A Practical Method for the Evaluation of Weak Gels

Abstract

Summary. In an in-depth permeability-contrast correction operation for EOR, dilute solutions of a water-soluble polymer and gelling agent are injected. In this treatment, weak gets form in situ in the subterranean formation. Although there are a variety of techniques devoted to evaluating strong gels, no simple technique is available for the evaluation of weak gels. In this paper, weak gels are evaluated in a quantitative manner with a modified screen viscometer. The apparent elongational viscosities of polymer and weak gel solutions are found with this viscometer from pressure and flow-rate data. The apparent elongational viscosity was found to be independent of shear rate in the range of 10 to 100 seconds used in this study; however, it is dependent on concentration and polymer type. The crosslinking of dilute polymer solutions with metallic ions, such as Cr and Al was found to result in substantial increases in the apparent elongational viscosity of the gel over that of the polymer, although the shear viscosity remained the same. Introduction Any fluid injected into a subterranean formation will be transported through the path of least resistance. For example, in waterflooding, the injected water will pass through the highest-permeability zones of the formation. With continued water injection, the high-permeability zones will become watered out, while the low-permeability zones will be bypassed and the oil in these zones will remain unswept. A major and emerging technology for EOR is permeability-contrast correction operations. In these operations, resistance to flow is created in the high-permeability zones, forcing any subsequently injected fluid to pass through the low-permeability regions. Flooding with water-soluble polymer solutions alone creates a temporary resistance to water flow by increasing the viscosity of the injected water. As soon as the viscosified water passes through a portion of the formation, the adverse mobility is restored and the reservoir heterogeneity takes over again. This is especially true in floods with biopolymer solutions. Flooding the formation with gels or gelable compositions, on the other hand, creates at least a partially permanent reduction in the permeability of the high-permeability zones. If a strong gel is injected, the penetration is generally not far into the formation because of the high viscosity and elasticity of the gel. To achieve in-depth permeability correction, dilute solutions of polymer and gelling agent are injected, either in one slug or in a sequential mode (polymer/gelling agent/polymer), allowing the gel to form in situ and to block the penetrated zone partially. In-depth permeability correction operations are preferred over polymer-alone floods and near-well gel treatments because a large portion of the formation can be treated at a reasonable cost and the permeability reduction lasts for some time. The larger the volume treated and the longer the treatment lasts, the larger the cumulative amount of oil recovered. Procedures for crosslinking water-soluble polymers with metallic ions in situ have been patented and applied in at least one commercial-scale flood (North Burbank Unit, Osage County, OK) . Gels obtained by mixing dilute solutions of polymer and gelling agent cannot be differentiated from the parent polymer solution by the naked eye. This is contrary to the strong gels made from concentrated solutions, where the gel is semisolid and does not flow, even if the vessel containing the gel is inverted. Evaluation of gels is a rather difficult task because any disturbance may alter the structure of the gel irreversibly. Several elaborate experimental techniques have been used to evaluate strong gels, such as light scattering, birefringence of gels, or viscometers capable of measuring viscoelastic properties. The evaluation of weak gels is even more difficult because the shear viscosity of the gel is the same as that of the parent polymer solution. What is needed is a method that can be used routinely to evaluate polymers and gels for field applications. This method has to be reliable, simple, and easy to operate and must give results that can be correlated in an informative manner. The commercial screen viscometer satisfies most of these requirements. The time needed to evaluate one weak gel sample is fairly long, however, which makes it an impractical method. A modification of the standard screen factor apparatus has been made, which is suitable for evaluating weak gels as well as polymer solutions. Fig. 1 shows a schematic of this modified screen viscometer. Details of the equipment and its operation are included in the Appendix. With this apparatus, one can evaluate several samples in a short time and compare the results obtained under well-defined conditions. The results obtained with this viscometer are very helpful in designing coreflooding experiments, which are the next logical step in evaluating a gelation treatment. JPT P. 195^