Advanced MWD improves wellbore positioning in Wolfcamp formation

The industry’s understanding of unconventional reservoir geology is still evolving, which makes precise well placement in shale and tight formations more difficult than in conventional reservoirs. The challenge is to provide the measurements and services to cost-effectively position the wellbore in the most productive portion of an unconventional reservoir. Anticipating the need for improved placement techniques, including geosteering in unconventional formations, led to the development of the AccuSteer Measurement Suite, Fig 1.  

Fig. 1. A new MWD system called AccuSteer, improves wellbore positioning in unconventional reservoirs.

Designed specifically for the challenge of drilling horizontal wells in unconventional reservoirs, the system provides all the measurements required to address drilling efficiency, directional drilling, geosteering, and fracture program design. The system has been used extensively in the Permian basin. 

The authors will present case studies that document how the new tool has brought visibility to the complexities involved with proper wellbore placement. Field experience has demonstrated that quality azimuthal gamma images and continuous inclination measurements in real time can be instrumental to achieving accurate well-
bore placement. 

GEOLOGY REVIEW 

The Wolfcamp formation is a siliceous mud source rock and is characterized by varying layers of softer, fast drilling, clay rich rock (high gamma) and bounded by harder, slower drilling (low gamma) formations. Regional dips are typically within a couple of degrees of horizontal–close to parallel with the wellbore, Fig. 2. 

Fig. 2. Image shows the highly stratified nature of the Wolfcamp lithology.

 
This scenario is ideal for demonstrating the advantages of geosteering with azimuthal gamma images and continuous inclination compared to geosteering with 90-ft surveys and bulk gamma measurements. Furthermore, it has been shown that, without the availability
of these measurements in real time, it is extremely difficult to identify precisely the wellbore position within the reservoir, and the perceived position is frequently incorrect.   

LOG INTERPRETATION

The basis of azimuthal gamma image log interpretation is simplified by using the smile and frown analogy, Fig 3. The smile indicates drilling up geological section, and a frown indicates drilling down geological section. 

Fig. 3. The basics of image log interpretation.

 
WOLFCAMP EXAMPLES

The following examples from a recently drilled horizontal well in the Wolfcamp demonstrate how difficult it is to accurately position the wellbore without adequate data. Figure 4 shows the vertical sequence of bedding within the zone, known as a “type log,” with the high gamma lobes marked from 1–3. The red, yellow, and blue colors are used to represent the gamma magnitude of the bedding planes within the zone, where the warmer colors are high gamma values and colder colors are low gamma values.

Fig. 4. Vertical sequencing in a type log.

 
In Figure 5, the available data is presented from a standard MWD gamma system. In this example, 90-ft surveys and a total gamma curve show three gamma peaks in the first 500 ft and three peaks in the second 500 ft. The variation in the magnitude of the gamma peaks is due to lateral changes within the zone.  The challenge is to accurately correlate the horizontally logged data to the type log and determine the correct stratigraphic position. 

Fig. 5. Data in a 1,000-ft horizontal section from a conventional MWD system with 90-ft inclination surveys and bulk gamma readings.

 
To make the interpretation, an assumption must be made about bed dip angle. In this case, geosteering engineers used 89.0° and this figure is represented by a thin red line.  Using that assumption, the following logic would typically be used to determine which lobe on the type log the well is currently drilling within. 

• If survey inclination is less than bed dip (89°) → wellbore is moving down the geological section (downwards on the type log).
• If the survey inclination is greater than bed dip (89°) → wellbore is moving up the geological section (upwards on the type log).

Given this logic, assumptions, and the available data, the geosteering engineer would likely interpret the gamma data to show moving down and then back up through all of the lobes, putting the wellbore back at the starting position just above lobe #1 on the type log,
as demonstrated by the numbering in Fig. 5. 

However, in Fig. 6, the interval from the same well is shown, but with the advantage of the high-resolution azimuthal gamma imaging and continuous inclination data. The continuous inclination shows character not captured in the 90-ft surveys, and the azimuthal gamma image shows that the last two gamma peaks are actually the same bed, completely changing the interpretation from that utilizing the data from the standard MWD system.  

Fig. 6. The same interval as shown in Fig. 5, but with high-resolution gamma imaging and continuous inclination readings.

 
Finally, Fig. 7 demonstrates the difference in final wellbore positon interpretation from the previous two examples. It demonstrates that the position interpretation with the standard MWD measurements resulted in going out of the top of the zone, when actually the wellbore is stratigraphically lower as depicted by the white line, which represents interpretation using azimuth gamma and continuous inclination. Given that these figures highlight only 1,000 ft of horizontal wellbore, it is clear these errors can accumulate over the course of a 10,000-ft lateral well and lead to significant misinterpretation of wellbore position. 

Fig. 7. Difference in well position interpretation between standard MWD system (in black) and one with azimuthal gamma imaging and continuous inclination measurements (in white).

CONCLUSION  

Although there is still much to be learned, the importance of precise wellbore placement within an unconventional reservoir is proving to be critical to the economic success of projects. Significant advantages can be realized by adopting workflows that use continuous survey measurements and azimuthal gamma imaging data.