CO2驱替对致密砂岩孔隙结构的影响:以鄂尔多斯盆地三叠系延长组长7段为例OA

[Objective]CO2 flooding is a crucial technology for enhancing oil recovery efficiency in tight oil reservoirs.CO2 injection reacts with in situ minerals in the formation,leading to changes in the pore structure of the rock.However,tight sandstones undergo complex diagenesis with significant differences in cement distribution,and it is therefore necessary to clarify the mechanisms of microstructural changes in tight sandstones with different cement characteristics during the CO2 flooding process.[Methods]In this study,CO2 flooding simulation experiments,cast thin sections analysis,scanning electron microscopy(SEM),nuclear magnetic resonance,and X-ray diffraction were adopted to investigate the changes in mineral composition and pore structures of tight sandstones before and after CO2 flooding.[Results]The results showed that the cements in tight sandstones were complex and diverse,mainly composed of clay minerals and calcite,with significant distribution differences among different cement types.Cements affected the mineralogical structure and pore morphology of tight sandstones.After CO2 flooding,the average porosity increment of tight sandstones reached 0.55%,the average permeability increase rate was 21.59%,and the average relative sorting coefficient decreased by only 0.01,indicating that CO2 flooding increased the porosity and permeability of the reservoir,but had a weak impact on the heterogeneity of the pore structure.During CO2 flooding,feldspar and calcite were dissolved,resulting in the precipitation of quartz and clay minerals.In tight sandstone with high clay content,CO2 dissolution was dispersed,and the newly precipitated minerals and clay detached from framework grains blocked pores,leading to minor changes in petrophysical properties.In tight sandstones with high calcite content,carbonic acid dissolved large amounts of calcite,creating new storage spaces and flow paths,significantly improving the petrophysical properties of tight sandstones.Therefore,CO2 flooding has different effects on the pore structure of tight sandstones with varying cement characteristics.CO2 flooding results in a much more significant improvement in the physical properties of tight sandstones with high calcite content than those with high clay content.[Conclusion]These results can provide new insights for the development and evaluation of tight sandstone reservoirs.