1课Fundamentals of Electrical Logging文档格式.docx
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1课Fundamentals of Electrical Logging文档格式.docx
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Withelectricallogging,parametersmeasuredinsitubyappropriatebottomholeinstrumentswerecontinuouslyrecordedatthesurface.Measurementswereperformedonlyintheuncasedportionsoftheboreholes.
Itbecameageneralpractice,whenaholewasdrilledoratintervalsduringthedrilling,
torunanelectricalsurveyforthepurposeofquicklysecuringacompleterecordoftheformationspenetrated.Thisrecordingwasofimmediatevalueforthegeologica1correlationofthestrataandalsoforthe,detectionandevaluationofpossibleproductivehorizons.Theinformationobtainedfromtheelectricallogwassometimessupplementedbysidewallsamplesoftheformationorbythetypesofinvestigationsperformedinboreholesdeviationsurveys,dipmeterssurveystemperaturesurveys,etc.
Severaldifferentkindsofresistivitycurvesorlogs,inadditiontotheSPcurves,wererecordedintheboreholes.Theselogs,obtainedbyusingdifferentresistivitymeasuringdevices,arecalledconventionalelectricallogs(normalandlateral),Laterolog,Microlog,etc.AtypicalelectricallogrecordedwiththeconventionaldevicesisillustratedinFig.4.ThelefttrackofthelogcontainstheSPcurve,andtherighttrackscontaintheresistivitycurves.Eachcurvesshowsasequenceofpeaks,deflections,andflatzonescorrespondingtothedifferentformationstraversedbythebore-hole.Aswillbeseeninthefollowingchapters,logsrecordedwithothertypesofresistivitymeasuringdeviceshaveasimilargeneralappearance,althoughthecorrespondingdevicesdifferbasicallyintheirprinciplesandintheirperformances.
RESISTIVITYMETHODS
Resistivitymethodsarenumerous,anditisconvenienttoclassifythemaccordingtothecorrespondingtypesofsubsurfacemeasuringdevices;
i.e.,macrodevicesormicrodevices.
Macrodevices
Themacrodevicesinvestigateratherlargevolumesofmaterialaroundtheboreholefromabout10to100cuftandmore.Theirmeasurementsareusedfordefinitionofformationforcorrelations,andforqualitativeandquantitativeanalysesofreservoirsintermsoffluid.saturationandporosity.Thisgroupincludestheconventionalresistivitydevicesandtheresistivitydeviceswithfocusingsystems.
WhenUsingtheconventionalelectricallog(normalandlateral),thepathofthecurrentusedforthemeasurementhasnorestrictionotherthanthelocationoftheelectrodesbetweenwhichthecurrentflows.Accordingly,themeasurementsareaffectedbythebedlocatedatthelevelofthedevice,themudcolumn,and,ifthebedisthin,theformationsaboveandbelowit.Therelativeeffectofthemudcolumnandoftheadjacentformationsdependsuponthephysicalconfigurationoftheelectrodesystemandthegeometryandtherespectiveresistivitiesofthemediainvolvedinthemeasurements.Informationsofmoderateresistivities,suchassandsandshales,thiseffectisnottooimportanttheconventionaldevicesaregenerallysuitableforacorrectinvestigationexceptwhenthebedsareverythin.Theresultsareusuallynotsogoodinhard-rockcountrieswheretheresistivitycontrastsbetweendifferentbedsandbetweentheformationsandthemudarehigh.
Thedevicesthathavefocusingsystems,suchastheLaterolog,forcethecurrenttoflowwithinahorizontalsliceofformationoflimitedthickness.Asaresult,theeffectsofthemudcolumnandoftheformationsaboveandbelowthelimitsofthehorizontalsliceareappreciablyminimizedandmaybenegligibleinmanycases.Thefocusingdevicesarebetterforinvestigatingthinbeds;
theLaterologischieflyappropriatetohardformations,particularlyinhigh-salinitymuds.
Microdevies
Thesepad-typedeviseinvestigateaverysmallvolumeofmaterial(afewcubicinches)behindandclosedtothewalloftheborehole,andtheeffectofthemudcolumnonthemeasurementispracticallyeliminated.Thedelineationofthebedsis,accordingly,muchmoreaccurateandmoredetailedthanwiththemacrodevices,eventhoseusingfocusingsystems.
WiththeMicroLogsystem,themeasurementsarequiteaffectedbythemudcakes,whichmakesitpossibletodetectpermeablebedsanddefinetheirboundariesexactly.
TheMicrolaterologtoolhasafocusingsystemthatminimizestheeffectofthemudcake,oreveneliminatesitwhenthemudcakesarethinenough.Undertheseconditionsthemeasurementsgivevaluesclosetotheresistivitiesoftheformationsflushedbymudfiltratebehindthewallofthehole.Thisprovidesanapproachtodeterminetheformationfactorand,therefore,porosity.
Whentheporosityoftheformationissufficientlyhigh,theformationfactorandtheporositycanalsobedeterminedbyusingMicroLogcuresandappropriatecorrectioncharts.
