Diffusion Oil Drainage And Abnormal Wetting Characteristics In Tight-Low Permeability Reservoirs

During the production of tight-low permeability reservoirs,due to its small pore throat,a lot of phenomena are different from conventional reservoirs and couldn’t be explained by classical theory.In this paper,experimental methods were used to study gas diffusion and wetting anomalies in tight-low permeability reservoirs.In this paper,the technical difficulties in controlling and manufacturing the physical parameters of tight cores were overcome.With the optimization of the core sand ratio,the amount of cement,the pressing pressure,and the curing pressure process,conditions for making dense cores were determined.High-pressure mercury intrusion experiments proved that artificial tight cores could well simulate the pore structure and permeability of natural tight cores;high-pressure gas flow experiments showed that artificial tight cores had the same abnormal seepage law as natural tight cores.CO₂ diffusion and oil drainage experiments were carried out with series of dense high-permeability cores made by ourselves.A theory was established to determine the effective action distance of CO₂ diffusion oil displacing according to the correlation between the diffusion oil displacing rate and core length.It was found from the experimental results that the CO₂ diffusion oil displacing rate and its effective action distance decreased monotonically as the core permeability decreased;while the relative diffusion oil discharge rate increased as the core permeability decreased.Based on this,some conclusions of CO₂ diffusion and oil displacing could be made.Compared with conventional reservoirs,the actual effect of CO₂ diffusion and oil displacing in tight-low permeability reservoirs is limited.From this,it can be confirmed that the actual effect of the CO₂ diffusion-oil discharge effect in tight-low permeability reservoirs is very limited,but the relative contribution rate to the total recovery effect is very high.According to the dynamic law of the CO₂ diffusion oil discharge rate changing with time,it is proved that CO₂ intermittent injection and production is an effective method to improve the production effect of tight-low permeability reservoirs.A high power optical microscope was used to measure the gas-liquid contact angle in SiO₂ microtubes with an inner diameter of 5 to 530μm.It is found that the gas-liquid contact angle in SiO₂ microtubes increases monotonically with the decrease of the tube diameter,and regularly deviates from the contact angle θ_p measured on the flat plate,showing a very strong wetting microscale effect.This wetting microscale effect causes the relative wettability of the liquid phase on the SiO₂ microtube wall to be significantly weakened.According to the relationship between the measured gas-liquid contact angle and tube diameter,an empirical model of gas-liquid contact angle in silica microtubes was established θt=α*d^β+θ_p.Based on the power law model of gas-liquid wetting angle in microtubes,the abnormal gas-liquid capillary force and pore throat droplet driving resistance in porous media were studied and found.The ratio of the capillary force determined by the flat gas-liquid wetting angle θ_p to the capillary force in the tiny pores(R_cf)was used to characterize the microscale effect of capillary force.For both air-water and air-oil（hexadecane,methyl silicone oil,crude oil）system,the R_cf value was significantly higher than 1.0,and increased monotonically as the pore diameter decreased.Under the same condition,the driving resistance of the gas to the droplet in the microporous throat was significantly smaller than the driving resistance determined by the plate gas-liquid wetting angle θ_p,and the ratio R_dr decreased monotonously as the pore diameter decreased.Due to the actual wetting microscale effect,the gas-liquid capillary force in the reservoir pores and the driving resistance of the gas to the pore throat droplets are lower than the value determined by θ_p,and the smaller the pore radius,the greater the deviation between the two.In tight-low permeability reservoirs,the gas-liquid capillary force and gas-driven droplet resistance are obviously overestimated by traditional calculation without considering the wetting microscale effect.The contact angle of silicone oil-water in SiO₂ microtubes with an inner diameter of 0.4μm to 3500μm was measured using a frozen double-beam scanning electron microscope and a high-power optical microscope.The wetting microscale effect is characterized by Rθ defined by the ratio of the fluid contact angle θt in microtube and the contact angle θ_p on the plate.It can be seen from the experiment results Rθ that when the diameter of the SiO₂ microtube was small enough,the oil-water contact angle began to deviate from its contact angle on the SiO₂ plate,showing a very strong wetting microscale effect.The relationship between the silicone oil-water Rθ and the tube diameter d was very complex and non-monotonic—taking the tube diameter de（1500μm）corresponding to the extreme point of Rθ as the boundary.When d<de,Rθgradually decreased from 1.0 to 0.4 as tube diameter decreased,while water wettability increased;when d>de,Re gradually increased from less than 1.0 to 1.94 as tube diameter decreased,causing hydrophilic wettability change to lipophilic wettability.The experimental results showed that the oil type had a significant effect on the magnitude of the contact angle,but under different conditions there was a consistent qualitative law between the contact angle and the tube diameter.The oil-water injection sequence would affect contact angle value due to microscale wetting effect.The contact angle when water injected first was smaller than the contact angle when oil injected first,while both of them showed the same relationship with tube diameter.Active agents have a great influence on the wetting microscale effect.In SiO₂ microtubes,when the cationic surfactant solution was injected first,the crude oil-water contact angle was greater than 90° and microtubes wettability inverted.The contact angle had a non-monotonic correlation with tube diameter that it increased first and then decreased with the decrease of the tube diameter.The anionic surfactant solution-crude oil contact angle was slightly smaller than the crude oil-water contact angle.Both contact angles have a monotonically increasing change rule with decreasing pipe diameter.In the reservoir pores,the oil-water wetting microscale effect leads to abnormal capillary force and pore-throat oil droplet driving resistance,and the correlation were very complicated.Different oil-water injection sequences and oil types had a great influence on the abnormal changes of capillary force and pore-throat oil droplet driving resistance.For hexadecane-water and water-silicone oil systems,there was a critical pore diameter dc.When pore diameter d>dc,capillary force and pore throat oil droplet driving resistance caused by wetting microscale effect was greater than the value determined by θ_p;When pore diameter d<dc,capillary force and pore throat oil droplet driving resistance caused by wetting microscale effect was smaller than the value determined by θ_p For the silicone oil-water system,capillary force and pore throat oil droplet driving resistance caused by wetting microscale effect was greater than the value determined by θ_p measured on the flat plate.