Determination of In-Situ Wettability from Laboratory and Well Log Measurements for a Carbonate Field
Authors: Spinler E.A. 
Published: December 2007

Laboratory measurements of wettability are compared with a novel method for independent determination of apparent wettability in the field. Wettabilities of core plugs were determined by either an Amott Water Index or a Spontaneous Imbibition Index (ratio of measured spontaneous imbibition to highly water wet imbibition). Apparent wettabilities for the reservoir were obtained from water saturations measured by conventional well logs in extensively water-flooded zones and connate saturations calculated by neural nets trained in nearby wells. Apparent wettability appeared to vary within the formation and with the porosity of the reservoir rock. The field data also provided a strong indication that capillary forces dominated the displacement of oil in this reservoir.

Introduction

Wettability is a parameter that can be measured in the laboratory on reservoir core material by a variety of methods, but no well logging tools have yet been developed which can provide a direct measure of wettability in the reservoir. The most ambitious efforts to develop such a tool have been in the area of Nuclear Magnetic Resonance (NMR) in which some studies using NMR to measure wettability have proven successful for chalk by Howard and the author in the laboratory. Field trials using NMR to provide a quantitative measure of wettability are much more difficult than laboratory studies and have yet to reach fruition.

However, opportunities can and have occurred that allow estimates to be made of the apparent wettability of reservoirs from conventional well logs. The apparent wettability can be considered as one measure to evaluate the efficiency of a water-flood. Previously the author had introduced methodologies by which the apparent wettability of a reservoir formation could at times be obtained from evaluating newly drilled formations containing hydrocarbons or formations that have been water-flooded. This paper will describe the application of this approach to the well logs of a portion of a reservoir which had been extensively water-flooded. It will examine individual zones to determine if the wettability behavior varies differently. The primary premise of capillary dominated water displacement needed for the calculation of wettability will be evaluated.

Experimental

Cylindrical plugs from the reservoir cores were extracted by mild solvents to remove wettability artifacts that can be introduced by reservoir crude oils when cores are brought from reservoir to ambient conditions. It was postulated that the strongly bound organic films control wettability behavior for the reservoir rock in this study and the extraction method was selected to preserve these films. Baldwin 4 stated that appropriate extraction can remove crude oil and still retain the more strongly bound organic films on pore surfaces. Some plugs were subsequently aged under crude oil to obtain less water-wet behavior, but most were used as extracted. The core plugs were re-saturated with n-decane and brine using the centrifuge with an angled head to obtain uniform initial water saturations. Magnetic Resonance Imaging (MRI) was used to verify the procedure for obtaining uniform saturations. Spontaneous imbibition was conducted on the bench-top using glass imbibition cells to volumetrically measure the volume of oil produced. Forced residual oil saturations were also obtained by centrifuge using an angled head. The final saturation states of the plugs were determined by extraction.

The reservoir saturations after water-flooding were obtained from well logs using an Archie calculation. The well logs used for interpretation consisted of formation density tools for porosity, and a dual induction log or dual laterolog for deep resistivity. P. 167^