The Effect Of Inaccessible Pore Volume On The Flow Of Polymer And Solvent Through Porous Media

Abstract

Abstract High molecular weight polymers used as mobility control agents in oil-recovery process do not occupy all of the pore volume in a porous medium. The part of the pore volume not contacted by polymer is termed "inaccessible pore volume" (IPV). It is known polymer is termed "inaccessible pore volume" (IPV). It is known that IPV affects the rate of movement of polymer molecules through the medium. It is also probable that polymer-solvent interactions affect the flow and dispersion of solvent in which the polymer is dissolved. The primary objectives of this research were 1) to investigate the variation in IPV with polymer concentration, 2) to develop a qualitative description of the mechanisms causing IPV and 3) to study the effect of polymer flow on the movement and dispersion of the solvent in which the polymer is dissolved. Fifteen linear displacement experiments were performed on a one-foot Berea sandstone core to study the effect of polymer concentration on IPV. Polymer (polyacrylamide) concentration varied from 51 ppm to 1069 ppm. Inaccessible pore volume was determined from interpretation of effluent concentration profiles and by material balance. The largest values (0-3 - 0.32 PV) were observed in the dilute solution region (less than 300 ppm). At higher concentrations IPV decreased with concentration from 0.32 PV to 0.21 PV. A qualitative explanation of concentration PV to 0.21 PV. A qualitative explanation of concentration dependence is provided in terms of solvent-solute-surface interactions. The movement and dispersion of solvent in the presence of polymer was inferred from behavior of an ethanol presence of polymer was inferred from behavior of an ethanol tracer which was added to the polymer solutions. Two distinct features were observed in the dispersion experiments in which polymer was present. First, an early breakthrough of ethanol polymer was present. First, an early breakthrough of ethanol occurred in each run, i.e., the 50 percent normalized ethanol concentration arrived at the effluent point after an injection of about 0.9 pore volume of fluid. Second, the concentration profiles had a sharp change in slope, near a normalized profiles had a sharp change in slope, near a normalized concentration of about 70 percent, which developed into "tails". The "tails" resembled dispersion results in systems having two or more different pore characteristics, such as porous media with dead-end pore volume. Possible explanations for the solvent behavior are discussed. Introduction High molecular weight polymer molecules used for mobility control in displacement processes do not flow through all of the connected pore volume in many reservoir rocks. Thus, the polymer moves faster than the solvent in which it was dissolved. The part of the pore volume not contacted by polymer is termed "inaccessible pore volume" (IPV). Although inaccessible pore volume has been pore volume" (IPV). Although inaccessible pore volume has been observed by several investigators there have been few studies of the mechanisms which cause inaccessible core volume. A recent study of IPV by Trushenski and Gupta was completed while the research reported in this paper was in progress. This paper describes an investigation of the movement of polymer and solvent through porous media. The primary objectives polymer and solvent through porous media. The primary objectives were 1) to investigate the variation in IPV with polymer concentration, 2) to develop a qualitative description of mechanisms causing IPV and 3) to study the effect of polymer on the movement and dispersion of the solvent in which the polymer is dissolved. THEORY Inaccessible pore volume occurs primarily because polymer molecules are large relative to solvent molecules and pores in a reservoir rock. Pore size distributions in reservoir rocks include pores which are a fraction of a micron in diameter to several hundred microns. The effective sizes of high molecular weight polymers, such as partially hydrolyzed polyacrylamides, have been estimated to range between 0.3 microns and 2.0 microns depending on salinity and concentration. Consequently, some pores are too small for polymer molecules to penetrate. These pores are too small for polymer molecules to penetrate. These pores are part of the IPV. pores are part of the IPV. Polymer molecules are also retained due to adsorption and/or mechanical entrapment. The plugging of pores by either of these mechanisms would contribute to IPV. Since polymer retention increases with polymer concentration, IPV should increase with concentration if it was primarily due to pores which become inaccessible because of retention.