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1、PetrophysicsMScCourseNotesElectricalLogs19.ELECTRICALLOGGING19.1IntroductionElectricallogsareperhapsthemostimportanttoolsavailabletoapetrophysicist.Thisisbecausetheyprovideamethodforcalculatingthewatersaturation,uponwhichcalculationsofSTOOIParebased.Theywerealsosomeofhe
2、firstlogstobeused,withMarcelandConradSchlumbergertestingoutanelectricallogforthefirsttimein1927inthePechelbronnfield,France.Thesefirstmeasurementswerecontinuousrecordingsusing2or3electrodesandadirectcurrent.Itwasdiscoveredthathighqualityrecordingsofapparentresistivityco
3、uldbeobtainedunderfavourableconditionsofsmalldiameterboreholes,highmudresistivitiesandshallowinvasioninthickreservoirs.Theseearlytoolsarecalledelectricloggingtools.Thedevelopmentofelectricaltoolshashenceforwardbeenintense.Therearenowtoolsthatcancopewithextremelyhighlyre
4、sistivemuds(oil-basedmudsorgasastheboreholefluid),whichrelyuponelectromagneticcouplingandaninducedalternatingcurrent(inductionlogs).Theinductionlogactuallymeasuresconductivity,andhenceissometimescalledtheconductivitylog.Themoderntoolformeasuringresistivityinhighsalinity
5、(lowresistivity)mudsisthelaterolog,whichfocusesitscurrentintoathinsheettoimproveverticalresolutionandpenetrationdepth.Thelaterologsmeasureresistivityintheconventionalsense,andareusuallyreferredtoasresistivitytools.Boththeinductionlogsandthelaterologscomeindifferenttypes
6、,whicharesensitivetodifferentdepthsofpenetrationintotheborehole.Henceresistivitydeterminationsfortheinvaded,partlyinvadedandundisturbedrockzonescanbemeasured.Inaddition,thereisarangeofsmallerelectricaldevices(micro-resistivitytools),whicharedesignedtomeasuretheresistivi
7、tyofmudcake.TherearealsoArrayLogs,whicharestateofthearttools,andelectricalmeasurementsareusedathighresolutions(smallscale)toimagetheinterioroftheboreholeelectrically.19.2PrincipleUsesofElectricalLogsThemainuseoftheelectricaltoolsistocalculatethewatersaturationofareservo
8、irformation,andhencetheSTOOIP.Chapter17coveredmostoftheimportanttheoryforthisapplication,andChapter1introduced
