The compensated dual resistivity (CDRSM) tool is an electromagnetic propagation tool for measurement while drilling. The CDR tool provides two resistivity measurements with several novel features that are verified with theoretical modeling, test-tank experiments, and log examples. IntroductionThe CDR tool 1 is a 2 ~ 106-cycles/sec electromagnetic propagation tool 2 . 3 built into a drill collar. This drill collar is fully selfcontained and has rugged sensors and electronics. The CDR tool is borehole-compensated, requiring two transmitters and two receivers. The transmitters alternately broadcast electromagnetic waves, and the phase shifts and attenuations are measured between the receivers and averaged. Phase shift is transformed into a shallow measurement, R ps ' and attenuation is transformed into a deep measurement, Rad' The CDR tool has several new and important features. I. Rad and Rps provide two depths of investigation and are used to detect invasion while drilling. For example, in a I-a· m formation, the investigation diameters (50% response) are 30 in. for Rps and 50 in. for Rad' 2. ROil and Rps detect beds as thin as 6 in.; however, these measurements are affected differently by shoulder-bed resistivities and both require corrections in thin resistive beds. Rps has a better vertical response than Rad' Rad and Rps cross over at the horizontal bed boundaries; this crossover can be used to measure bed thickness.3. Both Rad and Rps are insensitive to hole size and mud resistivity in smooth boreholes. Borehole corrections are very small even for contrasts of 100: I between formation and mud resistivities. Rugose holes and salty muds together, however, can cause larger errors than indicated by the borehole-correction charts. In these conditions, borehole compensation is essential for an accurate measurement.An extensive research program was conducted to verif'y these features and to ensure that the CDR tool provides a high-quality log. To achieve wireline quality, the CDR tool's physics was studied thoroughly, and its environmental effects were modeled and experimentaly measured. Two theoretical models are used for the CDR tool. The first model treats the tool geometry in detail but assumes a homogeneous medium outside the tool. This model is verified by test-tank experiments and by air measurements. The second model assumes a simplified tool geometry but treats boreholes, caves, beds, and invasion in detail. This model is used to study environmental effects and to prepare correction charts. Experiments with artificial boreholes, caves, step-profile invasion, and horizontal bed boundaries verif'y the predictions of the second model. Finally, CDR logs are compared to wireline logs to demonstrate the new features.
To address the increasing number of slim holes being drilled in the range of 5.75 in. to 6.75 in., a mixed-boreholecompensated (MERC) 2-MHz array resistivity tool with a 4.75-in. collar diameter is described for resistivity measurement-while-drilling (MWD) applications. MERC is a unique technique developed to minimize the effects of borehole rugosity. The new MWD array resistivity tool makes multiple MERC phase shift and attenuation resistivity measurements using five transmitter-to-receiver spacings.Five independent MERC phase shift and attenuation measurements are made at 2 MHz. The five phase shift resistivities have similar vertical responses but with increasing radial depths of investigation. The five attenuation resistivities are also vertically similar but coarser and are radially deeper than the phase shift resistivities. This new array resistivity tool effectively exploits the technology and the interpretation methodology recently developed for wireline induction resistivities and the benefits of multispacing probes for formation evaluation. Most of the petrophysical applications developed in conjunction with the wireline resistivity AlT· Array Induction Imager Tool are illustrated from MWD array resistivity logs acquired while drilling.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractAll commercial 2-MHz and 400-kHz propagation resistivity logging-while-drilling (LWD) tools use an assumed value or relationship for dielectric permittivity to derive independent resistivities from phase shift and attenuation measurements. This methodology has been used for over ten years. For resistivities below 100 Ω-m, the phase shift resistivity is not strongly affected by the assumed value of dielectric constant. In contrast, the attenuation resistivity is sensitive to the assumed dielectric constant above 10 Ω-m, and extremely sensitive above 50 Ω-m. A new empirical relationship between relative dielectric constant and resistivity is proposed to improve the phase shift and attenuation resistivities above 100 Ω-m. However, in high resistivity formations, and in unusual lithologies with exceptionally large dielectric permittivities, it is better to invert the measured phase shifts and attenuations together to obtain an apparent resistivity that is independent of the dielectric constant. This "dielectricindependent" resistivity will be free of possible errors caused by assuming a wrong value for dielectric permittivity. Even though phase shift and attenuation resistivities exhibit different vertical and radial response characteristics, simulations show that bed boundaries and moderate invasion do not adversely affect the computation of the dielectricindependent resistivity. In oil based mud, it is possible to read resistivities above 1000 Ω-m. This provides the ability to differentiate among high resistivity formations, but not necessarily the ability to measure a quantitative value. Three field examples illustrate the strengths and limitations of the dielectric-independent resistivity.
The multiple phase shift and attenuation resistivities from a 2-MHz array resistivity tool provide the log analyst with the measurements needed to resolve complex interpretation problems associated with high relative dip formations typical in horizontal and highly deviated wells. With the support of modeling, productive zones can be located that may otherwise be missed.In high relative dip formations, the interpretation of multispacing 2-MHz resistivity tools and the comparison with wireline logs are not straightforward. Therefore, it is imperative for the log analyst to understand the unique response characteristics of these measurements in both vertical and horizontal wells. In high relative dip formations, the primary factors that may cause the resistivity curves of different spacings to separate are borehole effects, invasion, bed boundary proximity and anisotropy. With the help of forward modeling codes and inversion algorithms, the log analyst can resolve most of the interpretation ambiguities.The responses of a 2-MHz array resistivity tool, with five measurement spacings, to the above factors are illustrated and discussed. Traditional rules of thumb based on vertical well must be modified significantly. Productive zones that would be missed by logs made in a vertical well can easily be detected by logs made in a high-angle or horizontal well through the same formation. A comparison of the resistivity responses of a 2-MHz tool in low and high relative dip formations helps explain the differences between logs made in low-angle and high-angle wells.
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