Filtration of Polymer Solutions

Introduction

Biopolymers (polysaccharides) have attracted considerable interest in mobility-control applications because of their low adsorption and shear stability characteristics. However, to prevent plugging at the wellbore, the biopolymer solutions must be conditioned. Filtration is one method of conditioning. Plugging is thought to be caused by gels or clumps of incompletely solubilized polymer, or residual proteinaceous material from the polymer, or residual proteinaceous material from the fermentation process.

DE Filter Evaluations

Before beginning a proposed field test involving both polymer and caustic-polymer solutions, several types of polymer and caustic-polymer solutions, several types of filters were evaluated for their applicability. The goal of the test was to achieve a submicron filtration. A technique using "millipore" filters as an analog of reservoir rock was adopted as a convenient means of obtaining a qualitative indication of solution injectivity. The method involved measuring the cumulative volume of fluid passed through a millipore filter at a constant differential pressure (5 psig) and plotting this value as a function of time. The plugging tendency of the fluid is inversely related to the volume of fluid that flows through the filter in a given time. Fluid conditioning with diatomaceous earth (DE) filters resulted in the best injectivity improvement of the polymer solutions. The studies with DE filtration involved polymer solutions. The studies with DE filtration involved systems with filter areas ranging from millipore filter holders of 0.01 sq ft to a 50-sq ft vertical leaf filter. Most of the work was conducted with a 0.65-sq ft vertical leaf unit. Most solutions evaluated had a concentration of 500-ppm polymer. The brine concentration was 4,000 ppm total dissolved solids. Unless otherwise stated, the ppm total dissolved solids. Unless otherwise stated, the filter operating conditions were (1) a precoat loading of 0.1 lb of DE/sq ft of filtering area, (2) a body-feed rate of 200 ppm, (3) a filtration flux rate of 0.25 gal/min/sq ft, (4) filtration at ambient temperature, and (5) the same DE used for both the precoat and the body feed. The initial attempt at DE conditioning of a polymer solution used a 50-sq ft filter. The goal for this particular test was to obtain as fine a filtration as possible without any significant loss in fluid viscosity. A precoat with a water permeability of 0.1 darcy, capable of removing particle as small as 0.1u, was used, A "crystal clear" particle as small as 0.1u, was used, A "crystal clear" filtrate was obtained from the hazy, untreated, polymer solution. This test pointed out some problems that might be experienced in the field. The very fine 0.1-darcy DE filter would not adequately bridge upon most porous supports. Also, excessively short cycle times would be encountered. The short cycle time raised the question of whether it was necessary to obtain the crystal clear filtrate. A review of the proposed polymer injection program resulted in the establishment of less stringent filtration limitations. In particular, we noticed that samples of polymer feed passed through 0.8 millipore filters were polymer feed passed through 0.8 millipore filters were still hazy. That is, solutions that satisfied the criterion of a submicron filtration were not crystal clear.