In the early 1980s, a need by operators for a more cost-effective technology to evaluate thin-bed and non-conventional reservoirs led the industry toward the development of measurement-while-drilling (MWD) resistivity tools. In 1983, NL Sperry-Sun introduced the first MWD propagating-wave resistivity tool to the industry. The original electromagnetic wave resistivity (EWR) tool operated at a 2 megahertz (MHz) transmitter frequency and provided a single transmitter-to-receiver (T-R) spacing and had an inter-receiver spacing of six inches, which provided a formation resistivity measurement with a relatively sharp vertical resolution. Eight years later, in 1991, Sperry-Sun Drilling Services introduced the first multi-spacing propagation resistivity tool, the EWR-Phase 4™ tool. The EWR-Phase 4 tool provides four T-R spacings and transmits a 2 MHz signal from the three shortest spaced transmitters and a 1 MHz signal from the longest spaced transmitter. The additional spacings provide multiple resistivity measurements, each having a unique depth of investigation. These multiple measurements aid in the determination of the actual formation resistivity. The inter-receiver spacing remained six inches, so the excellent vertical resolution was preserved. The EWR-Phase 4 tool has been very reliable from an application viewpoint.

(Click image to enlarge)

The most recent development in this resistivity tool evolution is the multi-frequency EWR-M5™ resistivity sensor. It is a third generation propagation resistivity sensor integrated into a new tool that also provides enhanced gamma ray data along with improved drilling optimization data (vibration/shock and mud pressure data) in a compact design. It is the only MWD resistivity sensor with symmetrical hardware compensation for all five T-R spacings.

The EWR-M5^TM Sensor MWD tool (Click image to enlarge) With pressure measurements (internal and annular) and vibration/shock measurements incorporated into the design along with a 25,000 psi pressure rating, this MWD tool addresses both high-pressure and difficult drilling environments. Geosteering ability has been greatly improved due to deeper reading resistivity data and an azimuthal gamma ray measurement located only 1.18 ft from the bottom of the tool. A gamma ray image is generated from these azimuthal data. Resistivity
In the past, the 2 MHz frequency was utilized to ensure good vertical resolution across a broad operating range. The 2 MHz, 500 kHz, and 250 kHz frequencies allow the advantages of the 2 MHz data (greater accuracy in high resistivities and better vertical resolution) to be preserved while adding the advantages of the lower frequency measurements. Some of these advantages are: greater depth of investigation and elimination of data spiking when the tool is eccentric in a borehole filled with OBM while logging a very low resistivity formation, as well as reduced polarization horns, dielectric effect, and effect of anisotropy.

Compensation Geometry
In the EWR-M5 Sensor, three receivers with six transmitters provide five geometrically compensated resistivity measurements. This unique three-receiver design reduces the number of transmitters required to provide five geometrically compensated measurement spacings. The spacing between R1 and R2 and between R2 and R3 is eight inches. By utilizing five compensated spacings with three different frequencies, and measuring both the phase shift and attenuation of the transmitted signals, the EWR-M5 sensor is capable of providing 30 unique compensated resistivity values. Multi-frequency data at multiple spacings provide more "mathematical leverage" for advanced interpretations, e.g., unraveling dielectric properties, anisotropy, and complex bedding/invasion conditions. Correlating a measurement with its known depth of investigation allows the generation of a radial resistivity profile, which can be displayed as an image. Advantage of Lower Frequency Data
In certain conditions, tool eccentricity can cause spikes in high frequency propagation resistivity data. Transmitting at a lower frequency will mitigate the magnitude of this problem. The log example below demonstrates the improved responses of the 500 kHz and 250 kHz data, as well as the increased depths of investigation of the lower frequency data. The extensive amount of data, from the five T-R spacings and three transmitter frequencies, enables the proper evaluation of the formation resistivity in a much broader range of operating conditions than was possible previously.

(Click image to enlarge)

Log demonstrating the increased depth of investigation and mitigation of data spiking (Click image to enlarge) Vertical Resolution
A benefit of the 2 MHz data is its inherent better vertical resolution. At 14,523 ft. in the log examples below, there is a thin resistive streak on top of a very conductive salt water-bearing zone. While this thin resistive streak is obvious in the 2 MHz data, its amplitude is significantly diminished in the lower frequency phase shift-derived data and it is impossible to resolve in the 500 kHz and 250 kHz attenuation resistivity data (not shown). The excellent vertical resolution of the 2 MHz phase data plays a significant role in the identification and development of thinly bedded reservoirs.
Logs showing the effect of transmitter frequency on vertical resolution (Click image to enlarge)

Anisotropy
The effect of formation anisotropy varies with transmitter-receiver spacing, transmitter frequency, and resistivity type. The graph below illustrates the magnitude of this effect, as a function of the relative dip angle, on several of the EWR-M5 phase shift measurements for a typical ratio of vertical to horizontal resistivity. The additional data provided by the EWR-M5 sensor improves the mathematical leverage available for various types of advanced analysis.
Graph showing the effect of formation anisotropy (Click image to enlarge) Bed Boundary Effects
The bed boundary effect on high frequency propagation resistivity data has been well documented and is utilized in some geosteering applications. Although the presence of a "polarization horn" can provide useful landing and geosteering information, it can cause problems with the determination of the actual formation resistivity. The magnitude of this polarization decreases with decreasing transmitter frequency.


(Click image to enlarge)

Azimuthal Gamma Ray
The new tool incorporates the new Azimuthal Gamma Ray (AGR™) sensor. The AGR sensor consists of two scintillation detectors oriented 180 degrees apart. This hardware redundancy is a carry-forward of the redundant detector design used in the dual gamma ray (DGR) sensor with the additional ability to azimuthally bin gamma ray data from both detectors while the tool is rotating. The AGR sensor provides a 60% increase in count rate; hence, better statistical precision, better data quality in fast-drilling environments, and a gamma ray-based image of the formation. Images derived from both real-time and memory (recorded) data. They can be referenced to either high-side, or magnetic north. Since the AGR sensor is located less than two feet from the bottom of the tool, these data can be useful for geosteering purposes and close-to-the-bit formation evaluation. Application
The deeper reading EWR-M5 sensor enhances the detection of distant bed boundaries; thereby giving an earlier indication of an approaching bed boundry in high angles and horizontal drilling scenarios. Below is an image that demonstrates how a client might use the EWR-M5 sensor to steer away from an approaching boundary. Note that the graphic shows the deepest reading curve detecting an approaching bed about 80 meters prior to the borehole entering the new formation.

(Click image to enlarge) Conclusion
The EWR-M5 resistivity sensor is a third generation propagation resistivity tool that provides enhanced resistivity data.  It has been combined with a new azimuthal gamma ray sensor, a new vibration/shock sensor, and some other sensors in a compact and robust tool design. The tool offers improved resistivity data accuracy and precision, resistivity data with reduced borehole effects, more resistivity data for advanced/complex interpretations, and deeper reading resistivity data. Also, the tool provides a mud resistivity measurement, as well as annular and internal mud pressures, azimuthal gamma ray data for imaging and geosteering. These high quality data along with an expanded downhole memory, a much improved data transfer rate, and a much larger battery provide a high quality, operationally efficient service package. Overall, this integrated MWD tool has been designed to incorporate the optimal combination of leading-edge formation evaluation capabilities and drilling efficiency sensors in a robust, high-pressure tool that addresses the most demanding applications.