X-Ray Computer-Assisted Tomography for Laboratory Core Studies

Abstract

Introduction X-ray computer-assisted tomography (CT) entered the petroleum scene about 13 years ago. The technology showed much promise, but was expensive, unproven, and inaccessible to most petroleum engineers. Today CT scanners are an integral part of most major oil company, service company, and research university laboratories. This paper gives an overview of the theory and typical applications of CT. Theory Although CT is used to determine a wide variety of oilfield rock and fluid properties, it measures only one property, the linear attenuation coefficient, µ. The linear attenuation coefficient is defined by Beer's law and is a measure of the fraction of X -rays that pass through an object. Applications CT measurements are nondestructive and relatively fast. Most petroleum applications are aimed at estimating lithology, porosity, and/or saturation. By conducting well-designed experiments, these three parameters can be used to obtain a wide variety of additional information. When plugs are to be taken from whole cores, the whole core can be scanned beforehand, enabling the plugs to be taken at appropriate locations. This can be done without removing the wrappings for preserved cores. Lithology and Porosity. Lithology determination is typically performed for one of two reasons. The first is the screening of cores. Cores are used in a wide variety of laboratory flooding experiments. Even if CT is not used during the flow experiments, it is a useful tool for selecting cores for these experiments. Flooding experiments can be expensive and time consuming, and their analysis typically involves an assumption of uniform porosity and/or permeability. Little is more disheartening than spending months on a project only to find that your analysis is suspect or wrong because a supposedly homogeneous core has a significant interior shale streak or other heterogeneity, unnoticed from the outside. Today, many samples are routinely CT scanned to check qualitatively for homogeneity before conducting coreflooding experiments. This should be standard procedure.