Waterflood Mobility Control: A Case History

Abstract

Performance of a pilot flood in eastern Kansas indicates improved recovery and accelerated production resulting from mobility ratio control obtained by adding a high molecular weight polymer to injected water. Ultimate pilot flood recovery of the 75-cp reservoir oil is anticipated to be 350 bbl/acre-ft during the six-year flood life. Cumulative recovery to date totals 242 bbl/acre-ft after 30 months of injection. Reservoir parameters and flood performance are discussed in detail to provide a case history relating to the feasibility of polymer flooding. Introduction The Vernon Polymer flood of the Brazos Oil and Gas Co., located in Sec. 2, 24 S., 16 E., Woodson County, Kans., was begun in late Oct., 1963. Injection of a 500 ppm solution of Pusher* polymer was initiated in a 15-acre pilot to determine feasibility of improving oil recovery by mobility ratio control. Performance to date and the anticipated recovery of 115,000 bbl from the 329 acre-ft pilot indicate efficient displacement of the viscous reservoir oil. Dependence of flood recovery on the displacing fluid-displaced oil mobility ratio has long been recognized. Displacement efficiency of the reservoir oil decreases with increasing mobility ratio. Therefore, economic reduction of this parameter is an attractive approach to improving oil recovery, particularly from viscous oil reservoirs where mobility ratios are often quite high. Waterflood mobility ratios can be controlled by dilute solutions of certain polymers which provide a displacing fluid with considerably reduced mobility. Reservoir parameters and flood performance are emphasized in this paper to provide data relating to polymer flood applicability. THEORY The polymer used in the Vernon pilot exhibits the property of unusually high resistance to flow through porous media. A small concentration in injected water effects a mobility reduction considerably beyond that predicted from increases in bulk viscosity as measured in a capillary or rotating cup viscosimeter. Water mobility in the subject flood is reduced by a factor of 7.55 with the addition of 500 ppm Pusher chemical to injected water while the bulk viscosity is increased from 1.03 to only 1.40 cp at 75F. The exact tow mechanism causing resistance to flow has not been established; however, it is observed only in tortuous flow systems and appears to be influenced by brine properties, rock properties and oil saturation. Consequently, mobility reduction is estimated from flow studies for a given sample of reservoir rock and is expressed in terms of a "resistance factor". This parameter is defined as the ratio of brine mobility to polymer solution mobility measured at residual oil saturation: .........(1) where RF = resistance factor (dimensionless)w and p = mobility of water and polymer solution, respectively kw and kp = water and polymer permeability, respectively (darcies)w and p = viscosity of water and polymer, respectively (poise). Eq. 1 is similar to the expression commonly used to relate the relative mobility of the water bank at residual oil saturation to oil-bank mobility at irreducible water saturation: ........(2) where Mw-o = mobility ratio of water to oil (dimensionless) o = mobility of oil ko = permeability to oil (darcies) o = oil viscosity (poise). From Eqs. 1 and 2 it is apparent that the polymer-solution-to-oil mobility ratio can be approximated by dividing the brine-to-oil ratio by the resistance factor RF: ..........(3) where Mp-o = mobility ratio of polymer solution to oil (dimensionless). This simplified presentation assumes no dilution of the polymer solution by connate water. Also, the presence of a connate water bank separating the polymer solution from the displaced oil is not considered. Displacement of oil by waterflooding is increasingly dominated by viscous forces as the viscosity of the oil displaced increases. When the oil mobility is less than that of the injected water, the oil-water interface is not a piston-like front. Instead, instabilities occur at the flood front in the form of viscous fingers which tend to penetrate the less mobile oil bank. JPT P. 1151ˆ