Influence of Microgels in Polysaccharide Solutions on Their Flow Behavior Through Porous Media

Abstract

Abstract Microgels existing in xanthan solutions, which are partly responsible for the poor injectability of such solutions, are retained around the injection wells. Therefore, most of the oil is swept by microgel-free solutions. This paper shows that even a few microgels can strongly modify flow behavior in porous media. Consequently, their careful elimination is required in laboratory tests for proper design of a polymer flood. Introduction The poor filterability of most xanthan gum solutions is explained by two mechanisms.Residual cellular debris or other insoluble particles coming from the fermentation process, which is responsible for dilute solution turbidity, cause partial plugging by adsorption and entrapment inside the pores. Various clarification procedures of more or less efficiency already have been proposed to prevent this type of plugging.Large but deformable soluble aggregates of macromolecules called microgels have been shown to remain even in well-clarified and thus translucid solutions. The main aspects of the plugging process by microgels and a laboratory testing method using Millipore filters to select the available products have been described in a previous paper. Recently, new enzymatic treatments have proved effective in destroying both these microgels and cellular debris. In the first part of this paper, filterability tests performed at low shear rates with different clarified biopolymer solutions under the same experimental conditions are described to compare plugging processes in Millipore filters and natural sandstones. In the second part, a more quantitative study using the same clarified xanthan solution with or without microgets shows how the presence of microgels can modify the behavior of solution flowing through different types of well-defined porous media having various pore sizes, shapes, and adsorption properties. Plugging Behavior Caused By Microgels Experimental Polymer Solution. Three industrial polysaccharides currently proposed for enhancing oil recovery were tested. Polymer A is a scleroglucan fermentation broth. Polymers B and C are industrial-grade xanthan gums - a powder and a fermentation broth, respectively. All solutions were prepared by a standard procedure. The polymer was dispersed or diluted under low shear conditions in 5-g/dm3 [5-g/L]-NaCl water previously filtered through a 0.22-um [0.22-micron] Millipore filter to obtain the same polymer concentration (400 ppm). These solutions were then clarified by successive filtrations through Millipore filters of 3, 1.2, and 0.8 um [3, 1.2, and 0.8 microns] under high differential pressure (1 bar [14.5 psi]). This clarification procedure gives translucid solutions that still contain microgels. Millipore Filters. Millipore filters currently are used to perform filterability tests because they are easy to use and offer quite good reproducibility. Their internal structure is similar to a three-dimensional network. Therefore. the pore shape is complex but the pore size distribution (PSD) is very narrow. Because their chemical nature is a mixture of cellulose esters, they are expected to be adsorbent for xanthan and scleroglucan. For the test, two filter holders were placed in series. The first contained one 3-um [3-micron] filter and the second, three on-line 3-um [3-micron] filters. Natural Sandstones Two types of natural sand-shines were, used for these filterability tests: Fontainebleau sandstones, which are purely quartzitic, and Vosges sandstones, which contain 5 to 10 % clay (mainly illite). Test Procedure, Clarified polymer solutions were injected continuously at low flow rates (0.25 and 0.33 m/d [0.82 and 1.1 ft/D], which are representative of velocities inside an oil reservoir far from injection wells. Temperature was kept constant at 30 deg. C [86 deg. F]. Pressure drops during polymer flow, Delta p, were measured and hence mobility reduction, alpha lambda, was calculated as in which Delta p is the pressure drop for water flow at the same, flow rate. SPEJ P. 361^