Advances in Well Logging
Authors: Timur Aytekin 
Published: December 2007

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.

INTRODUCTION
Well logging is an integral part of formation evaluation inthat borehole measurements provide the largest source of data. These are usedwith core and fluid data to determine reservoir depth and thickness, porosity, lithology, hydrocarbon saturation, and permeability. This porosity, lithology, hydrocarbon saturation, and permeability. This information also aids indrilling, completion, and operation of wells; geological and geophysicalexploration; and development of reservoir models for efficient production.Borehole measurements of geophysical parameters involve downhole sensors(sondes), means for transmitting data to the surface, and recording andprocessing systems. Physical properties are measured and recorded as functionsof depth to obtain well logs. Then the measured physical parameter logs areinterpreted to obtain logs of derived formation parameter logs are interpretedto obtain logs of derived formation parameters. These include fundamentalproperties such as porosity, parameters. These include fundamental propertiessuch as porosity, permeability, saturation, and any other subsurfaceinformation needed in permeability, saturation, and any other subsurfaceinformation needed in geological, reservoir, and economic analyses. Explorationfor hydrocarbons in high-cost areas and enhanced recovery operations with long, uncertain lead times have increased the demand for more accurate evaluation ofnew formations and re-evaluation of some old ones. This demand for accurateformation evaluation is increasing not only the number of well-logging sensorsneeded but the amount of data required of each. For example, the data rate fora conventional induction/ sonic log is 200 bits per borehole foot, two ordersof magnitude higher for a high-resolution dipmeter, and three times more for anacoustic full waveform log (60 kilobits per foot). Estimated data rates forfuture acoustic and induced gamma ray spectroscopy tools are two orders ofmagnitude higher than these (i.e., on the order of megabits per foot). To putit in a familiar perspective, in an average well the acoustic full waveform logrecords roughly the same amount of data that a conventional seismic surveyrecords over a 1-mile section. Several significant advances have been developedto meet the challenge of data explosion. Logging tools have been equipped withdownhole computers for digital transmission of data to the surface, where othercomputers are used for recording and real-time processing. Combination loggingtools have been developed to reduce rig time during logging; some can recordsix or more petrophysical measurements concurrently. For example, a combinationtool consisting of gamma ray, dual-induction, compensated-neutron, andcompensated-formation-density logs provides, in a single run, a complete suiteof logs for formation provides, in a single run, a complete suite of logs forformation evaluation. Wellsite computers provide quick-look processing of theselog data for immediate completion decisions. They also allow transmission bymeans of satellites or phone lines to central computer facilities. Well loggingmethods usually do not measure directly the primary rock properties needed forreservoir engineering purposes: porosity, properties needed for reservoirengineering purposes: porosity, permeability, fluid saturation, and fluidmobility. These parameters are permeability, fluid saturation, and fluidmobility. These parameters are inferred from borehole measurements ofelectromagnetic, acoustic, and nuclear properties. Nevertheless, these methodsdo provide the best, sometimes the only, opportunity for measuring theseproperties in place in a relatively undisturbed state. Well logging is arapidly developing technology encompassing far too many methods to describe.This article highlights some of the technology in a more general fashion. Ref.1 gives a more detailed overview; references cited therein give furtherdetails.

ELECTROMAGNETIC WAVE METHODS(Resistivity Logs).
Properties ofelectromagnetic waves are measured in boreholes in a broad frequency rangevarying from DC to gigaHertz, primarily for evaluation of hydrocarbonsaturation. Numerous tools and techniques involving focused resistivity andconductivity and microresistivity have been developed to measure the electricalresistivities of the formation, both within the invaded zone () and beyond(true formation resistivity,). However, none of these alone can measure trueformation resistivity under all borehole and formation conditions. Theselection of a proper combination of deep and shallow investigating tools fordetermining depends on formation conditions such as connate water resistivityand porosity as well as borehole conditions such as mud salinity and holesize.
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