Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering. Summary. This paper reviews published results of the use of polymers toimprove oil recovery, A discussion of the capabilities of the available typesof polymers and where they have been successful is coupled with the principlesof the mechanisms of polymer flooding to serve as a guide for futureapplications. The scope of this review is limited to case histories wherefull-scale polymer floods were applied, as opposed to near-well treatments. Introduction The purpose of this paper is to describe briefly the principles involved inpolymer flooding and to review field experience. Earlier reviews by Jewett and Schurz and Chang have covered much of this same ground. Chang, in particular, presents an extensive review of the polymer flooding literature. Therefore, wehave updated the list of literature rather than repeating those included inthese previous papers. We have tried to summarize the major points, particularly in relation to the most recent field case histories. The scope ofthis review is limited to what we refer to as "full-scale" polymer floods. Thisincludes those cases where crosslinking agents have been used to produce anin-depth permeability contrast correction, but excludes near-well, low-volumepolymer gel treatments. Consequently, all results of treatments of producingwells have been excluded from this review. Definition and Mechanisms of Polymer Flooding Oil and water are immiscible fluids. As a result, neither can completelydisplace the other from an oil reservoir. This is reflected in the irreduciblewater and residual oil saturations (ROS's) on a relative-permeability curve. Regardless of the amount of water cycled through the system, the oil saturationwill not be reduced below the ROS. In polymer flooding, a water-soluble polymeris added to the flood water. This increases the viscosity of the water. Depending on the type of polymer used, the effective permeability to water canbe reduced in the swept zones. Polymer flooding does not reduce the ROS, but israther a way to reach the ROS more quickly or to allow it to be reachedeconomically. There are three potential ways in which a polymer flood can make the oilrecovery process more efficient:through the effects of polymers onfractional flow,by decreasing the water/oil mobility ratio, andbydiverting injected water from zones that have been swept. Fractional Flow. The way in which a section of reservoir approaches itsultimate ROS is a function of the relative permeability relationships and ofthe viscosities of the oil and water phases. These are combined in the conceptof fractional flow. By applying Darcy's law to the oil and water phases flowingsimultaneously through a segment of a porous medium, the fractional flow of oil, fo, can be derived as (1) Any change that reduces the ratio / will improve the rate of oil recovery byincreasing the fractional flow of oil. Polymers can do this by increasing theviscosity of the water, . Once they have flooded a zone, some polymers alsoreduce the relative permeability to water, kw. This effect applies to any part of the reservoir where there is a mobile oilsaturation-i.e., anywhere that the relative permeability to oil is greater thanzero. However, if ko is already small because the mobile oil saturation is low, then fo will remain small at any achievable kw or . The fractional flow effecttherefore is more significant for polymer floods conducted early in the life ofa waterflood while the mobile oil saturation is high. An additional consideration is the oil viscosity, . All else being equal, the fractional flow of water will be greater in reservoirs where the oilviscosity is high. This leads to early water breakthrough and relatively highwater production when there is still a significant mobile oil saturation. Fractional flow effects are thus likely to be more significant in viscous oilreservoirs. Mobility Ratio. Real reservoirs cannot be swept uniformly. Even ahomogeneous reservoir suffers from less than 100% areal sweep at waterbreakthrough and at economically achievable water/oil ratios (WOR's). JPT P. 1503^

Oil acid and base numbers influence wetting through their effect on electrostatic interactions with the mineral surface. An improved nonaqueous potentiometric titration has been developed that correctly quantifies weak bases in crude oils. In crude-oil/silica systems, wetting behavior correlates with base/acid ratio and is consistent with wetting theories based on disjoining pressure.

This paper describes th.e properties of synthetic water-soluble polymers that are stable for extended periods of time in hard brines at very high temperatures. Several copolymers of vinylpyrrolidone (VP) and acrylamide (AM) were prepared and evaluated in our laboratories for EOR application in hostile environments. VP in the copolymer composition protects AM against extensive thermal hydrolysis, which otherwise will result in loss of viscosity and precipitation. A range of VP/AM copolymer compositions was found to tolerate the harsh conditions of 250°F [121°C] in seawater for extended periods of time and to be suitable for EOR application under these conditions. The performance of these polymers in porous media was evaluated by extensive coreflood experiments in Berea sandstone at 250°F [121°C] with synthetic seawater. The results indicate that these copolymers can easily be injected into porous media and that they can be effective polymers for EOR application in hostile environments.

