Introduction to wireline methods

Wireline logs provide a survey of the formations drilled by the bit. These recordings enable geoscientists and engineers to determine reservoir characteristics such as lithology, porosity, fluid saturations, pressure, formation dip, hydrocarbon type, and their associated depth. Logs are an extremely important element in the characterization of subsurface formations. However, logs are not capable by themselves of providing full and perfectly accurate reservoir characterization. The best characterizations occur when logs are combined with cores and their associated analysis, mudlogs, measurement while drilling (MWD) data, seismic data, well tests, analysis of cuttings, and production tests. The characterization of reservoir properties from logs only is commonly called the sciene of log analysis. The characterization of reservoir properties from the analysis of all these measurements is commonly called petrophysics.

Part 4 of the Manual focuses primarily on the logging tools, logs, and their associated analysis. The first three chapters address basic open hole and cased hole logging tools and their uses, limitations, and advantages (Alberty). The next three chapters discuss speciality tools and their interpretation, in particular, the formation tester (Smolen), the dipmeter (Goetz), and imaging devices (Luthi). These tool discussions are followed by a series of chapters on the interpretation of logging measurements. The series includes preprocessing (Patchett), determination of water resistivity (Rw) (Peveraro), lithology (Hancock), standard interpretation (Alberty), difficult lithologies (Hashmy and Alberty), and fractured reservoirs (Augilera).

The variety of wireline measurements are affected by the environment in which the log is recorded. Borehole size, mud properties, and invaded zone can all influence the measurements. These environmental factors can significantly alter the apparent responses of the logs, frequently leading to erroneous analysis. Two steps can be taken to minimize the environmental perturbations. First, collect the logging data under conditions that minimize the influence of the environment. Second, be sure to correct the logs for the residual influence of the environment before analysis. Never assume that corrections after the fact can compensate for running logs under less than ideal conditions. Minimizing this influence of the environment at the time of collection can be accomplished by doing the folowing: optimize mud properties, use appropriate centralizers and standoffs, combine only those tools that require the same positioning within the borehole (e.g., centralized, decentralized, or stood off), and select the types of tools most optimum for the expected formation properties and hole conditions. Corrections after the acquisition requires an understanding of the correction required, the order in which they may best be applied, accurate knowledge of the borehole environment, and identification of the appropriate borehole corrections for the particular tool used.

Analysis of logs is almost always a problem of more unknowns than measurements. The "art" of this science comes from knowing which assumptions can best be made when and causing the least amount of uncertainty in the answer. Usually, these assumptions are best made when an analyst is experienced in a given geological area. The best analysts continue to expand their knowledge as they analyze more and more logs. Beware when you think you have conquered the art of log analysis.