Polymer Flooding Review

Abstract

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^