Determination of Waterflood Residual Oil Saturation From Routine Core Analysis

Abstract

Proposal Proposal Rathmell et al. proposed a method for estimating water-flood residual oil saturations from core saturations where the cores must he taken with water-base muds having a filtrate loss in excess of 5 cc at bottom-hole conditions. Their equation is (So)res = (So)coreBoE .........................(1) where (So)res = average post-waterflood residual oil saturation in the flooded region of the reservoir (So)core = average oil saturation from cores BO = oil formation volume factor at the time of coring E=bleeding factor- proposed to he 1.11. In my opinion. Eq. 1 must be corrected for waterflood displacement efficiency through multiplication by the reciprocal of the conformance factor. The conformance factor may be obtained from a reservoir simulator that accounts for reservoir heterogeneity, capillary effects, and mobilities of oil and water. in the absence of a reliable reservoir simulator, the conformance factor is estimated by (1 − V2)/M, as reported by Craig. Using this empirical factor, I propose Eq. 2 as the most practical and the most convenient method to calculate waterflood residual oil saturation from the average of the oil saturations obtained from routine core analysis (provided the cores are taken with water-base mud): (So)res = (So)core BoE M .........(2) 1 − V2 where M= mobility ratio, the ratio of the mobility of water at the average water saturation in the reservoir at breakthrough to the mobility of oil in the oil bank ahead of the displacing front. V= permeability variation calculated from reservoir core samples as described in Ref. 2. Example This example applies to the 119-R Project of the Henry reservoir, which has been reported elsewhere. The data are obtained from Marathon Oil Co. files or from Ref. 3. The average core oil saturation was obtained from cores taken at the end of waterflooding and before the application of the Maraflood process. The pertinent data are (So)core = 0.248 SWc = 0.25 M = 1.3 V = 0.325 Using Eq. 2 we get (So)res = (0.248)(1.01)(1.11) 1.3 (So)res 1 − (0.325)−2 The post-waterflood residual oil of 0.404 compares favorably with the results of other methods reported in Ref. 3. In fact, the "best" estimate of post-waterflood residual oil for the 119-R Project, as reported in Refs. 4 and 5, is 0.40.