Effect of Core Surfacing and Mud Filtrate Flushing on Reliability of Core Analysis Conducted on Fresh Cores

Reliability of core analysis information for use in formation evaluation studies is essential if results of such studies are to predict reservoir behavior accurately. This paper presents results of an experimental study of factors which might cause the behavior of cores to be nonrepresentative of the reservoir. For this study, extracted cores were brought in contact for 12 to 16 hours with bottom-hole samples of East Texas crude and formation water at the East Texas reservoir temperature and pressure; their water-flood behavior at these conditions was observed. The effects on core properties of reduction of temperature and pressure from reservoir to surface conditions, both with and without prior flushing of the cores with drilling mud filtrates, were investigated. The water-flood behavior of the cores was no significantly altered by a reduction of temperature and pressure from the environments of the East Texas field to atmospheric conditions nor by flushing with the filtrates of two caustic quebracho muds or with a high-pH saline water. Introduction An evaluation of oil reserves may be separated into two parts, an estimate of total reserves and an estimate of recoverable reserves. Each of these estimates is based on laboratory information obtained from samples of the reservoir rock. This paper presents a laboratory study of reliability of core analysis information used in estimation of recoverable oil reserves. In the estimation of recoverable reserves of a water-drive reservoir, water-oil relative permeability relations are the data of prime importance. For this reason, laboratory determination of these functional relations has received considerable attention in the past. Most of the early activity, however, was confined to measurements made on rock samples previously cleaned by solvent extraction.