Conventional LWD propagation resistivity tools have proved useful in helping to land and maintain well bores within hydrocarbon reservoirs by providing resistivity measurement of

Figure 1. The ADR is a single collar with six transmitters and three tilted receivers. (All figures courtesy of Halliburton Sperry Drilling Services)
the formation being drilled. Resistivity measurement accuracy has steadily improved, but even the latest-generation LWD tools lack azimuthal sensitivity that provides directional information and data necessary for geosteering and evaluating complex reservoirs.
New LWD technology with an inherently “azimuthally sensitive” design that provides both directional geosteering and formation evaluation while drilling. Using tilted-receiver antenna technology, the Azimuthal Deep-Reading Resistivity tool (ADR) combines compensated formation resistivity and directional measurement.

The tool is a single 25-ft (7.6-m) collar with six transmitters and three receivers in various
Figure 2. Computed response of a traditional wave propagation tool. The top graph shows the well trajectory, and the bottom graph shows the traditional wave propagation resistivity response.
spacing configurations. The tilted antenna configurations produce directionally sensitive measurements and have greater sensitivity to formation anisotropy, which can help determine horizontal and vertical resistivity, dip angle and azimuth with increased accuracy and reliability.

Algorithms developed for this tool are used to calculate formation anisotropy, as well as dipping angle and azimuth, providing more robust estimates of formation parameters. The shoulder-bed effect is automatically corrected, resulting in a more accurate determination of formation resistivity and anisotropy. The tool offers more complete information for evaluating deviated and horizontal wells by providing a directional mapping of the formation parameter around the borehole at different depths of investigation.

Sensor characteristics
The ADR transmitter and receiver array (Figure 1) uses multiple spacings and frequencies to cover the entire range from shallow- to very deep-reading, and generate multiple resistivity images for fracture and fault direction, dip direction and angle, and advanced invasion profiling and anisotropy estimation.

The ability to acquire azimuthal resistivity measurements at different depths of investigation extending from several inches from the borehole wall to several feet into the formation allows detailed, three-dimensional characterization of the near-wellbore environment.
The ADR capitalizes on the high frequency and short spacing to map the near-wellbore properties. The longer spacing and lower frequencies are used to measure the formation properties of the uninvaded zone.

With operating frequencies of 2 MHz, 500 kHz, and 125 kHz, the ADR retains the advantages of high frequency data — such as greater accuracy in high resistivities and better vertical resolution — while gaining the advantages of the lower frequency measurements, including greater depths of investigation, thus mapping the formation parameters near the borehole up to 18 ft (5.5 m) deep. Obtaining such comprehensive multifrequency data at multiple spacings generates a better picture of the reservoir that allows more advanced interpretations of anisotropy and complex bedding/invasion conditions.

Geosteering and well placement
Figure 2 shows a computed response of a traditional LWD wave propagation tool in a payzone of 20 ohm-m sandwiched between two conductive shale beds of 1 ohm-m. The top graph shows the well trajectory, and the bottom graph shows the corresponding tool response. As the tool approaches the bottom conductive shale, it starts to read lower resistivity, and as the tool approaches the top conductive shale, it also starts to read a lower resistivity. That is, the tool reading is the same whether it approaches the conductive shale from the top or bottom. This is due to the lack of azimuthal sensitivities, thus making geosteering in the payzone unpredictable and uncertain.

Figure 3 shows a computed response of the ADR tool. The top graph shows the well trajectory; the middle graph shows the high-side and low-side resistivites, and the bottom
Figure 3. Computed response of the ADR tool. The top graph shows the well trajectory, the middle graph shows the high-side and low-side resistivities, and the bottom graph shows the directional geosteering signal.
graph shows the directional geosteering signal. The geosteering signal is a difference between measurements determined at opposite azimuthal orientations of the tool.
Note that as the tool approaches the bottom conductive shale, the bottom quadrant resistivity reads a lower resistivity, indicating that the tool is approaching a conductive bed from the bottom of the payzone. As the tool approaches the shale from the top of the payzone, the top quadrant resistivity reads a lower resistivity, indicating that the tool is approaching the shale from the top of the payzone.

Azimuthal resistivity and geosteering signals allow for precise geosteering using real-time interpretation of boundary locations and layer resistivities.

The increased depth of investigation allows much earlier detection of bed boundaries and more accurate determination of the drilling direction relative to reservoir boundaries. This ability to “look around” not only reveals in which direction to steer but also allows sufficient time for the driller to make adjustments to ensure optimal wellbore placement.

Because it is an azimuthally sensitive tool, the ADR provides deep-reading resistivity with maximum geosteering flexibility; that is, geosteering is possible using all spacing and frequencies configurations, enabling navigation of even very thin reservoirs.

Advanced formation evaluation
Resistivity measurements can provide indications of hydrocarbon concentrations and other useful information. However, such measurements can exhibit boundary-related artifacts that
Figure 4. Illustrative graph of computed phase resistivity as a function of rotation angle in an anisotropic formation for different signal frequencies.
make interpretation difficult, particularly in situations where the borehole penetrates the formation at an angle. The azimuthal deep reading resistivity tool does not suffer the same problem since so many azimuthal measurements are made around the borehole. Using all the azimuthal measurements, an accurate resistivity log with no artifacts can be produced with minimal processing.

Many reservoirs, especially shale and thinly laminated sand-shale sequences, can exhibit microscopic or macroscopic anisotropy. Microscopic anisotropy manifests itself in the orientation of the clay minerals in the shale. Macroscopic anisotropy can occur in a sequence of lamination of shale and sand.

Traditional propagation tools have some response to anisotropy at high deviation. However, with transmitters and receivers oriented in a non-parallel plane, the ADR exhibits a very good response to anisotropy. Figure 4 shows a modeled response of the ADR tool in an anisotropic formation. Using multispacing and multifrequency azimuthal data, inversion algorithms simultaneously determine horizontal resistivity, Rh, vertical resistivity, Rv, and relative dip angle, corrected for shoulder and invasion effects. These are the essential parameters to accurately estimate hydrocarbons in place.