The Dependence of Water Permeability On Core Cleaning Methods In the Case of Some Sandstone Samples
Authors: Grist D.M. , Langley G.O. , Neustadter F.L. 
Published: March 2010

Large discrepancies in core analysis data are often noted when results from different laboratories are compared. This paper highlights the effect that core cleaning methods can have on water mobility following waterflooding.

INTRODUCTION

DATA obtained by two independent laboratories showed a consistent ten-fold difference in effective water permeability of sandstone samples at almost identical "residual" oil saturations. The experimental techniques used apart from core cleaning methods, were similar; also, core material was taken from similar locations in the sandstone formation.

The purpose of the work described here was to investigate possible reasons for the large discrepancy in effective water permeability values with particular reference to core cleaning methods.

It has long been recognized that core drilling, handling and cleaning techniques can significantly affect the flow and distribution of oil and water in reservoir rocks(1,2). Attempts have also been made to isolate the oil fractions responsible for changing the wetting conditions of reservoir rocks from their natural water-wet state(3). This work indicated that different solvents remove different amounts of surface-active crude-oil components from rock Samples. Other investigations have shown that, at a given water saturation, as the degree of rock preferential water wettability decreases the relative permeability to water increases(1). The extent to which wetting conditions affect the relative permeability to water is dependent on the pore structure of the rocks(2). This paper highlights the effect that core cleaning methods can have on water mobility following waterflooding.

Core Samples

Four plug samples were cut from core material which had been recovered from the consolidated sandstone formation in question. In addition to these, a synthetic (alundum) plug was used for reference purposes. Capillary pressure (by mercury injection).

(Figure in full paper)

The reservoir rock samples consisted mainly of cemented quartz grains; some chert was also present and illite clays occupied part of the pore space.

Test Methods

Three flood tests were carried out on each of the samples, the basic method being the same for each test. In addition, samples 2 and 3 were subject to a fourth flood test.

BASIC METHOD

The samples were:cleaned by reflux extraction using the solvents described below;oven dried for 24 hours at a temperature close to the boiling point of the solvent used; andevacuated for several hours before being saturated, under vacuum, with a deaerated sodium chloride solution (2.5 per cent weight per weight), hereafter referred to as brine.

After saturation, the permeability to brine (Kn) was measured. The samples were then drained of brine by dynamic displacement with a refined oil. The oil had a viscosity of about 40 cp, this being chosen to encourage a good displacement efficiency. Oil flow continued until brine displacement apparently ceased, after which the 40-cp oil was miscibly displaced by a refined oil of lower viscosity. This was done in an attempt to match the reservoir oil/water viscosity ratio. ‘Immobile’ brine saturation (Sw1)was determined and the effective permeability to oil (KCD) measured.