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^

A newly developed process using a combination of water-soluble polymers and multivalent cations to reduce water mobility has been tested in the laboratory. The process treats the porous media with a sequence of three slugs, porous media with a sequence of three slugs, the first and third being polymer solutions and the second being a solution of a selected multivalent cation. Calcium, magnesium, and aluminum cations have been used with partially hydrolyzed polyacrylamides or copolymers of acrylamide in laboratory flow tests. For example, treatments of Repetto sandstone cores having brine permeabilities of 200 to 400 md with a polymer solution resulted in a residual resistance factor to brine of about 3; whereas, residual resistance factors of 16 to 18 were obtained by using either calcium or aluminum cations in the three-slug combination treatment with the same polymer. Based on laboratory evidence that polymer concentrations of 250 ppm can produce residual resistance factors in excess of 10 in porous media having brine permeabilities greater than 1 darcy, applications are evident in polymer flooding as well as in the high concentration-low volume treatment of oil-producing and water injection wells. These processes are being field tested in the treatment of producing wells and water injection wells to reduce water mobility and increase oil-producing rates. Introduction The action of polymers in porous media to reduce the water mobility has been extensively studied and, in general, is attributed to the increased viscosity of the polymer solution and to a reduction in permeability (residual resistance effect) to water of the porous media. For successful operations, the relative importance of these two methods of controlling mobility largely depends upon the application. In polymer flooding both methods of reducing the water mobility are important, with the permeability reduction effect probably being more permeability reduction effect probably being more significant where light oils and high variations in permeability are present. In the small volume treatment of water-injection wells to improve injection profiles and in treatment of production wells to reduce water-oil ratios, the production wells to reduce water-oil ratios, the permeability reduction effect is essential and permeability reduction effect is essential and the viscosity effect negligible. Although the various roles of polymer treatments are not clearly delineated for use with chemical flooding, the permeability reduction effect will perhaps be of dominant importance for perhaps be of dominant importance for pretreatment of reservoirs to achieve more uniform pretreatment of reservoirs to achieve more uniform injection of the chemical slug. By comparison, the viscosity effect is probably important in the slug proper, and both effects for use as a mobility buffer behind the chemical slug.

Summary This work studies the mixing of injected water and in-situ water during waterfloods and demonstrates that the mixing process is sensitive to the initial water saturation. The results illustrate differences between a waterflooded zone and a preflooded zone during, for example, water-based EOR displacement processes. The mixing of in-situ, or connate, water and injected water during laboratory waterfloods in a strongly water-wet chalk core sample was determined at different initial water saturations. Dynamic 1D fluid-saturation profiles were determined with nuclear-tracer imaging (NTI) during waterfloods, distinguishing between the oil phase, connate water, and injected water. The mixing of connate and injected water during waterfloods, with the presence of an oil phase, resulted in a displacement of all connate water from the core plug. During displacement, connate water banked in front of the injecting water, separating (or partially separating) the injected water from the mobile oil phase. This may impact the ability of chemicals dissolved in the injected water to contact the oil during secondary recovery and EOR processes. The effect of the connate-water-bank separation was sensitive to the initial water saturation (Swi). The time difference between breakthrough of connate water and breakthrough of injected water at the outlet showed a linear correlation to the initial water saturation Swi. The results obtained in chalk confirmed earlier findings in sandpacks (Brown 1957) and thus demonstrated the generality in the results.

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In the process of analyzing a large amount of dynamic adsorption data for hydrocarbons a number of existing mathematical models have been used. Even though many of these can be used to "fit" the data to varying degrees, they involve assumptions that are incompatible with the proved physical mechanisms involved and/or are incapable of describing the concurrent adsorption of several adsorbates. Therefore, the ability of a model to fit the data has only empirical significance. Data are presented which illustrate