Correction Factor for Residual Oil Saturation From Conventional Cores
Authors: Egbogah Emmanuel O. , Amaefule Jude O. 
Published: December 2007

Abstract
Knowledge of initial-residual saturation values are required to estimate recovery efficiency and the economic viability of an oil recovery project. Accurate knowledge of the oil content of a reservoir is critical in the economic evaluation of any prospect for enhanced oil recovery after conventional recovery by primary means and any injection of water and gas. The accuracy of these data is very critical for implementing post-waterflood techniques because project economics are very sensitive to oil saturation in a partially depleted reservoir. Consequently, attempts have been made to determine oil saturation values from various techniques. However, the commonest and most economical method has remained the routine analysis from cores cut with water-base muds. But these render inaccurate results due to inherent flushing during coring as well as oil shrinkage and expulsion due to gas expansion during core lifting to the surface. Typically, such results are corrected for shrinkage and the resulting value is arbitrarily increased again by a bleeding factor of 10%–15%. There are no published industry correction data to correctly account for flushing and blowdown in conventional cores. This paper presents the core analysis results of a highly controlled, bland water-base sponge coring program with a PDC maximum-flushing corehead designed to simulate a waterflood The results from 29 full-diameter core analysis from 200 feet of recovered core yielded an average bleeding / expulsion factor (E) of 1.062 which we believe to the first such published data set to provide a measured E factor for correcting residual oil saturation values from conventional cores. The results are supported by a well designed and properly implemented quality control program. The results of the sponge coring program also compare very well with residual oil determined from other sources.

Introduction
Oil saturations determined from analysis of cores that are cut with water-base muds are always less than in-situ saturations because of drilling-fluid invasion and oil flushing during coring, and oil shrinkage and expulsion due to gas expansion as the core is lifted to the surface. The core is subjected to pressure and temperature reduction as it is brought to the surface. Thus, the gases released from solution with pressure decline cause shrinkage of the oil volume, and as the gases expand and escape from the core, they expel some of the core fluids. Use of pressure-retaining core barrel or sponge coring techniques can eliminate this second effect but do not eliminate flushing. Two well-known flow processes can occur to cause oil flushing during core cutting and retrieval:mud filtrate invasion caused by overbalance pressure at the drill bit during coring operations andfluid expansion caused by a drop in pore pressure during core retrieval.
Jenks et al provided information indicating that overbalance pressure is the prime factor in oil stripping during coring operations. Consequently, accuracy in reserves determination requires that residual oil saturations be corrected for the effect of high pressure gradients around a wellbore which can arise during both drilling and production.
Rathmell et al point out, however, that routine core analysis oil saturations that are adjusted for bleeding and shrinkage, can give reliable values for residual oil saturation after waterflooding in many sandstone reservoirs, particularly those containing low-viscosity oils. However, Rathmell and group have not provided a correction factor value for adjusting the oil saturations for bleeding and shrinkage. In consideration of this problem, Kazemi proposed the following equation:
(1)
P. 445^