Research On Collapse Pressure Of Shale Oil Formation Considering Of Influence By Countercurrent Imbibition

With the exhaustion of conventional energy sources,shale oil has attracted more and more attention due to its huge amount of resources,and its production technique has developed rapidly.Water-based drilling fluid is the necessary way of realizing efficient and economy development of shale oil.The current,the shale oil of China are mostly using oil-base drilling fluid,water base drilling fluid used in drilling for shale oil is relatively small.The main reason is the problem of instability of using water-based drilling fluid during drilling shale oil formation.In the overbalanced drilling of the mud shale formation,the filtrate invaded formation,expansion of shale in water,its strength is reduced,resulting in wellbore instability.When using water-based drilling fluid underbalanced drilling mud shale formation,drilling fluid can also be countercurrent imbibition into shale formation in capillary force and the chemical potential and the other effect of factors.The formation is the cause of the wellbore collapse.Based on the characteristics of shale oil formation,this paper analyzes the law of countercurrent imbibition invasion of water base drilling fluid under the combined action of capillary force and chemical potential,and establishes a collapse pressure model of shale oil formation considering the influence of water based drilling fluid countercurrent imbibition.The main research results are as follows:(1)Based on oil-water two phase flow theory,combined with the capillary force equation,while the introduction of diffusion coefficient of borehole wall,water saturation is modified.established the underbalanced drilling process of water-based drilling fluid invasion shale oil formation model,using this model,the systematic analysis of drilling time,underbalanced pressure difference,original water saturation and residual oil saturation on the influence of drilling fluid invasion depth are studied.Main conclusion included:after the diffusion coefficient is introduced,the water saturation decreases and the water invasion distance decreases.The water invasion distance and water saturation increased with the increase of drilling time and original water saturation,and decreased with the increase of the underbalanced pressure difference,and residual oil saturation.(2)Before considering the effect of capillary force,ion concentration difference and shale semi permeable membrane effect,the establishment of force-chemical potential coupling under countercurrent water invasion models.The influence of the ion concentration difference,temperature and the efficiency of the Semi-permeable membrane on the invasion depth and water saturation of the countercurrent are analyzed.Main conclusion included:before considering the chemical potential,water invasion distance and water saturation increased;under pressure is low,the more significant the effect of the chemical potential;water invasion distance and water saturation increased with ion concentration,temperature and semipermeable membrane efficiency increased.(3)The model for collapse pressure of shale oil formation considering the influence of countercurrent imbibition is established.The influence rules of hydration stress,chemical potential,drilling time,underbalanced pressure difference,residual oil saturation and original water saturation on collapse pressure are systematically analyzed.Main conclusion included:after considering the effect of hydration stress and chemical potential,the collapse pressure increase.The collapse pressure increases with the increase of time and original water saturation,and decreases with the increase of underbalanced pressure difference and residual oil saturation.The research results of this paper have enriched the basic theoretical research of shale formation collapse pressure and provided theoretical support for the safe and efficient development of shale oil formation.