Dielectric-Independent 2-MHz Propagation Resistivities

Abstract

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.

Prediction and Early Detection of Karsts—An Overview of Methods and Technologies for Safer Drilling in Carbonates

“…The coil-type transmitter emits an electromagnetic field at certain frequencies. Generally, for LWD tools, this is 2 MHz and 400 kHz [48]. Receiver coils are placed in the electromagnetic field and voltage is induced to each of the receivers as the transmitter fires.…”

“…Once the phase shift and amplitude are registered, two rock properties can be estimated-namely attenuation and phase shift resistivities. To convert raw attenuation and phase shift measurements, the relative dielectric constant is approximated to a constant function of resistivity, obtained from empirical testing of hundreds of core samples [48].…”

A Retrievable and Reseatable Propagation Resistivity Tool for Logging While Drilling and Logging While Tripping

“…The same relationship between dielectric constants and formation resistivities that is used with other propagation tools is used here. 7 The resistivity transform together with the raw signal amplitudes and intrinsic random noise of the receiver determine the resistivity measurement range. The measurement error caused by this random noise dominates measurement error at each end of the range.…”

Real-Time Detection of Karstification Hazards While Drilling in Carbonates

“…For example, borehole acoustic reflected survey (BARS) [21], and ultradeep resistivity tools [22] are among the most promising. Although effective in certain cases [23,24], the main drawback of LWD tools is that the vast majority of them provide look-around measurements (also usually at a great distance from the bit) and do not look ahead of the bit. Thus, the area ahead of the bit cannot be investigated by direct measurements.…”