Evaluation of a New Cost-Effective Organic Gel System for High Temperature Water Control

Abstract

Abstract Due to their thermal stability, organically crosslinked gels have been used in conformance treatments for high temperature applications. Most of these gels consist of a polyacrylamide-based polymer and an organic crosslinker. Polyethyleneimine (PEI) has been used as an organic crosslinker for polyacrylamide-based copolymers. In a previous study (SPE 104071), we have shown for the first time that PEI can form ringing gels with polyacrylamide homopolymers (PAM) at temperatures up to 130oC (266oF). In addition, the gelation kinetics of this system was reported up to 140oC (285oF). However, the performance of this system in porous media was not studied. In this study, the performance of the PAM/PEI gel system was examined in porous media. The displacement of this gel system was visualized with Computed Tomography (CT) in sandstone cores. A permeability reduction of more than 94% was realized indicating the possibility of using this system in water shut-off treatments. In addition to CT visualization, this study reports on the bulk testing of this system over a wide range of concentrations of PAM and PEI. Visual observations revealed the formation of thermally stable gels. These gels were rigid and non-flowing at PAM concentrations of 7 and 9 wt%. The bulk tests were conducted at 100oC (212oF) for three weeks. Introduction Polymer gels have been used in water control treatments. These treatments are referred to as Conformance Improvement Treatments (CITs).1 A polymer gel system is based on a polymer and a crosslinker mixed at the surface. The mixture (gelant) is injected into the water producing zone where it should form a barrier to unwanted or bad water. In cases where the water producing zone produces a significant amount of oil, relative permeability modifiers are used. In that type of treatments, polymers are used to decrease the permeability to water more than that to oil through the disproportionate permeability reduction (DPR) effect.2 The success rate of chemical water shut-off treatments depends strongly on the identification of the unwanted water production problem. Characterization of the reservoir and the wellbore can help building part of the correct problem description.3 In high temperature applications, thermally stable gels are needed and should be used. Polyacrylamide-based polymers crosslinked with organic crosslinkers were reported to be applicable for reservoir temperatures up to 121oC (249.8oF).4 A widely applied organic crosslinker is polyethyleneimine (PEI). PEI is reported to form stable gels (through covalent bonding) with different types of polyacrylamide-based copolymers. For example, polyacrylamide tert-butyl acrylate (PAtBA),5 mixtures of acrylamide and acrylamido-2-methylpropane sulfonic acid (AMPSA),6 mixtures of acrylamide, AMPSA and N,N-dimethyl acrylamide.7 There are some environmental concerns for the use of the PEI crosslinker in some parts of the world, hence, other alternatives were explored.8 The mechanisms governing the formation of PEI crosslinked gels are reported elsewhere.9 More recently, studies indicated the possibility of using simple polyacrylamide polymers (PAM) with PEI to form thermally stable gels.10 The gelation kinetics of this system was examined at temperatures up to 140oC (284oF). However, the performance of this class of gels in porous media was not reported.