The Effect of Ultrasonics On the Injectivity And Viscosity of Xanthan Gum For Enhanced Recovery Applications

Abstract

Abstract This paper outlines preliminary studies which have indicated that the injectivities of xanthan gum solutions can be improved by ultrasonic treatment without adversely affecting formation viscosities in enhanced recovery methods. Introduction One of the major concerns in applying polymer flooding to reservoirs with relatively tight oil sands (less than 100 md) is to have good injectivity while maintaining high solution viscosities(l). If the solution is mixed well and the molecules are dispersed thoroughly without being mechanically degraded, then this optimum situation is reached. Ultrasonic waves have been used to disperse particles of the micrometre Size(2). The capability of ultrasonic waves to disperse particles or agglomerates stems from the cavitation caused by ultrasonics. Cavitation occurs in the regions of the liquid which are subjected to rapidly alternating pressures of high amplitude(3). Besides dispersion, ultrasonic waves have been utilized to break up polymer molecules. The ultrasonic degradation of polymers in solution is of a mechanical nature. The stresses set up in the polymer molecule are caused by the friction forces generated by the relative movement of the molecules of solvent and the polymer molecule as a result of the collapse of cavitation bubbles(4). The permanent degradation can be measured by determining viscosities and by chemical reactions. A recent review of the work in this area is presented by A. M. Basedow and K. H. Ebert(4). Ultrasonics were also found to be effective in liquefying gels(5). The polymer molecules that are used as- mobility control agents in enhanced oil recovery are between 0.1 µm and 0.5 µm in diameter(6). Xanthan gum molecules (Kelzan -XC) have been reported to be around 0.3-0.4 µm(7). These molecules have bonds between them that are weaker than the covalent bonds between the units of the chains. Morris suggests that these weak non-covalent associations between aligned molecules build up a tenuous gel-like network. He also proposes that this network is progressively broken down with increasing shear rate(8). These bonds lead to an increase of entanglements between the polymer chains. Moorhouse et al. characterize the bonds between the molecules as hydrogen bonds(9). If one can break the hydrogen bonds and decrease the degree of entanglement without destroying the covalent bonds, then he can increase the injectivity of the solution and still maintain the high viscosity level. It has been suggested that the larger polymer aggregates present more resistance to flow and accumulate greater shear stresses; hence they are more vulnerable to rupture than the smaller species or the dispersed molecules(4). The principle of utilizing ultrasonic waves to improve the flow properties of polymer solutions through porous media without decreasing the viscosities is examined in this investigation. Procedure In a previous study, the optimum blending of xanthan gum solutions was obtained by the shearing of the solution through several orifice plates, with 100 psi pressure drop each plate(7). Excessive pressure drops across the orifices leads to mechanical breakdown of the molecules. Some of these solutions prepared by optimum blending were subjected to ultrasonic radiation; their injectivities and viscosities were determined.