Restoration of the Natural State of Core Samples

Abstract

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Introduction The wettability of a reservoir rock plays an important role in oil recovery during a water drive. It influences the residual oil saturation, the relative permeabilities and the capillary pressure. Since it is impossible to measure all these parameters in situ, laboratory experiments are used, thus raising the problem of the representivity of samples. Most scientists working on the problem have concluded that it is difficult to obtain valid measurements without operating under conditions as close as possible to those in the reservoir. It is easy to reproduce reservoir temperature and pressure conditions in the laboratory. It is also possible to obtain reservoir fluids. Unfortunately, obtaining porous-medium samples that are representative from the standpoint of the surface state is more delicate. To obtain such samples, we can try either to preserve the original surface state or else to restore it. At the moment the former method is the most widely used, but requires special precautions while the sample is being taken. precautions while the sample is being taken. Nonetheless, these precautions are not sufficient to prevent a temperature and pressure drop when the sample is being brought up. This drop may change the distribution of fluids on the surface of pores and be accompanied by the deposition of asphaltenes or other heavy fractions. In summary, the preservation technique as is now practiced is far from being perfect. Likewise, only modest confidence is generally placed on the quality of preservation. placed on the quality of preservation. It would be thus advantageous to have another method that could be applied whether the sample had or had not been cored with precaution and protected from any contamination, oxidation, fluid loss, etc., or whether or not it had already been used for physical measurements in the laboratory. The restoration method meets these requirements. It consists of two steps:cleaning the surface of the solid to remove as much as possible if not all the products that had become possible if not all the products that had become fixed there, whether they came from crude oil or mud. It is not possible to separate the products initially adsorbed on the rock in the products initially adsorbed on the rock in the reservoir from those that had become fixed there afterward.