Time-Lapse Logging, A Valuable Reservoir Evaluation Technique
Authors: Murphy R.P. , Owens W.W. 
Published: December 2007

Abstract
A logging procedure is described which utilizes a comparison of two logs of the same type to define a reservoir property or condition of interest. Interpretation is based on a natural or induced change in reservoir conditions during the lapse occurring between the time the two logs are run.In a gas injection project in a wet gas reservoir, a comparison of neutron logs run before and after three months of dry gas injection accurately defined the zones of gas entry in an injection well. Electric and neutron logs run before and after gas injection in a water-bearing formation were used to define the zone of injection and the magnitude of gas saturation developed. Finally, in a pressure depleted reservoir, electric and neutron logs run before and after water injection were used to compute the oil, water and gas saturations of the formations prior to water injection. A list of other possible applications of this logging procedure is presented.

Introduction
Well logging devices are available which can provide both qualitative and quantitative information on the chemical and physical properties of underground formations and their contained fluids. The primary use of these devices to date has been to identify and characterize potential oil or gas productive horizons in exploratory (and developmental) drilling operations. However, with the ever increasing change in emphasis from exploration to secondary and tertiary recovery operations for increasing reserves, the utility of logs must be extended to provide information about the reservoir throughout its entire productive life.
It has long been recognized that the response of some downhole logging devices is influenced by changes in the formation liquid saturations. In fact, this sensitivity demands that some logs be run soon after cessation of drilling operations if true formation properties are to be determined. Similarly, however, this sensitivity has made it possible to locate zones containing mobile hydrocarbons. It is also this sensitivity of log response to changes in formation saturation conditions that can be used to advantage to provide data necessary for application to or interpretation of secondary recovery operations.
As an example, prior to the initiation of a water or solvent injection project in a reservoir, a knowledge of the vertical and lateral oil saturation distribution in the reservoir is needed. Logs might not normally be considered as a means for obtaining this information because their interpretation is difficult, or even impossible, when three immiscible phases are present in the reservoir. However, interpretation may be possible if the change in reservoir saturations with time, injection, or production can be coupled with known initial or final saturation conditions. The change in saturation may be determined by a direct comparison of the same log run before and after the change. The result of this comparison may thus provide information about the reservoir that may otherwise be unattainable. Since there is a lapse in time between the logging runs, this logging procedure has been called "time-lapse logging". To demonstrate the utility of this logging procedure, detailed discussions of three field applications are presented in the following section.