Research On The Mechanism Of Shale Oil Mobilization With CO₂ Huff-n-Puff Under The Influence Of Imbibition Water Shield

There is a great strategic significance to explore and develop shale oil efficiently for ensuring national energy security.However,after volume fracturing,a large amount of fracturing fluid flows into the matrix pores under the strong imbibition effect of shale reservoirs,forming“imbibition water shield”,which would affect the utilization of remaining shale oil by CO₂ huff and puff.Therefore,it is necessary to conduct further analysis and research on the laws and mechanisms of shale oil utilization by CO₂ huff and puff under“imbibition water shield”.Firstly,based on a series of core imbibition experiments conducted on the Chang 7continental shale oil reservoirs with developed micro-nano pores and fractures and ultra-low initial water saturation,it was found that both fractures and pressure can enhance the imbibition effect of the shale,and the imbibition water saturation increased significantly.The proportion of macro-pore imbibition water rose more than 4%under the fractures,while that remained stable in micro-pores and decreased slightly in meso-pores.When the pressure was larger than 10 MPa,the pressurization had a little effect on the amount and distribution of imbibition water.Furthermore,combining the circularity and the fractal theory of porous media,the cross-sectional size of“imbibition water shield”can be determined by the irregular hydraulic radius r_h（δ,T₂）and established a formula for characterizing the height of“imbibition water shield”on a core scale.The experimental results showed that geometric of“imbibition water shield”was micro-nano scale cross-section and macroscopic scale height.Under the effect of fractures and pressure,the height of“imbibition water shield”raised overall,and that increased more obviously in micropores and mesopores.Secondly,based on the physical model of CO₂-water-oil diffusion and combined with the quantitative characterization of the"imbibition water shield",the models of CO₂diffusing through the“imbibition water shield”and the“imbibition water shield”being extrusion and retreating were established.When the imbibition effect reached equilibrium in shale reservoir at 19.25 MPa and 60°C,in the pores of radii were 0.1μm and 0.2μm,the time of CO₂ diffusing through“imbibition water shield”were about 1200 d and 262d respectively,and the time of the shale oil breaking separately through“imbibition water shield”were approximately 1.05×10⁶ d and 1.88×10⁵ d,it reflected that the“imbibition water shield”has strong shielding effect on CO₂ huff and puff.Then,the shale oil utilization characteristics of CO₂ huff and puff under“imbibition water shield”were revealed by physical simulation experiments,the shale oil recovery factor decreased,and the cumulative shale oil recovery factor of different pores required more time to achieve dynamic equilibrium.Additionally,the single-cycle factor reduced significantly,and were generally less than 10%and fluctuated sharply.And there was no obvious regularity among different pores.Then it displayed the obvious two-way shield resistance of“imbibition water shield”.Finally,the impact of the“imbibition water shield”on the shale oil flow process was described under CO₂ huff and puff by numerical simulation.With the length of the“imbibition water shield”,the difficulty of CO₂ diffusing through“imbibition water shield”grew,and the shield resistance enhanced,then the shale oil was more difficult to product.Moreover,with the time of injection-soaking-production,the sweep region of CO₂diffusion front expanded,the water saturation cut down significantly after multiple cycles of CO₂ huff and puff,then the“imbibition water shield”was destroyed and its shield effect weakened.At last,the shale oil deep in the pores was produced effectively and enhanced shale oil recovery.In summary,this paper further improves the basic development theory of CO₂ huff and puff after volume fracturing in continental shale oil reservoirs,which has certain theoretical reference significance for promoting the application of CO₂-EOR technology in continental shale oil reservoirs.