Inaccessible Pore Volume in Polymer Flooding

We have found that solutions of typical waterflooding polymers do not occupy all of the connected pore volume in porous media. The remainder of the pore volume is inaccessible to polymer. This inaccessible pore volume is occupied polymer. This inaccessible pore volume is occupied by water that contains no polymer, but is otherwise in equilibrium with the polymer solution. This allows changes in polymer concentration to be propagated through porous media more rapidly than propagated through porous media more rapidly than similar changes in salt concentration. At the front edge of a polymer bank the effect of inaccessible pore volume opposes the effect of adsorption and pore volume opposes the effect of adsorption and may completely remove it in some cases. This paper presents three experimental polymer floods showing the effect of inaccessible pore volume in the presence of varying amounts of adsorption. Results of these floods clearly show that about 30 percent of the connected pore volume in the rock samples used was not accessible to The polymer solutions. The changes required to include polymer solutions. The changes required to include inaccessible pore volume in mathematical models of polymer flow and in held prediction methods are discussed. Introduction One way o improving the mobility ratio during waterflooding operations is by addition of a water-soluble polymer to the flood water. Several different polymers have been proposed and a number of investigators have presented results on the behavior of these polymer solutions in porous media. In addition, mathematical models have been developed for predicting the field behavior of polymer flooding. In all these studies movement polymer flooding. In all these studies movement of the polymer bank through the reservoir rock is of great importance. One phenomenon that has been repeatedly observed in polymer flooding is the removal of polymer from solution by adsorption on the reservoir rock. As a polymer bank propagates through porous media, the polymer bank propagates through porous media, the front edge is gradually denuded of polymer. The amount of polymer lost from a bank may be large or small, depending on the nature of the polymer and rock surface. This loss of polymer must be measured and included in any realistic mathematical model of polymer behavior. It has been widely assumed that polymer behavior. It has been widely assumed that adsorption is the most significant factor causing polymer to propagate through porous media at a polymer to propagate through porous media at a velocity different from that of water. In this paper we present data that demonstrate that all of the pores may not be accessible to polymer molecules and that this "inaccessible polymer molecules and that this "inaccessible pore volume" can affect polymer propagation pore volume" can affect polymer propagation significantly. In addition to the experimental results, we discuss the changes in interpretation and in mathematical models that are required to include this phenomenon. EXPERIMENTAL The experiments described in this paper were single-phase displacement of polymer solutions through consolidated sandstone. All the cores were prepared by evacuating and saturating with brine; prepared by evacuating and saturating with brine; the pore volumes of the cores were measured at this time. The experimental floods reported here were then done in three steps.An "initial solution" was injected until the core was at complete equilibrium with that solution.A bank of a different solution was injected into the core.Injection of the initial solution was resumed and continued until the end of the experiment. During each experiment the effluent from the core was collected in small samples; the analyses of these samples for polymer and salt content gave the basic data which is presented here. In plotting the results we used a "concentration fraction" defined as (Ce -Ci)/(Cb -Ci), where C is concentration and the subscripts e, i and b refer to the effluent, initial inlet and bank inlet values, respectively. All the solutions used were mixed in distilled water; concentrations are given in weight percent or in ppm by weight. Two polymers were used; one was a polyacrylamide (Pusher 700, The Dow Chemical Co.); the other a polysaccharide (XC biopolymer, Xanco, Div. of Kelco Co.). SPEJ P. 448