Polymer flooding improved sweep efficiency for utilizing IOR potential Force seminar April 2016
Classic polymer screening Viscosifying effect Solution preparation Bulk rheology Flow properties in porous media Filterability Screen factor Mobility reduction Permeability reduction Inaccessible pore volume Retention Stability Shear stability Thermo chemical stability
IOR mechanism Improve sweep by reducing mobility ratio Water cut depends on polymer viscosity and permeability fw, kro 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Sw 0.4 0.2 krw kro Brine Polymer RRF = 1.5 Polymer RRF = 2.0 Polymer RRF = 2.5 fw=0.22 Lab data kro fw - Brine fw - Polymer krw Lab data krw 1.00 Will polymer alter Sor? Lab scale correctly interpret fw = 1, if not recovery increases by reducing M Field scale the existence of critical fw at which above production is not economic Recovery 0.75 fwc=0.95 fwc=0.99 0.50 fwc=0.999 fwc=0.9999 0.25 0.01 0.1 1 10 100 1000 Mobility ratio
How to optimize mobility ratio Polymer viscosity depends on Mw, concentration and salinity 1 Intrinsic viscosity, Intrinsic viscosity depends on effective salinity, C Intrinsic viscosity 100 10 Non Newtonian fluids Rheology in porous media differs from bulk rheology Slip flow Depleted layer Fåhræus Linquist effect 1 0.001 0.01 0.1 1 10 Effective salinity, MIS, M
How to optimize Viscosity, mpas Mobility reduction 1000 POL 1 HMW POL 1 LMW POL 2 MC POL 2 LC 100 10 1 0.1 1 10 100 1000 10000 Shear rate, 1/s 10000 POL 1 HMW POL 2 MC 1000 POL 3 HMW 100 10 POL 1 LMW POL 2 LC POL 3 LMW Polymer 1 Regular HPAMbased polymer Relatively shear stable viscsosity at moderate shear rates Polymer 2 Biopolymer Shear thinning polymer Polymer 3 HPAM based polymer with hydrophobic co monomers (Associative polymer) Highly shear thinning at moderate shear rates 1 0.1 1 10 100 1000 10000 Shear rate, 1/s
Shear degradation in porous media Synthetic polymers are shear senstive Onset of degradation above critical shear rate, which depends on Mw LMW polymers are more shear stable than HMW Replacing HMW polymer with LMW will not improve viscosity, only injectivity Viscosity 20 1 15 0.75 10 0.5 5 0.25 LMW MMW HMW 0 0 1.0E+02 1.0E+03 1.0E+04 1.0E+05 Shear rate, 1/s Normalized viscosity
Polymer transport in porous media Polymer retention Assume Langmuir isoterms Adsorption depends strongly on wettability Inaccessible porevolume (IPV) Fraction of pores too small for polymer invasion, depleted layer Here, IPV = 0.20 Effective transport properties Oil wet reservoir (low adsorption) v p /v T > 1 for c > 500 ppm Water wet reservoir v p /v T < 1, critical only at ultra low concentration (e.g., in low salinity water) Minimize produced polymer Use retention and injected concentration as design criterion Adsorption, g/g rock Polymer to tracer velocity 60 50 40 30 20 10 0 1.25 1 0.75 0.5 0.25 0 Water wet Oil wet 0 500 1000 1500 2000 Polymer concentration, ppm Water wet Water wet Langmuir Oil wet Oil wet Langmuir 0 500 1000 1500 2000 Polymer concentration, ppm
Vertical sweep efficiency Delay breakthrough time Example: = 100, at unit mobility reduction, 1 0.505 and at infinity viscosity 1 0.55 Selective viscosity will dramatically improve sweep efficicency Selectivity exploited by salinity, temperature and permeability contrasts Displacement in low permeability layer at bt Selective 1 0.5 0.2 0.1 0.05 0.02 1 0.1 0.01 0.001 0.1 1 10 100 1000 Mobility reduction
The new class of EOR polymers Hydrophobically modified water solubles copolymers Hydrophobic groups added to regular polymer backbone reacts with each other leading to intermolecular polymer network Mobility reduction can in porous media due to formation of polymer network increase significantly Mobility reduction depends at least on amount of associative groups, Mw, salinity and temperature
Mobility reduction in porous media Constant rate vs. constant differential pressure Flow behaviour at low flow rates deviates strongly from classic Darcy law flow Demonstrate the possibility of maintaining nearly constant differential pressure at flow rates varying more the two order of magnitude and the behaviour is reversible
Mobility reduction effect of oil In presence of oil the associative interactions are weakend resulting in less mobility reduction and lower RF compared to Sw = 1 (dotted lines)
Effect on oil recovery High mobility reduction will improve the sweep efficiency towards piston like displacement and reduce the tail end production High mobility reduction may be utilized to increase the capillary number /, with the possibility of lowering Sor Exp I Brine, followed by regular ATBS followed by 1000 ppm associative polymer Exp II Brine followed by 500 ppm associative polymer
Oil recovery vs. capillary number
Optimization Define assosiative polymers which at injection condition behave as regular polymers (low mobility reduction and good injectivity) while high mobility reduction is triggered by temperature
Conclusions Main mechanisms for EOR polymer flood are understood Sweep improvement by lowering mobility ratio The wide variety of EOR polymers allowing for optimization, e.g., Injectivity vs. mobility reduction EOR potential vs. mobility reduction Type of injection brine Polymer loss vs. produced polymer Why always choose HMw HPAM polymer? Commercial simulators are not fully ready for polymer does however only partly explain lack of field experience