Characteristics And Mechanism Of CO₂ Huff And Puff In Fractured Low Permeability Reservoirs

At present,there is little research on CO₂ huff and puff in fractured low permeability reservoirs.The existing results are difficult to explain the mechanism of CO₂ huff and puff in fractured low permeability reservoirs.Therefore,it is of great significance to systematically study the characteristics of CO₂ huff and puff in fractured low permeability reservoirs and to explain the stimulation mechanism of CO₂ huff and puff by conducting the diffusion experiments in porous media.In this dissertation,the solubility characteristics of CO₂ in heavy oil and light oil under high temperature and pressure were measured and compared through physical experiments.By conducting CO₂ huff and puff experiments in fractured low permeability cores,the characteristics of CO₂ huff and puff in fractured low permeability reservoirs were obtained,and some of the mechanisms were revealed.According to the improved pressure drop method,the CO₂ diffusion coefficient in light oil was measured.In addition,the diffusion coefficient of CO₂ in light oil saturated porous media was measured according to the radial diffusion model.The visualized analysis of the core was carried out with the improved low-field NMR imaging device to further reveal the mechanism of CO₂ huff and puff in fractured low permeability reservoirs.The results show that:compared with heavy oil,CO₂ can be more dissolved in light oil,which can obtain larger dissolved gas-oil ratio,volumetric coefficient,while the property of viscosity reduction was not obvious.During the process of CO₂ huff and puff in fractured low permeability core,the fractures can store CO₂,increase the elastic energy of the formation,reduce the flow resistance and increase effective cycles.In addition,the recovery factor of CO₂ huff and puff in fractured low permeability reservoirs can be enhanced by increasing the soaking time and CO₂ injection volume.Due to the influence of CO₂ solubility,the diffusion coefficient of CO₂ in light oil was larger than that in heavy oil,and the diffusion coefficient increased with temperature and pressure.During the diffusion of CO₂ in the fracture-pore dual media,CO₂ first dissolved in the crude oil on the surface of core and fracture,and the mass transfer rate was faster than that of the non-fracture porous media,which reduced the influence of low permeability on diffusion and increased the swept volume of CO₂.The remaining oil of fractured core after huff and puff mainly distributed at the end of the core far away from the injection end,and the remaining oil saturation was generally low.The remaining oil saturation of non-fracture core was relatively high,and only the light oil near the injection end was extracted.The visualized experimental results well demonstrated that fractures can increase the volume of CO₂ in the core and increase the swept volume of CO₂.