Evaluation of Polymer-Injection Projects in Brazil

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 94898, "Evaluation of Polymer-Injection Projects in Brazil," by M.A. de Melo, C.R.C. de Holleben, I.P.G. da Silva, A. de Barros Correia, G.A. da Silva, A.J. Rosa, A.G. Lins, and J.C. de Lima, Petrobras, prepared for the 2005 SPE Latin American and Caribbean Petroleum Engineering Conference, Rio de Janeiro, 20-23 June. Petrobras has implemented three polymer-injection pilot projects during the last 2 decades in the Canto do Amaro, Carmópolis, and Buracica onshore fields. The first one is in the middle of the operation, and the last two are finished. The main objective of the pilot projects was to gain practical knowledge of the process for possible future expansion to other reservoirs and even to offshore fields. Introduction The objective of polymer-injection projects is to increase the oil-recovery factor by reducing the mobility ratio between the water and the oil. The reduction in mobility ratio is achieved by increasing the viscosity of the aqueous phase, which increases the sweep efficiency and, hence, the oil recovery. Depending on the type of polymer used, this increase in viscosity also can cause a reduction in the effective permeability to water in the swept areas. This reduction in permeability has a secondary effect of restoring part of the reservoir pressure after polymer passage. The technical and economic success of the process depends on the correct selection of the reservoir and the specification and design of the polymer bank to be injected. Reservoirs and Pilot Areas The selection of Carmópolis, Buracica, and Canto do Amaro reservoirs was based on the criteria defined in the literature. By comparing average reservoir properties with the adopted screening guide, it can be seen that they fit all the criteria except for heterogeneity and clay content. Temperature and water salinity are two important process parameters. If the temperature and water salinity are greater than the specified values, the polymer can degrade. Oil saturation has to be above a minimum value for the process to be economically feasible. High clay content can increase adsorption of the polymer in the rock. This problem can be eliminated or minimized through laboratory tests to find a polymer compatible with rock conditions. The worst problem is the presence of faults, fractures, and preferential-flow channels. These heterogeneities can be identified by use of a tracer before using the polymer. After identification, these channels can be plugged by use of blocking polymers. Mobility ratio is the most important parameter to be considered because it will be affected directly by the polymer solution to increase the sweep efficiency and oil recovery. Polymer Selection In this phase, market research was performed to verify available products for this application. Synthetic and natural polymers were analyzed, and on the basis of cost, a partially hydrolyzed polyacrylamide was selected. After selection of the polymer, laboratory tests were performed to select the proper molar mass and degree of hydrolyzation on the basis of rock/fluid-interaction parameters. These parameters should be defined for each reservoir through specific laboratory tests. These tests were performed to evaluate compatibility of the polymeric solution with the reservoir rock and fluids and to deter-mine the parameters of rock/fluids interaction [e.g., adsorption, resistance factor (RF), and residual resistance factor (RRF)].