Summary Laboratory data are used to show that commercial polyacrylamides hydrolyzeto an equilibrium degree that depends on the temperature of hydrolysis but islargely independent of the brine composition. At greater than 20 ppm hardnesslevels, polyacrylamide solutions pass through a sharp cloud point as theirtemperature is raised. This cloud-point temperature depends primarily on thehardness level of the brine and the degree of hydrolysis of the polymer, withlesser dependency on polymer molecular weight and polymer concentration. Indications are that these cloudy solutions cause pluging of porous media. Therefore, a polymer solution is potentially useful only below its cloud-pointtemperature. For application in a given reservoir, the temperature andfrequently the hardness of the water are fixed. If a polyacrylaniide hydrolyzesat reservoir conditions to where its cloud point in the field water falls belowthe reservoir temperature, it is not suitable for polymer flooding in thatreservoir. Cloud-point data, in conjunction with rate-of-hydrolysis data, indicate a "safe" limit of approximately 75C [167F] for brines containing 2,000ppm hardness and above, increasing to around 88C [190F] at 500 ppm, 96C [205F]at 270ppm, and at least 204C [400F] at 20 ppm and below. Most unsoftenedinjection waters will limit polyacrylamide use to below 93C [200F]. Mostproduced waters will limit applicability to below 82C [180F]. Introduction Partially hydrolyzed polyacrylamides used for EOR are Partially hydrolyzedpolyacrylamides used for EOR are known to be sensitive to temperature anddivalent ions. The amide groups present in these polymers will hydrolyze inaqueous solutions to an extent that depends on pH and temperature. Theresultant more hydrolyzed poly-acrylamide may have a degree of hydrolysissufficient to poly-acrylamide may have a degree of hydrolysis sufficient tocause precipitation in the reservoir or injection water used. Several investigations have dealt with this precipitation mechanism ofpolyelectrolytes in hard brines. It is widely accepted that precipitation isthe result of interaction between divalent cations and the carboxylate groupspresent within the hydrolyzed polymer. Strong site present within thehydrolyzed polymer. Strong site binding apparently occurs between the divalentcation and two carboxylate groups on the polyion. Whether actual precipitationoccurs, however, depends on temperature. precipitation occurs, however, dependson temperature. If the temperature of a solution of polyacrylamide in watercontaining divalent cations is gradually raised, the solution generally willsuddenly turn cloudy at a well-defined temperature. If the temperature israised further, precipitation follows. precipitation follows. This cloud point, then, represents a stability limit for that particular polyacrylamide in thatparticular brine. For EOR purposes, the particular polymer/brine combination ispotentially useful only below its cloud point. As polymer flooding becomes morewidespread, there is a polymer flooding becomes more widespread, there is atendency for polyacrylamides to be applied at higher temperatures, sometimes inbrines containing significant hardness levels. It has not been clear how farthe temperature/hardness limits can be pushed without polymer precipitation. precipitation. We attempt to define some of these limits through an extensiveseries of cloud-point measurements for commercial polyacrylamides, coupled withdata on the rate of hydrolysis as a function of temperature. These are combinedto predict precipitation times as a function of hardness level and temperature, thereby providing guidelines for the use of polyacrylamides in EOR. Experimental Materials. All the polymers tested were acrylamide polymers of commercialorigin, which were used as received polymers of commercial origin, which wereused as received or were thermally hydrolyzed to desired extents before use. The salts used in this study were reagent grade. Sample Preparations. Because thermal stability of polyacrylamides isimpaired in the presence of dissolved polyacrylamides is impaired in thepresence of dissolved oxygen, precautions were taken to exclude oxygen frompolymer solutions. A glove box maintained under an polymer solutions. A glovebox maintained under an oxygen-free helium atmosphere was used for all samplepreparation and handling. The presence of oxygen in the preparation andhandling. The presence of oxygen in the box was monitored with a solution ofdiethyl zinc. A 5 % sodium chloride solution was sparged for 15 to 20 minuteswith nitrogen from which all oxygen had been removed. This purging time wassufficient to reduce the dissolved oxygen content of the water to less than 10ppb as measured by a Chemet TM dissolved-oxygen kit.

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