Numerical Study Of Oil Snap-off In Micro Pore-throat Of Reservoir

For China’s industrial and social development,oil is an important energy source,which is known as the blood of industry.In recent years,a lot of primary old oilfields in China entered the middle and late stages of exploitation due to persistent exploitation,which caused continuous drop in the production of crude oil.Therefore,improving the recovery capacity of residual oil in high-water-cut reservoir is an important way to maintain the energy security.In the processes of crude oil exploitation,it is common for residual oil to pass through the pore-throat of oil reservoir under the action of external forces.The residual oils may snap-off when it passed the pore-throat because capillary instability and complexity of pore-throat in the reservoir.The formative daughter droplets by snap-off can be trapped,and then reduced recovery factor.Previous studies indicated that snap-off was controlled by capillary instability and viscosity,and which was main reason for the generation of residual oil in reservoir.Thus,it is crucial to capture the critical conditions of snap-off for enhance oil recovery.Based on the hydrodynamic assumption of uniform thickness direction,twodimensional areal pore-throat was usually used to simulate snap-off processes.The actual pore-throat has complex spatial configurations and three-dimensional flow effects,so the snap-off phenomenon in three-dimensional pore-throat and twodimensional areal pore-throat may be inconsistent.Besides,Newtonian fluid is ubiquitous and has been widely used to explore snap-off processes.Nevertheless,in the processes of polymer flooding,the displacement fluid and crude oil also shows non-Newtonian characteristics,such as shear-thinning and viscoelasticity.Researchers main focus on the effects of polymer concentration and solution elasticity on snap-off in polymer flooding.However,the shear-thinning can also significantly affect the apparent viscosity and viscous force.Thus,it is necessary to investigate the snap-off phenomenon in shear-thinning continuous fluid and obtain its critical conditions for improving the recovery factor of polymer flooding processes.In this thesis,the snap-off processes of fluid in the pore-throats were firstly simulated under different capillary numbers,viscosity ratios and pore-throat ratios.We found that fluids were more prone to snap-off in the three-dimensional porethroats,owing to the capillary instability in the three-dimensional models is stronger.The two-dimensional models are not suitable for simulating the snap-off processes.The phenomenon of snap-off of oil phase in non-Newtonian continuous fluid was explored,and compared with Newtonian continuous fluid under different capillary numbers and viscosity ratios.At the same time,the influence of different rheological parameters and polymer concentration on snap-off was explored.The results indicated that the shear-thinning property promotes the snapoff phenomenon,and snap-off is more likely to occur at low zero-shear-viscosityμ0 and power-law index nc,high relaxation time λ conditions;The effect degrees of different rheological parameters on snap-off is:nc>μ0>λ.The phenomenon of snap-off of non-Newtonian oil phase in non-Newtonian continuous fluid was explored,and the results were compared with those of Newtonian oil phase under different capillary number and viscosity ratios.The results suggested that the shear-thinning property of oil phase can also promote the snap-off;at the same time,the influences of rheological parameters of oil phase on snap-off is investigated.The influence of nd on snap-off is greater than the consistency coefficient K;When the power-law index of two-phase fluid changes respectively,which causes the change of nc/nd,the change trend of the critical condition for snap-off is opposite with the increase of capillary number.The increase of nc of displacement fluid leads to the increase of nc/nd,which has a stronger inhibition effect on snap-off.The research results can provide an important reference for understanding the snap-off mechanism in actual reservoir and designing polymer flooding.