FORMATIONRESISTIVITIES
Theresistivitiesoftheformationsarcimportantcluestotheirprobablelithologyandfluidcontent.Earthformationsconductelectriccurrentonlybymeansofmineralizedwatertheycontain.Themineralsthatconstitutethesolidpartsofthestrataareinsulatorswhenabsolutelydry.Inasimilar.manner,anypureoilorgasintheformationsiselectricallynonconductive.Thefewexceptionstothisaremetallicsulfides,graphite,etc.,whichconductelectricitylikemetals.
Formationsoflowestresistivityhaveincreasesinbothamountofwateranditsmineralization.Otherimportantfactorinformationresistivityaretheshapeandtheinterconnectionoftheporespacesoccupiedbythewater.Thesedependprimarilyonthelithologyoftheformation.
UnitofResistivity
Laterologsandelectricallogsrecordtheformationresistivity,andtheunitistheohm-meter(ohm-m).Whenauniformelectricalcurrentissentthrougha1-mcubeinadirectionparalleltoanyedge,theresistanceinohmsisequaltotheresistivityofthesubstance.Thesameunitisalsocommonlydesignatedohm-metersquaredpermeter(ohm-m2/m),whichgivesagoodpracticalrangeofnumericalvaluesfortheusualresistivitiesencounteredintheearth,from1ohm-morlesstoseveralthousand.
RelationofWaterResistivitytoSalinityandTemperature
Theresistivityofaformationdependsupontheresistivityofthewatercontainedinitspores,which,inturn,isafunctionoftheconcentrationofaformationwaterquiteoftenhastobededucedfromaknowledgeofitssaltcontent.Also,mostinterpretationsinvolvecomparisonofdifferentresistivitymeasurements,suchastheresistivitiesofthedrillingmudandoftheformationwater.Inmakingsuchdeterminations,thefollowinglawsofelectrolyticconductanceshouldbekeptinmind.
(1)Theconductivityofanelectrolyteincreasesinproportiontotheamountofchemicalsinsolution;
theresistivitydecreaseswithincreasingsalinity.Formationwatersanddrillingmudsusuallycontainseveralchemicalsinsolutionthatdifferintheirabilitytoconductelectricalcurrent.Sincethepredominantchemicalissodiumchloride(NaCl),theresistivitygraphforNaClsolutions(Fig.5)canbeusedconvertsalinityintoresistivity.Ifthewatercontainslargeamountsofotherchemicals,theamountsofthevariouselectrolytespresentmustbeconvertedtotheirequivalentweightsofNaCl.TheresultingequivalentconcentrationisexpressedinpartspermillionorgrainspergallonofNaCl.
(2)Theresistivityofanelectrolytedecreasesasitstemperatureincrease.Thisisofgreatimportanceinelectricalloggingbecausethetemperatureintheearthincreasewithdepth.Itisfrequentlynecessarytocomparetheresistivityofadrillingmudmeasuredatsurfacetemperaturetothatofaformationmeasuredatamuchhighertemperatureinadeepborehole.Thetwovaluesofresistivitycanbecomparedonlyaftertheyhavebeenconvertedtovaluesthatwouldhavebeenobservedatacommontemperature.
ThisconversioncanbemadebyusingthechartinFig.5.Forexample,ifthedrillingmudhadaresistivityof1ohm-mat
F,thechartgivesitscorrectedresistivityas0.5ohm-mat
F.
Itiscommonpracticeinwirelineloggingtomeasurethebottomholetemperature(BHT)withamaximumthermometercontainedinthebodyofthesonde.(Thevaluereadonthethermometergivesthemaximumtemperatureofthemud,whichidgenerallyatthebottomofthehole.)
Incaseathermometerisnotavailable,theBHTcanapproximatedbyusingoneofthetwofollowingequations.Thesewereestablishedempiricallyfromtheaverageofnumerousmeasurementsmadeinvariousgeologicalareas.
Forcoastalbelts,suchasLouisiana,TexasGulfCoast,andCalifornia,
(2-1)
Where
T=temperatureindegreesFahrenheit
And
D=depthinfeet.
Fortheinlandarea,suchasmid-continent,RockyMountains,westTexas,Mississippi,andIllinois,
(2-2)
RelationofFormationResistivityToLithology
Theabilityofanearthformationtoconductcurrentisdirectlyaffectedbytheamountofwaterinthepores:
i.e.,bytheporosityoftheformation.Ifthevoidsinsidetheformationwereparallelcylindricalchannels,theresistivityoftheformationwouldbeinverselyproportionaltotheporosity.Becauseoftherelativesizesandshapesofthemineralgrainsandthepresenceofcementingmateria1,anelectricalcurrentcrossingtheformationhastoflowalongmultipletortuouspaths.Thecrosssectionsofthesepathsvaryrapidlyformcomparativelargevaluesintheporestoverysmallvaluesintheintervalslinkingthepores.Theformati
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