A Review Of Over 100 Polymer Gel Injection Well Conformance Treatments In Argentina And Venezuela: Design, Field Implementation and Evaluation

Abstract

Abstract During the last 15 years, polymer gels have become an accepted technology for improving volumetric sweep efficiency in heterogeneous waterfloods. Water injected subsequent to the gel treatment ideally enters previously unswept zones with significant mobile oil saturation. Results from several field projects in four hydrocarbon basins in Argentina and Venezuela are described based on the application of two available polymer gel technologies: Marcit and Unogel. The types of reservoirs and reservoir conditions where polymer gels have been successful, and unsuccessful, are illustrated. Fundamental reservoir rock and fluid characteristics, reservoir temperatures, polymer gel designs, and project evaluation are presented for each of the field projects. A high temperature (275ºF) reservoir is included. In multi-layered reservoirs where crossflow is believed to be limited, one strategy is to inject a small gel volume in order to improve the vertical profile in the near wellbore region. If crossflow is believed to exist between layers or within a layer, signficant gel volumes are recommended for deeper placement in the offending zones so that water cannot easily bypass the gel treatment. Gel formulation is a fundmental issue. Traditionally, minimum polymer concentrations of at least 3000 ppm have been recommended for injection well gel treatments. Lower polymer concentrations were believed to be ineffective. A large scale and ongoing field project is presented in which low concentration polymer gels have been successful. In the same field project, the results of multiple gel treatments in the same injection well are discussed. Introduction This paper will summarize polymer gel applications in three basins in Argentina and the Lake Maracaibo basin in Venezuela (Figure 1 and 2), using two of the most widely applied polymer gel technologies: Marcit[1] and Unogel.[2] Several of the case studies presented in the following paragraphs included extensive pre-treatment diagnostics. Due to the space limitations, those procedures are not discussed in detail. Marathon's patented Marcit gel technology was developed for application in naturally fractured reservoirs and was first applied in the Tensleep and Phosphoria reservoirs of northwest Wyoming. The Unogel technology was developed and patented by Union Oil Company of California (Unocal), primarily for use in high temperature (>250ºF) reservoirs. Although somewhat less versatile than the Marcit gels, Unogel has shown promise in high temperature reservoir applications. Both Marcit and Unogel gelants are typically formulated using a partially hydrolyzed polyacrylamide polymer (PHPA). The primary difference lies in the crosslinking mechanism. Marcit gels are crosslinked with a metal ion (Chromium III) while the Unogel technology requires an organic crosslinker and a stabilizing agent for delayed gelation. Although polymer gels evolved from the application of polymers for mobility control, gel treatments are not designed to improve an adverse mobility ratio. One of the primary criteria for polymer gel applications are reservoirs with low oil recovery efficiency and, in many cases, such reservoirs exhibit an adverse mobility ratio. However, the primary objective of any polymer gel treatment is selective permeability reduction due to reservoir heterogeneity. Oil viscosity is not, in itself, an important consideration for well selection or treatment design.