Charles Flaum and Philippe P. Theys
Schlumberger
Abstract
The advertised geometrical characteristics of a given logging tool commonly change over time as the original specifications are modified by field experience. This change often results from the vagueness of the definitions of these characteristics.
Depth of investigation, usually quoted as a single number, is a parameter that attempts to summarize too much information. In fact, this parameter depends on detailed geometry of the tool and varies with the properties of the formation. Two tools with the same quoted depth of investigation may behave quite differently and illuminate the formation in significantly different ways. A single parameter is inadequate to differentiate between tools and help the user find the measurement most appropriate for a particular problem.
Quantification of vertical resolution suffers from similar limitations. No definition is universally accepted by the industry. Complicated tool behavior may preclude simple, one-parameter definitions of vertical behavior. Interdependence between vertical and radial responses further complicates the description.
This paper addresses the limitations of the current definitions of these parameters. The problems inherent in the combinations of measurements with different geometrical behaviors are discussed.
From these observations, new definitions of geometrical parameters are proposed. Expressing tool behavior in terms of these parameters leads to a more objective comparison of logging devices.