by Duncan McLeod, Product Manager and Dag Billger, Business Manager at Inertial Sensing One AB
Running continuous gyro surveys while conducting borehole surveying within mining applications is becoming increasingly popular.
There are multiple reasons for this trend, but chief among them are the following four points.
- Considerably faster survey time when compared to traditional multishot surveys resulting in reduced drilling down time and cost;
- Greater accuracy as reducing overall gyro survey time lessens the impact of inherent gyro drift;
- Ease of use for operators with less button clicking during a survey;
- Potential to generate finer results, yielding greater understanding of the borehole.
Continuous gyro surveys have been used for a long time within the oil and gas industry, for example to perform so-called ‘micro-dogleg’ studies. By changing from traditional multishot surveys with a typical station interval of 50 ft (15 m) or 100 ft (30 m) to continuous surveys with resolution down to as low as 1 ft (0.30 m), it is possible to view the detailed structure of how the borehole bends and curves. Such surveys routinely show how a typical borehole is often much more doglegged than what can be seen at a 100-foot scale. This is made possible through a continuous survey, as attempting a multishot survey at 1 ft (0.30 m) intervals would take a prohibitively long time.
Conducting continuous gyro surveys saves time and money and is therefore attractive to anyone in the drilling sphere. Specific dogleg investigations are less common in mining, although they are still important in directional drilling.
However, it would be a false economy to rely on continuous gyro surveys for all the benefits they can yield if survey results are less accurate or reliable than by using a traditional multishot survey. This article presents findings using Inertial Sensing’s TwinGyro™, demonstrating that accuracy can be maintained in even the most demanding survey applications using continuous surveying. Specifically, the focus was on repeatedly surveying the same hole in two applications:
- A vertical coring project at a mine located in Sweden’s far north.
- A micro-dogleg study of a vertical gas hole with kick-off in Texas.
Figure 1 – Survey equipment. From left to right: Bluetooth wireless depth tracker, TwinGyro™ survey tool and the LiPAD-100™ gyrocompass.
The TwinGyro™ was an excellent platform for these studies, generating two simultaneous yet independent surveys during each run. The independent, continuous surveys not only double data collection, they also reveal the very fine details of the borehole’s true character. The TwinGyro™ is unique in this important respect.
Table 1 – Summary of results from a 512-meter vertical coring hole.
Figure 3 – A Dynapar optical encoder with a serial cable interface to Surveyor™ software mounted onto the winch, just ahead of the cable drum.
It should be noted that these results only apply to gyro survey tools with both a high dynamic and high rate range such as the TwinGyro™ and SlimGyro™. The results don’t apply to tools that measure deflection using optical, strain or other deflectometric principles, as their sensors lack the bandwidth and sensitivity to cope with measuring while moving. Likewise, results from gyro systems that use low dynamic range sensors, or that generate results from simple averaging while moving, don’t yield sound results in more challenging survey conditions.
In order to perform a continuous gyro survey, the survey tool depth must be continuously measured and integrated with the gyro’s measurements. There are two solutions – wired and wireless. The TwinGyro™ utilizes Surveyor™ control software to interface with most solutions, including Bluetooth and serial cable connections to standard industrial encoder units.
Figure 2 – Detailed results from the 12 surveys of the vertical coring hole.
The hole was a 512-meter deep vertical coring hole located in an underground mine in Sweden’s Arctic far north. The hole was drilled using WL46 rods and surveyed using two-inch centralizer blades. The hole was somewhat challenging from a survey perspective, as it was near vertical over its entire run with a distinct bulge which revealed repeated survey accuracy. Depth was continuously measured using the Bluetooth-enabled Wireless Depth Tracker™ which was mounted onto the hole’s collar. Figure 1 shows the two TwinGyros™ used to survey the hole in three in and out run pairs, generating a total of 12 surveys.
Table 2 – Average misclose results: continuous vs multishot surveying within a gas test well.
To add a further element of rigor to the study, each survey was independently initialized using the LiPAD-100™ gyrocompass system by Northrop Grumman LITEF. This system is the most accurate by far – lightweight with an easy to use north-finding gyrocompass, tailored for borehole survey applications with reliable accuracy at high latitude. Each survey in this study was given an independent vertical reference direction simply by holding the LiPAD-100™ against the vertically mounted running gear.
The survey results are summarized in Table 1. The average end-of-hole misclose was an impressive 0.05 % (0.24 m over the 512-meter depth). The survey with the greatest deviation had a misclose of only 0.07 %. The detailed plots of the inclination and azimuth of the hole for all 12 surveys are shown in Figure 2.
It is clear that the TwinGyro™ results in highly repeatable surveys, even in a comparatively deep vertical hole while in continuous survey mode.
Figure 4 – Micro-doglegs are visible in continuous surveys between 200 ft (61 m) and 650 ft (198 m).
The second study was in a Texas gas test well with a depth of 1300 ft (396 m). The hole was drilled for gyro tests and began at vertical but soon rose sharply to a final inclination of 20°, simulating directional kick-off drilling. Depth was measured using a wired Dynapar optical encoder mounted onto the winch, just ahead of the cable drum, as shown in Figure 3.
The TwinGyro™ was operated in both survey modes – continuous and traditional multishot. Continuous mode consisted of 10 runs, generating 20 surveys with a 2 ft (0.61 m) result interval to show micro-doglegs. Multishot surveys consisted of 12 runs with 24 surveys and a 50 ft (15 m) station interval during surveying. The data is compiled into misclose results for the average, as well as with the position deviations at the end of hole and presented in Table 2. The results demonstrate that there is no significant difference between the two surveying methods, in terms of survey accuracy at end of hole.
Figure 4 shows micro-doglegs between 200 ft (61 m) and 650 ft (198 m), where the driller deliberately steered the drilling to kick-off the hole. The traditional multishot surveys show the hole’s correct average behavior, yet the 50-foot resolution cannot capture the detailed micro-dogleg effects visible in the continuous surveys.
This study was conducted using the TwinGyro™ within a vertical coring hole and a vertical gas well with kick-off. Results indicate that continuous gyro surveying provides the same high level of accuracy as traditional multishot surveying, while continuous gyro surveying includes the following benefits:
- Faster survey time. At least twice as fast as a multishot survey;
- Greater deep survey accuracy. Less time, less accumulation of gyro drift;
- Operator ease of use – fewer button clicks, less room for error;
- Yields higher resolution of borehole details and position;
- The unique lightweight LiPAD-100™ gyrocompass provides fast, direct vertical survey referencing, avoiding cumbersome ground alignment and transfers to the hole;
- With both gyro and depth data recorded and saved, the TwinGyro™ survey can be reprocessed to give results at any desired resolution without re-surveying the hole;
- Minimal added cost when compared to multishot surveying. A wireline depth encoder can either be mounted at the hole collar or in front of the winch in most situations.
Footnote:The studies shared within this article were first distributed and revealed as a technical paper and presentation at the AIMMGM XXXIII International Mining Convention in Acapulco, Mexico in October 2019.
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