Azimuthal gamma ray optimizes shale development

For the thousands of shale gas wells drilled in the US, drillers have typically deployed an MWD gamma-ray tool as the primary and often only logging technology. While the data obtained from these logs provide information on both lithology and the general location of the drill bit, design limitations of conventional gamma-ray tools leave a sizable portion of additional information trapped in the reservoir.

FIGURE 1. The SpectralWave gamma-ray tool is placed outside of the collar for improved sampling. (Images courtesy of Weatherford)

Many gamma-ray logging sensors use sonde-based tools, in which the gamma detector is housed in the center of a logging collar. In this configuration formation gamma rays must travel through the mud in the annulus, the bore, and the drill collar. Because drilling mud weighting material and the collar absorb formation gamma rays, these tools generally have poor statistical accuracy and are limited to total gamma-ray measurements, which impairs both real-time interpretations for wellbore positioning and post-well petrophysical evaluation.

Broadening the spectrum

The fact that many drillers are already accustomed to running gamma-ray logs in unconventional reservoirs prompted Weatherford to investigate whether a more advanced gamma-ray tool could be developed to sharpen the view of the subsurface. The result is the LWD SpectralWave tool (Figure 1) that provides more information, including real-time and recorded spectral gamma-ray measurements and azimuthal data for high-quality imaging.

The sensor uses large scintillation detectors containing 1.5-in. by 8-in. gain-stabilized sodium iodide detector crystals. Unlike most gamma-ray tools, the technology has its detectors mounted on the outside of the drill collar, which significantly reduces the attenuating effect of the collar material on naturally occurring radiation.

FIGURE 2. This log example shows imaging data from the SpectralWave tool vs. a standard gamma-ray logging tool.

The outer mounting of the detectors also affords a wider range of measurements. In addition to total gamma ray, the tool acquires spectral gamma-ray data – particularly potassium (K), uranium (U), and thorium (Th) – of the in situ formation. The K and Th values provide valuable insight into the clay content of the shale reservoirs and are typically found in calcareous and silty zones. Knowledge of the clay content allows one to estimate the brittleness (the receptiveness to hydraulic fracturing) of different parts of the reservoir. The presence and concentration of U correlates strongly with the total organic carbon (TOC) in the reservoir. Conventional natural gamma-ray logs are not adequate to identify U-rich zones, making accurate evaluation of the organic content of the shale difficult.

The tool also incorporates X-Y magnetometers to track the azimuthal position of the detectors as the tool rotates. This feature allows for measurement in four quadrants (up, down, left, and right) and acquisition of 16-bin azimuthal total gamma-ray data for real-time and recorded borehole imaging. The enhanced imaging – at count rates that are up to 50 times higher than a standard LWD gamma ray – allows for more accurate dip determination, improved bedding plane identification, and geosteering capabilities.

The tool is currently available in 4?-in. and 6?-in. sizes, with the smaller diameter tool employing a single scintillation detector and the larger tool employing three detectors positioned 120° from each other. Both tools are approximately 4 m (13 ft) long; rated to 150°C (301°F); and contain their own downhole processor, memory, and self-contained power supply.

Field deployments show promise

The tool has been run in many unconventional reservoirs in the US, including the Marcellus, Bakken, Niobrara, and Eagle Ford shale reservoirs, where it enhanced log interpretations by providing more accurate total gamma-ray measurements and improved imaging capability, dip-picking solutions, and TOC analysis.

The technology provides a higher resolution image than standard gamma-ray tools, as Figure 2 illustrates. This log, from a horizontal well in the Eagle Ford shale in South Texas, compares results from the tool with a standard gamma-ray type tool (denoted HAGR, for high-temperature azimuthal gamma ray). The data illustrate how the mechanical configuration, along with the high accuracy of the sensors, provides a superior gamma-ray image in real time while drilling (Dynamic Image, Track 5). The superior gamma-ray image also is a result of the standard tool having an 8-bin image, which is not as azimuthally focused as the SpectralWave due to collar shielding. The SpectralWave sensor also has more precise vertical resolution than a standard gamma-ray tool due to detector configuration. Geosteering and subsequent bedding dip determination are enhanced by a superior gamma-ray image, and the SpectralWave image removes some of the ambiguity associated with bed analysis and dip-picking interpretation.

Ultimately, the depth of information provided by these types of LWD tools helps operators optimize the economics of shale well development by keeping the wellbore within the targeted formation interval and positioning fracturing stages to maximize production rates and recovery.