Study of Problems Related to the Restoration Of the Natural State of Core Samples
Authors: Culec Louis 
Published: March 2010

In the field of reservoir engineering, it is important for the rock samples used in laboratories to have a surface state as close as possible to that found in the reservoir. To this end, the natural state of the samples can be either preserved or restored.

For several reasons, the existing techniques of sampling used for preservation are not always reliable. This is why we have concentrated on the restoration which seems more promising.

In restoration, any products which have been adsorbed onto the solid surface, whether they come from the original crude, from the mud, from oxidation during handling or from deposits during sampling, are first removed. Then the sample is left to age under reservoir conditions with reservoir fluids.

For the present study of some problems related to this restoration method, two typical porous media (sandstone, limestone) and various crudes were used. In order to have a clearly defined reference state, outcrop rock samples that had never contained any oil were chosen. Properties of the solid surface (acid or basic) and of the adsorbable products in the crudes guided the choice of the solvents used to clean the porous media which had previously been made oil wet.

The study also concentrated on the aging conditions of the rock-fluid system; i.e. the influence of the rock and oil natures and of the aging time.

The variations in surface properties of the rocks were checked by spontaneous- and forced-displacement experiments.

Data were thus obtained concerning the interactions between rocks and crude oils and the specific effects of some solvents used for cleaning oil-wet surfaces.

Introduction

WETTABILITY plays an important role in oil recovery by water drive, influencing the residual oil saturation, the relative permeabilities and the capillary pressure. Because these parameters cannot be measured in situ, laboratory tests are used, and the question of the validity of the measurements is thus raised. Most researchers have concluded that valid results cannot be obtained without using reservoir rock and fluid samples and unless temperature, pressure and surface equilibrium conditions are comparable to actual reservoir conditions. Although it is relatively easy to obtain reservoir fluids, it is quite difficult to have core samples with a surface equilibrium representative of the reservoir. There are, in general, two ways of overcoming this difficulty:obtaining fresh preserved cores through special coring and handling procedures;restoring the wettability of non-preserved cores by aging the previously cleaned cores' by means of reservoir fluids.

Special cores are often not available from reservoirs, and special drilling and coring are expensive. However, even when preserved cores are available, the representativity of the solid surface state is questionable. Indeed, temperature and pressure drop while the cores are being brought to the surface, and this may cause deposition of asphaltenes or other heavy fractions on the surface of the pores of the samples, bringing about changes in wettability. Consequently, restoration is often the only possibility, but it is desirable to restore the original surface state with some degree of reliability.