The oil displacement effect of polymer flooding is mainly governed by the rheological property of polymer in the deep reservoir, while the mechanical degradation is one of the key factors affecting its rheological properties . Polymer molecular chains can be mechanically degraded in shear flow and extensional flow, which both involves in the flow through a porous medium. Mechanical degradation includes shear degradation and stretch degradation. The questions are which one dominates, the shear or the stretch, how much it degrades, and how to simulate.
Maerker  conducted degradation of partially hydrolyzed polyacrylamide (HPAM) through consolidated sandstone plugs and reported that the degradation of HPAM through porous media was caused by large viscoelastic normal stress in elongational flow field and larger flow rate, longer flow distance and lower permeability induced more severe degradation. Dupas et al.  carried out HPAM degradation experiments through an API (American Petroleum Institute) capillary system, indicated that after polymer was degraded at velocity 8m/s, shear viscosity drops by 10%, while extension viscosity decreases up to 60%.
The objective of this work is to develop a device to simulate the mechanical degradation of polymer in micro pore throat, identify the shear effect and elongational effect, examine the viscosity loss of polymer solution through different pore throat model and present the relationship between viscosity loss and extensional rate.
Mechanical degradation is induced by forcing fresh polymer solution through a device which consists of a series of pore throat models with 4 kinds of throat length (15mm,35mm,70mm,100mm)and 3 kinds of diameter(100μm,300μm,500μm). Viscosity measurement of native and degraded polymer solution is made with Haake RS6000 rheometer at 25℃. The throat flow rate varies from 6.37 m/d to 3821.66 m/d. Polymer is HPAM with intrinsic viscosity 2510 dL/g. The concentration of polymer is 250mg/L, prepared in brine with salinity 32868 mg/L. Viscosity loss caused by extensional degradation can be obtained by calculating the viscosity loss of polymer solution degraded through pore throat model with throat length 0 mm. Accordingly, viscosity loss caused by the shear degradation through pore throat model with different throat length is calculated.
The results show that there is no obvious correlation between viscosity loss of HPAM in the pore throat model and throat length. More than 95% viscosity loss is caused by stretching degradation in pore throat, which is the main mechanism of mechanical degradation. The relationship between viscosity loss in pore throat and extensional rate (ε) has two characteristic values (ε0 and εL).ε0 is the critical extensional rate, εL is the ultimate extensional rate. When ε <ε0, the viscosity of polymer solution decreases slowly; whenε0 <ε < εL, the viscosity drops sharply; when ε>εL, viscosity loss keeps unchanged.ε0 and εL can be used as index parameters to evaluate anti-mechanical degradation ability of oil displacement polymer.
James J. Sheng, Bernd Leonhardt, Nasser Azri. Status of Polymer-Flooding Technology. Journal of Canadian Petroleum Technology, 2015(3):116-126
J. M. Maerker. Shear Degradation of Partially Hydrolyzed Polyacrylamid Solutions. Society of Petroleum Engineering Journal, 1975(8):311-322
A. Dupas, I. Hénaut, D.Rousseau, P. Poulain, R. Tabary, J. F. Argillier, T.Aubry. Impact of Polymer Mechanical Degradation on Shear and Extensional Viscosities: Towards Better Injectivity Forecasts in Polymer Flooding, the SPE Internatlonal Symposium on Oilfield Chemistry held in the Woodlands, Texas, USA. 8-10 April,2013
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