At the time of publication of this book, nuclear magnetic resonance (NMR) logs are still not widely used, and their use in some geologic and drilling environments is still problematic. At present, while they create estimates of porosity that are equivalent to the other porosity logs (and have some advantages over those logs), they are most powerful when used in conjunction with that other data rather than instead of it. This chapter is focused on the interpretive applications of NMR logs.
Conventional porosity and resistivity logs provide the geologist with a first look at the reservoir, but there are practical limits to the accuracy and usefulness of these data. Neutron, density, and sonic porosities are more sensitive to lithology than to reservoir fluids (this is not always the case in a gas-bearing formation). As a result, even small amounts of clay minerals may cause errors in porosity measurements. In addition, reservoirs commonly contain clay-bound, capillary-bound, and moveable water. Resistivity tools are sensitive to all water, and Archie water-saturation calculations can overestimate the amount of producible water. Thus, potential pay zones might remain elusive with conventional logs.
Porosity measurements with neutron-density combinations provide an estimate of total porosity, which is all the pore space in the reservoir whether the pores are interconnected or isolated. Still, with volume of shale (Vshaie) corrections, it is possible to mostly eliminate the shale effect and arrive at an estimate of effective porosity or producible pore space. What remains to be resolved in this situation is
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Basic well log analysis
This publication is a general introduction to common openhole logging measurements, both wire line and MWD/LWD, and the interpretation of those measurements to determine the traditional analytical goals of porosity, fluid saturation, and lithology/mineralogy. It is arranged by the interpretation goals of the data, rather than by the underlying physics of the measurements. The appendix files contain digital versions of the data from the case studies, a summary guide to the measurements and their interpretation, and a simple spreadsheet containing some of the more common interpretation algorithms. This Second Edition of Basic Well Log Analysis delivers a great impact on training and self-training along with superior workbook exercises, newer measurements, borehole imaging, and nuclear magnetic resonance in separate chapters, all directed to provide a guide through the lengthy and sometimes ambiguous terminology of well logging and petrophysics. It provides readers with interpretation examples (and solutions) so that the techniques described here can be practiced.