A review of thermally stable gels for fluid diversion in petroleum production

The use of water-soluble polymers coupled with proper concentration of cross-linker(s) as flow-diverting agents have become a common practice in recent years for oil recovery applications. In such practice a solution containing the polymer and cross-linker(s), referred to as gelant, is injected in desired zones and allowed sufficient time to set into a solid gel. These gels are used in injection wells to divert the flow of injected water or gas (CO2) to un-swept zones where additional oil can be recovered. The gels are also used to shut off the flow of water that strongly interferes with hydrocarbon production and substantially reduces the profitability of wells. There are a number of gelling systems available for treatment of lower temperature reservoirs. However, gels that can tolerate the harsh conditions of elevated temperatures and high salinity and divalent cations commonly present in deeper reservoirs are limited. When high molecular weight polyacrylamides are cross-linked to treat these hot reservoirs, their acrylamide groups will thermally hydrolyze. The resulting gel will further cross-link with the divalent cations available in the media, shrinking it to a fraction of its original volume. This process, which is referred to syneresis, can be avoided by selection of acrylamide-based polymers that are protected from extensive thermal hydrolysis. While other remedies such as lower-molecular-weight polyacrylamides, retarding agents or cooling of the target zones are attempted; these options often create unintended results. Recent studies include gelation of high molecular weight polyacrylamides with hydroquinone (HQ) and hexamethylenetetramine (HMTA) or terephtalaldehyde, terphthalic acid with hyroquinone, dihydroxynaphthalene and dibasic esters. These gelling systems are often prepared in seawater and require 2% sodium bicarbonate for their stability. Due to health and environmental concerns, the use of compounds such as HQ and formaldehyde is discouraged. Other recent studies describe the use of a modified polyacrylamide with polyethyleneimine. While the resulting gels are stable, their use is prohibitive due to requirement of a large polymer content (3- to 7-fold), as well as the high price of chemicals needed. This paper reviews the gelling systems available for high temperature well treatments. (C) 2000 Elsevier Science B.V. All rights reserved.