Ahmad Moradi-Araghi

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

Abstract 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 (CO 2 ) 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.

Hydrolysis and Precipitation of Polyacrylamides in Hard Brines at Elevated Temperatures

Laboratory data are used to show that commercial polyacrylamides hydrolyze to an equilibrium degree that depends on the temperature of hydrolysis but is largely independent of the brine composition. At greater than 20 ppm hardness levels, polyacrylamide solutions pass through a sharp cloud point as their temperature is raised. This cloud-point temperature depends primarily on the hardness level of the brine and the degree of hydrolysis of the polymer, with lesser dependency on polymer molecular weight and polymer concentration. Indications are that these cloudy solutions cause plugging of porous media. Therefore, a polymer solution is potentially useful only below its cloud-point temperature. For application in a given reservoir, the temperature and frequently the hardness of the water are fixed. If a polyacrylamide hydrolyzes at reservoir conditions to where its cloud point in the field water falls below the reservoir temperature, it is not suitable for polymer flooding in that reservoir.

Application of Freshwater and Brine Polymer Flooding in the North Burbank Unit, Osage County, Oklahoma

A freshwater polymer-flood project was implemented in a 1,440-acre area of the North Burbank Unit (NBU) in 1980 with sequential injection of 4.2 million Ibm of polyacrylamide and 4.0 million Ibm of a 2.9% aluminum citrate crosslinking solution. Response to polymer flooding has been very pronounced, with ultimate incremental oil recovery projected to exceed 2.5 MMSTB of oil and total project oil expected to be 4.5 MMSTB. A crosslinked polymer-flood process for use in brine was developed that displays equally favorable performance characteristics as the freshwater polymer-flooding system.

Determination of anionic high-molecular-weight water-soluble polymers by size-exclusion chromatography

Abstract A method was developed for the determination of anionic high-molecular-weight water-soluble polymers in oil field brines using size-exclusion chromatography with ultraviolet detection. The stationary phase is coated controlled pore glass and the mobile phase is 0.1 M potassium dihydrogen phosphate. Various polymers were determined by this technique. The detection limits are 1–2 ppm with typical relative standard deviations less than 2% and linear response in the range 0–200 ppm. The polymer samples in oil field brines can be analyzed without prior dialysis, shearing, or solvent extraction.

High Temperature and Hardness Stable Copolymers of Vinylpyrrolidone and Acrylamide

The preparation and solution properties of poly(vinylpyrrolidone-co-acrylamide), synthetic water-soluble copolymer which has non-precipitating behavior in hard brines at high temperatures, is discussed. One sample of the copolymer has remained in synthetic seawater for over six years at 121°C (250°F) without precipitation. In that time the viscocity of its seawater solution has remained constant. It is recognized that while polyacrylamide is a highly effective water thickener, it rapidly hydrolyzes at high temperatures. In the presence of divalent cations, such as Ca++ or Mg++, hydrolyzed polyacrylamide will precipitate and lose viscocity. The usefulness of polyacrylamide is restricted to applications in which temperatures of less than about 75°C are encountered.