Micellar Shear Degradation, Formation Plugging, And Inaccessible Pore Volume
Authors: Szabo Miklos T. 
Published: December 2007

Abstract
Aqueous solutions of petroleum sulfonates can undergo severe shear degradation while flowing through restrictions or porous media. Viscosity losses as high as 90% have been observed. The degradation is not permanent, and after stress relief, viscosity is permanent, and after stress relief, viscosity is recovered in time intervals varying from a few hours to several months. The rate of recovery is dependent on a number of factors, such as sulfonate type, cosurfactant, salinity, and degree of shear degradation.
A number of electron microscope pictures are provided, showing that gel-like micellar-aggregates are provided, showing that gel-like micellar-aggregates are deposited from sulfonate solutions. These aggregates are deformable, extrudable, and can be ruptured under high shear conditions. They are also shown to reform during passage through a given pore size, and are capable of plugging pores considerably larger than those previously traversed. The fraction of pore volume accessible to the flow of micelle-aggregates is dependent upon the shear conditions. Physics-chemical properties of sulfonate solutions, such as interfacial properties of sulfonate solutions, such as interfacial tension and adsorption, are also shown to be altered as a result of shear degradation.

Introduction
Some of the preparatory work for a surfactant flood field test involved injecting 5 wt.% solutions of Stepan 269, 420 and 465 sulfonates, and mixtures thereof, through "preserved state" core plugs. Various alcohols were used as cosurfactants, and salinities varied between 1 and 2 wt.%.
Core plugging invariably occurred in these tests. Three to tenfold increases in injection pressures were typical over injection periods varying between several hours and several days, and at rates from 30–90 ft/d.
In order to improve injectivity, some of the solutions were filtered through Millipore filters of selected pore sizes. Pressure build-up still occurred, however, with the rate of delta P increase dependent on the pore size used in filtering the solution. Surprisingly, even though filters with pore sizes as small as 0.22 mu m were used, those solutions still generated several fold increases in delta P in a few hours. It is important to note here that the time lapse between filtration and the beginning of injection was not recorded in these tests because it was not recognized to be a factor at that time.
The above results were unexpected since analysis of a typical formation core, from the field under consideration, shows that only 12% of the pore volume is accessible through pore throat diameters less than 0.22 mu m. (Figure 1). Therefore, only a small permeability reduction should have occurred, followed by permeability reduction should have occurred, followed by permeability stabilization after a pore volume of permeability stabilization after a pore volume of injected fluid.
It has been previously reported that sulfonates interact with clays in the rock, causing a reduction in permeability. This led to one hypothesis which proposed that the sulfonate might be gradually proposed that the sulfonate might be gradually interacting with clay constituents of the formation rock samples. A second hypothesis suggested that gradual, in-depth, sulfonate adsorption might be taking place. To test these hypotheses, long duration (5–10 days) injections of sulfonate were carried out through formation core plugs. Both "ultrafiltered" (0.1 mu m or smaller pore size) and unfiltered solutions were used. When "ultrafiltered" solutions were directly introduced into the core after the filter, no "continuous" increase in delta P was observed, although some permeability reduction in the cores was noted, probably due permeability reduction in the cores was noted, probably due to the clay-sulfonate interaction.
When unfiltered solutions were injected, continuous increases in injection pressure resulted. However, when the effluent was immediately reinjected into the core, no further pressure increases were observed.