pertaining to A-B period of the middle diagenesis stage.(2)Results of porosity evolution show that initial mean value of unconsolidated sandstone porosity Chang 6 reservoir is 38.15%,the porosity loss compaction is 13.01%,cementation loss is 7.45%,increasing in dissolution is 7.86%.The secondary dissolution diagenesis is an important diagenesis to improve the reservoir properties in the research area.(3)Intergranular pore,laumonite dissolution pore and feldspar dissolved pore are the main pore types in the study area,and the flake-curved lamellar throat is the main throat type.On basis of the high pressure mercury experiment,reservoirs can be divided into four categories,and the development effect of all kinds of reservoirs is different from each other.(4)According to seepage characteristics of reservoir layer by nuclear magnetic resonance,the reservoirs can be divided into four types of fluid flow ability of different sensitivity by the characteristics and morphological analysis.Sensitivity experimental analysis shows that the reservoir belongs to the weak speed sensitivity,alkali sensitivity,water sensitivity,medium to weak-medium strong acid sensitivity and stress sensitivity of fractured.Fractures developed within the reservoir mainly strike east-west direction.(5)The expansion of clay minerals is caused by entering the reservoir of foreign fluid which is also contributes to changes of the pore structure.Low initial confining pressure can not be consumed due to the compression,but high confining pressure will release the confining pressure due to the compression.It is hard to reach quasi linear seepage in low permeability reservoir on the basis of the analysis of pressure gradient.(6)Formation mechanism of foam mainly includes liquid film hysteresis,necking separation and thin film separation.Residual oil of foam-gas flooding around the reservoir is the main formation of the residual oil in the flow field.If then implement the water flooding,the effect will be poor.Results of low temperature oxidation experiments show that the safe oxygen content is 11.42%.The safe oxygen content can be acquired after 1400h of function between reservoir and crude oil.(7)Air-foam flooding includes front region,middle region and rear edge region.The ideal of foam plugging can be achieved when the gas-liquid ratio fall in between 1:1 and 2:1.The effect of foam plugging will became worse when the water flooding reaches 0.4 PV,and it will result in gas channeling if gas flooding was carried out after water flooding at this situation.Air-foam flooding has a significant effect on EOR(enhanced oil recovery)usually between 11%and 20%.