Use of Dynamic Pore Network Modeling to Improve Our Understanding ofExperimental Observations in Viscous Oil Displacement by Polymers

摘要

Any aqueous solution viscosified by a polymer(or glycerol)should improve the recovery of a very viscousoil to some degree,but it has long been thought that the detailed rheology of the solution would not play amajor role.However,recent heavy oil displacement experiments have shown that there are clear differencesin incremental oil recovery between aqueous polymeric or Newtonian solutions viscosified to the sameeffective viscosity.For example,synthetic polymers(such as HPAM)recover more oil than biopolymers(such as xanthan)at the same effective viscosity.In this paper,we use dynamic pore scale network modelingto model and explain these experimental results.A previously published dynamic pore scale network model(DPNM)which can model imbibition,hasbeen extended to include polymer displacements,where the polymer may have any desired rheologicalproperties.Using this model,we compare viscous oil displacement by water(Newtonian)with polymerinjection where the"polymer"may be Newtonian(e.g.glycerol solution),or purely shear-thinning(e.g.xanthan)or it may show combined shear thinning and thickening behaviour(e.g.HPAM).In the originalexperiments,the polymer concentrations were adjusted such that the in situ viscosities of each solution werecomparable at the expected in situ average shear rates(see Vik et al,2018).The rheological properties of theinjected"polymer"solutions in the dynamic pore network model(DPNM),were also chosen such that theyhad the same effective viscosity at a given injection rate,in single phase aqueous flow in the network model.Secondary mode injections of HPAM,xanthan and glycerol(Newtonian)showed significant differencesin recovery efficiency and displacement,both experimentally and numerically.All polymers increasedthe oil production compared to water injection.However,the more complex shear thinning/thickeningpolymer(HPAM)recovered most oil,while the shear-thinning xanthan produced the lowest oil recovery,and the recovery by glycerol(Newtonian)was in the middle.In accordance with experimental results,at adverse mobility ratio,the DPNM results also showed that the combined shear-thinning/thickening(HPAM)polymer improves oil recovery the most,and the shear-thinning polymer(xanthan)shows the least incremental oil recovery with the Newtonian polymer(glycerol)recovery being in the middle;i.e.excellentqualitative agreement with the experimental observations was found.The DPNM simulations for the shear-thinning/thickening polymer show that in this case there is betterfront stability and increased oil mobilization at the pore level,thus leaving less oil behind.Simulations forthe shear-thinning polymer show that in faster flowing bonds the average viscosity is greatly reduced andthis causes enhanced water fingering compared with the Newtonian polymer(glycerol)case.The DPNMalso allows us to explore phenomena such as piston-like displacements,snap-off and film flow,which atthe pore level may have impact on the overall efficiency of the various fluid injection schemes.The DPNMmodels the effect of polymer rheology which changes the balance between the viscous/capillary forces thatallows fluid microscopic diversion,and hence improved incremental recovery,to emerge.