Rheology of Polyacrylamide-Based Systems Under Near-Well Bore Conditions

Abstract

Introduction Excessive water production adversely affects the economics of oil and gas producing wells: being generally responsible for both a rapid productivity decline and an increase in operating costs caused by the handling and treating of large quantities of water. To face these problems and to extend the lifetime of the wells, different treatments were proposed such as injection into the water producing zones of cements, resins as well as organic or inorganic strong gels. These treatments effectively stop the flow of all fluids in the treated layers, but require zone isolation for their placement. More recently systems were developed to reduce selectively water production without significantly affecting the flow of hydrocarbons. These so-called "permeability modifiers" are based on the use of either single high-molecular-weight water soluble polymers, generally polyacrylamides, or of "weak" gels obtained by addition of a crosslinker. Their placement is performed over the entire perforated interval. In this paper are described some basic characteristics of the rheology in porous media of these permeability modifiers. Experimental Neutral and partly sulfonated polyacrylamides of high molecular weight (5.106 < Mw <8.106 daltons) were dissolved in water containing 20 g/L of NaCl and 50 ppm of NaN3 as a stabilizer. pH value was kept constant between 6.5 and 6.8. The fraction of acrylate groups was around 1%. while sulfonation degree was 5, 13 and 25%. In the experiments aimed at investigating gel formation in flow through porous media, a zirconium complex, which reacts with acrylate groups, was added immediately before injection. Polymer solutions were injected at constant flow rate in Vosges sandstone cores (0.24 < k < 0.40 m2) at 60 C. Differential pressure transducers were used to monitor pressure drop across the cores. Adsorption was estimated from the difference in effluent concentration profiles between the first and the second polymer front. Results and discussion Polymer mobility in the dilute regime: The dilute regime is defined as the concentration range where macromolecules are not confined by the others so that their size remains constant. Strictly dilute regime requires c* [ ] < 1 where c* is the overlap concentration and [ ] is the intrinsic viscosity but significant size reduction is observed only above c** [ ] = 3–4, c** characterizing the beginning of entanglements and thus of the semi-dilute regime. With the sulfonated polymers used in this study. c** is around 1 g/l. The Figure 1 shows the behavior of a dilute polymer solution (c =0.5 g/L<< c**) flowing through a 0.24 m2 Vosges sandstone. The various rheological characteristics are plotted under a dimensional form versus shear rate. The relative viscosity, , which is the polymer solution-to-solvent viscosity ratio, varies with shear rate as usual. Together with , are also plotted rheological characteristics in porous media, namely the mobility reduction Rm (the water-to-polymer mobility ratio or apparent viscosity) and the permeability reduction Rk (the ratio of permeabilities before to after polymer adsorption) versus the shear rate at pore throat wall estimated as in Ref. 6 where the a constant has been taken equal to 4.