FIELD EXPERIENCE OF NMR LOGGING THROUGH FIBERREINFORCED PLASTIC CASING IN AN EOR OBSERVATION WELL

摘要

A method for measuring the spatial and temporal changein a reservoir resulting from injecting an enhanced oilrecovery (EOR) agent is time-lapse saturation logging ofan observation well positioned between the injectors andproducers. The saturation logs reveal the conformance ofthe EOR flood vertically across the reservoir.Conventional observation wells are completed incemented steel casing; however, this limits the choice oflogging measurements that can be used. If fiberreinforcedplastic (FRP) casing is used, then it is possibleto log nuclear magnetic resonance (NMR), which is afluid-volume-sensitive measurement that can beobtained only if the casing is nonconductive andnonmagnetic. This measurement is necessary if the EORprocess makes it difficult to interpret the response of theconventional saturation logs. An EOR fluid that changesthe aqueous phase salinity by an unknown amount willmake the interpretation of resistivity and sigma logsuncertain, and one that changes the wettability will alsointroduce errors in the interpretation of a resistivity log.For certain medium-viscosity oils in porous formationsNMR-monitored corefloods have shown that thismeasurement can resolve remaining oil saturations to aprecision of 5 saturation units. An advantage of NMR formonitoring EOR is the possibility of using a commonphysics of measurement at multiple scales.In this EOR project, laboratory corefloods, a single pilotwell, and the observation well logging were allmonitored with NMR. Laboratory experiments wereconducted on a low-field bench-top NMR magnet with afluid injection sequence that matched the EOR scheme.In-situ measurements of the oil and brine saturationsduring the coreflood were conducted by using diffusioneditingprotocols and by relaxation measurements aloneand were in quantitative agreement with gravimetricassays of recovered oil. This was followed by a loginject-log single-well in-situ evaluation, in which thealkaline surfactant polymer (ASP) fluid was transporteddownhole in a sample chamber and injected into theformation through a pencil-sized hole. Boreholeelectrical images, dielectric and high-resolution NMRlogs were recorded before and after fluid injection. Thefinal stage of EOR screening was a multiwell pilotprogram that included FRP casing in the observationwell designed for NMR logging. The NMR tool employsa fixed magnetic-field gradient and operates at multiplefrequencies and thus multiple depths of investigationfrom the casing wall. The different frequencies createthin “shells” of sample volume. The deeper shellsmeasure entirely within the formation, which is beyondthe casing and cement. The NMR logs recorded in theobservation well prior to injection of the ASP agreedwith the NMR logs at the waterflood stage of thelaboratory and single-well pilot results. The results alsoshowed that computed saturation is repeatable to within5-s.u. or less.