Characterization of a Heavy Oil Reservoir Using Computer Assisted Tomography of Core Material

ABSTRACT

The characterization of reservoir rock, using Computer Assisted Tomography (CAT) with X-rays has been an objective of increasing popularity in the petroleum industry. In the present study, a modified medical X-ray CAT scanner was used for the study of care material from the “B ” member of the Clearwater Formation of the Lower Cretaceous Mannville Group in the Cold Lake area. Two pieces of core, each approximately 0.3 m long, were scanned at 0.01 m intervals at a resolution of 0.00075 m x 0.00075 m x 0.005 m. The raw CAT scan data were analyzed using a post-processing computer package developed in our laboratories at Novacor Research & Technology Corporation (NRTC). This analysis gives a detailed picture of the various heterogeneities within each core section including cracks, shale barriers and clay clasts, and also allows determination of core properties such as bulk density and porosity. Bulk densities determined the CAT scan data are in good agreement with direct measurements obtained from several plugs taken from the core, The CAT scanning of the core successfully identified the locations for good plug selection. In addition, various types of heterogeneities were mapped in three dimensions. Significant core damage was observed which is attributed to the coring and freezing processes. This paper presents the methodology of the scanning and the subsequent data analysis.

Introduction

Computer Assisted Tomography (CAT) X-ray scanners have become a useful tool for the petroleum scientist and engineer. A detailed literature survey has been published elsewhere (1) while various applications are described in a variety of papers (2,3). Of particular relevance to this work are various papers describing applications to heavy oil recovery (4-6). The quantitative representation of core properties has been the focus of our recent work with emphasis on consolidated media and particularly in the Alberta dolomitic reservoirs. In this paper we address the potential of using the same technique for unconsolidated media. The CAT scanner can measure bulk density and effective atomic number directly by doing the so-called dual scanning, i.e., one scan at an energy which is predominantly producing-Compton scattering phenomena and one energy where photoelectric absorption is dominant. Fluid saturations can be measured in a core if the core is scanned while saturated and while dry at different energy levels, and if the fluids are scanned separately (1,3). This is very important when flow processes are monitored (1,2,4,6).

In the case of heavy oil unconsolidated sands, the core is drilled and frozen immediately in an effort to preserve it at conditions as close to the reservoir as possible. When the core reaches the core laboratory, plugs are taken at different intervals and core analysis is commencing. The recovery of unconsolidated sand during coring involves using a rubber or plastic sleeve core barrel. Rubber sleeve cores may be stabilized before sampling by freezing the rock and/or by immobilizing the core by surrounding it with wax, plaster of Paris foam, or other suitable materials. Test samples are often drilled from frozen core using liquid nitrogen as the bit lubricant.

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