Abstract
Abstract
Petrobras has implemented three polymer injection pilot projects during the last two decades, in the onshore fields of Canto do Amaro (Rio Grande do Norte state), Carmópolis (Sergipe state) and Buracica (Bahia state), all of them in Northeast of Brazil. The first one is in the middle of the operation and the last two are already finished.
Polymer flood in oil reservoirs has been performed for several decades around the world. The polymers act basically increasing the viscosity of the injected water and reducing the porous media permeability, allowing for an increase in the vertical and areal sweep efficiency of the water injection, and, consequently, increasing the oil recovery.
The main drive for the pilot projects in Petrobras was to gain practical knowledge of the process, for a possible future expansion for other reservoirs and even for offshore fields.
This paper describes the experience of Petrobras in implementing and managing those polymer flood pilot projects. The laboratory tests, the designing and programming of the field operation, as well as the analysis and interpretation of the results, were all carried out by Petrobras personnel and are covered in this paper. The tracer injection for reservoir characterization purposes, which preceded the polymer floods, is also described.
In this work we performed a technical evaluation of the whole polymer injection process, investigating the reservoir response in terms of oil recovery and time. The economical study will be made hereafter in another work.
Introduction
The idea involved in a polymer injection process for mobility correction has been the objective of enough investigation in the past and is well known. However, field application of polymer injection is directly related to the correct selection of the reservoir, as well as to the selection, specification and design of the polymer bank (1,2). Other critical point of this kind of process is the evaluation of the results (3,4,5,6,7), since normally the project involves several years of operation and a multidisciplinary team.
The objective of polymer injection projects for mobility control is to increase the oil recovery factor, by reducing the mobility ratio between the water and the oil. The reduction of the mobility ratio is achieved by increasing the viscosity of the aqueous phase, which increases the sweep efficiency and, as a consequence, the oil recovery (8,9). Depending on the type of polymer used, this increase in the viscosity can also cause a reduction in the effective permeability to the water in the swept areas (9). This reduction of permeability acts favorably in the process as an additional secondary effect, restoring part of the pressure of the reservoir after the passage of the polymer (residual resistance factor). This fact can cause an injectivity profile control of the injection wells through the rearrange of the resident fluids (10). On the other hand, the injection of polymer does not reduce the residual oil saturation, that is, it does not influence in the displacement efficiency. Therefore, polymer injection for mobility control affects only the sweep efficiency, allowing the residual oil saturation to be reached more quickly or in a more economical way (8–11). Besides that, the increase of the injected fluid viscosity results in an increase of the saturation of the displacement front, and therefore in an anticipation of the oil production.
The technical and economical success of the process depend on the correct selection of the reservoir, preferentially by observing a screening guide, specification and design of the polymer bank to be injected, as described in previous works (1,2,9,10,12–17). In this work, the candidate reservoirs were chosen by using prerequisites presented in the literature (1,13–18) and the selection of the polymer involved an analysis of the intrinsic properties of the product associated to the characteristics of the rock and of the resident fluids (1,2).