Design And Basic Research On The Pad-mounted Multi-frequency Electromagnetic Well Logging

The most common well logging method based on rock electricity isresistivity logging.The resistivity logging measures the difference ofelectric conductivity, which requires a certain degree of salinity. Comparedwith resistivity logging method, electromagnetic well logging measurement isbased on the difference of dielectric property, which can measure formation’sconductivity and permittivity simultaneously around borehole, and do notrequire high salinity of formation water. Electromagnetic well logging canbe used in borehole with oil based mud and glassteel casing pipe, water layerwith high resistivity and oil layer with low resistivity, and water floodedlayer, which aloso has significant advantages over normal resistivity loggingin secondary recovery of crude oil and enhanced oil recovery.Electromagneticwell logging is gaining more and more attention, which stimulates thedevelopment of electromagnetic tool.There are many instruments with variablefrequency and variable antenna array, because of single operating frequencymost of them can not widely used in industry. The common electromagnetic welllogging tools are invented by America. The classification of electromagneticwell logging tools is made up of sigle frequency logging tools, dual-frequencytools and multi-frequency tools. The most representative instruments we cansee are EPT (operating frequency is at1.1GHz) and DPT (operating frequencyis at25MHz) of Schlumberger. EPT mainly measures dielectric properties ofthe flushed zone as the depth of investigation of such measurement isrelatively shallow. The measurements of DPT are influenced by mud and formationresistivity, then DPT can investigate the dielectric properties of virginformation when mud and formation resistivity meet certain conditions.Subsequently, double frequency dielectric instrument was invented. Onerepresentative of this instrument is Atlas dual frequency dielectric tool withoperating frequency at47MHz and200MHz. The apparent dielectric dispersioncauses the difficulty in interpretation. In recent years, multi-frequencyelectromagnetic tool were manufactured with wide band frequency, which canmeasure formation’s conductivity and permittivity simultaneously atdifferent depths around borehole, is fit for oil reservoir detection.Schlumberger’s Dielectric Scanner instrument is a representative machine,which have exhibited huge advantage and satisfied the requirement of industry,so that the application foreground is broad. Dielectric Scanner (ADT), whichworks at four discrete frequencies from20MHz up to1GHz, can allow a continuousmeasurement of the dispersion and muti-spacings high frequency measurements,moreover, transmitter and receiver antennas have collocated longitudinal and transverse polarizations. In practice their interpretation model considersthe effects of rock texture, the presence of clays and so on; people can getpore-fluid analysis and carbonate oil reservoir analysis.Our country has made great progress in single-frequency and dual-frequencyelectromagnetic well logging tool area. However, in respect of multi-frequencyelectromagnetic well logging, we are still in the initial stage. Therefore,people should pay more attention to and strengthen theoretical research ofmulti-frequency electromagnetic well logging instrument. Numerical simulationand inversion interpretation of multi-frequency electromagnetic well loggingare also especially important. In this paper, combining with the new type ofmulti-frequency electromagnetic well logging instrument, we design a new devicewith articulated pad. And a multi-spacing measurement with two transmitters andeight receivers is performed to minimize near well-bore effects. The antennasare mounted on a fully articulated short pad allowing optimal pad contact evenin rough boreholes.First,three Dimension Finite Difference Time Domain (3D FDTD) method withsubgridding scheme is used to simulate electromagnetic well propagating inthis model, which is the critical technology of forward modeling in this articl.Being limited by the small size of antenna unit and in order to ensure thecalculation efficiency at same time, we separate the whole domain into corsegrid domain and fine grid domain. The grid refining factor must be odd, antennaunit and other fine structure will be calculated in the fine grid domain, otherhomogeneous stratigraphic area using coarse grid. That not only meets theaccuracy of the simulation, and also can save computer memory and CPU executiontime.Second, the antenna in multi-frequency electromagnetic well logging toolis an important part.It plays the role of transmitting and receivingelectromagnetic wave. Because of the limitation of borehole’s diameter,electromagnetic well logging instrument can not be too large. Combined withthe development of the resonator antenna technology currently, this paper putsforward using dielectric resonator antenna as sending and receiving antenna,the outshoot characteristics of dielectric resonator antenna are small size,low electromagnetic radiation loss. Then we studied the influence of receivedsignal affected by resonant cavity size.Third, the logging data is influenced by downhole measurement environment.The analysis of influence factors can guide people avoid the influence ofunfavorable factors effectively in field logging. Combining with the actualsituation, this paper mainly studies the influence of borehole factors on thedetection results. Under the homogeneous formation and complex condition, westudy the signal changes at the four discrete frequencies, including:1) togain the change of amplitude ratio and phase difference under the situationof borehole diameter changes, rough borehole, holes in borehole wall;2) theinfluence on detecting companies with mud conductivity and permittivitychanges;3) the influence on measurement from the thickness of mud cake. Fourth, detection depth is one of the most important aspects in logging.We studied the detecting depth of the antenna array, and it could reach50cm from the amplitude ratio and phase difference at four frequencies.Fifth, we give the charts converting the amplitude ratio and the phasedifference to the conductivity and relative permittivity at four frequenciesunder two kinds of mud properties. Namely under the condition of this kindof mud, we can obtain formation’s conductivity, permittivity according tothe measured amplitude ratio and phase difference in conversion chart. Dueto the dispersion phenomenon, the dependence from dielectric constant andconductivity to the amplitude ratio and phase difference will change withfrequency.Sixth, there are high requirement for formation layer definition demand,especially for thin oil/water bed layer. This paper chiefly simulated the twolayer model of oil/water bed, sand shale alternating layer, so that the bedresolution on thin layer of the new type pad-mounted multi-frequencyelectromagnetic well logging can be obtained under different frequencies andspacing. In addition, a vertebral formation model with bed thickness anddifferent resistivity-Oklahoma model has been simulated, in order to testthe bed resolution of the new device. Studies show that if we focus on layeredeffect, amplitude ratio and phase difference data fully meet the requirements.The sensitivity of formation with low resisitivity became high along withfrequency increases. The horns appearing at formation interface is mainly dueto the amplitude ratio corresponding to dielectric constant, which is affectedby transmitting frequency simultaneously. The appearance of horn is affectedby frequencies. This instrument can identify the thin layer under1inche.