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Comparison of Image Logs to Nuclear Magnetic Resonance Logs


Image logs have proven their effectiveness in delivering insight about the complexity of deposition and alteration within many reservoirs. Bedding events can be observed and depositional environments determined using these enhanced logs. Secondary porosity events can also be observed in many circumstances with vugs, natural fractures, and drilling-induced fractures all apparent from the image and available for accurate interpretation by specialized geologists. The contribution of this data to reservoir quality is not easily determined. Observed secondary porosity cannot be quantified from these images and attributed to changes in permeability. In addition, in many cases, textural changes within the reservoir cannot be observed or determined. On the other hand, Nuclear Magnetic Resonance (NMR) logs provide additional insight into the textural alterations, both depositional and evolutionary, that affect the deliverability of a well. Porosity events can be accurately measured and characterized using the Bray-Smith permeability equation. Drilling-induced events are ignored due to the depth of the measurement involved. Contributing reservoir alterations or secondary porosity, can be quantitatively evaluated as a portion of effective porosity. This paper compares observed textural changes within various reservoirs and the characterization of permeability using this Bray-Smith algorithm. Examples of pore size distribution changes will be presented along with natural and drilling-induced fractures. Vugs in carbonate reservoirs will also be considered. These results will be compared to image logs whenever possible. In all cases, permeability characterization provides additional clarity as to the quality of the reservoir. Permeability is accurately compared to production data and core data when available. An assessment of the value of each of these enhanced logging techniques in evaluating and understanding difficult reservoirs will be presented.