Errors in Core Oil Content Data Measured by The Retort Distillation Method
Authors: Rathmell J.J. 
Published: December 2007

Abstract
Core analysis results may yield information concerning the magnitude of reservoir oil content. Under conditions where the oil content of a core is at the waterflooding residual value prior to lifting, core data can be combined with an adjustment for oil expulsion by gas drive during lifting and for oil shrinkage to yield an estimation of reservoir residual oil saturation. The effect of analysis errors in oil content values determined by retort distillation of oil from core plugs has been examined. Saturations of crude oil and brine were established in 14 1 1/2 - X 3-in. core plugs from three sandstone and one limestone outcrops and one limestone reservoir. These saturations were known within 3 percent. The core plugs were submitted to another laboratory where saturations were estimated by routine core analysis procedures involving retort distillation of the fluids from a portion of the core plug. Results of these studies indicate that errors in the oil content of cores expressed as oil volume per unit bulk volume of rock are small in comparison to the natural variations of oil content from one sampling point to the next in the cores of most sandstone reservoirs. It is suggested that the common practice of analyzing one plug-size sample of core per foot of reservoir depth may be inadequate, particularly for thin sand sections or sands of a highly inhomogeneous nature.

Introduction
The level of waterflooding residual oil content of a petroleum-producing reservoir is important not only to waterflooding economics but, more critically, to the economics of tertiary recovery operations. A priori economic evaluation of tertiary recovery processes often indicates marginal economic success. For this reason a reliable estimate of the amount of oil ‘in the reservoir is of great importance. Core oil content data measured on cores taken with water-base muds could be used as a source of information on reservoir residual oil content. Reliability of the average core oil content obtained from producing interval cores will be limited by these factors:precision of the average core oil content of the interval,mud filtrate flushing of the core to residual oil,adjustment of the surface oil content of the core for flow of oil from the core during lifting (hereafter referred to as bleeding),adjustment of the surface oil content of the core for shrinkage andcareful core preservation at the surface.
The precision of the average core oil content for retort analysis procedures (Factor 1) will be considered in detail. The last four of the above conditions are assumed for the purpose of this article to be met or achievable with many cores. It is recognized that the validity of this assumption will depend on the coring conditions as well as the rock and reservoir fluid properties in the cored interval. One of the more common means by which core oil contents are measured is the retort distillation technique. The experimental approach in this investigation was to determine retort-analysis core oil content precision by submitting core plugs having a known oil content measured gravimetrically in our laboratory to a reputable commercial laboratory for retort analysis.