Wellbore surveying with gyro tools | Part 2 of 2
In part one we talked about the general differences between magnetic tools and improvements in gyro technology.
In part two we will discuss error sources and error elimination to optimise survey accuracy.
In order to give the client something to compare different tools we have error numbers and error models.
For most gyro suppliers an error in degrees per 100m is what is published for the mining industry. This however cannot take into account all of the relevant survey environmental factors and is usually a best case number.
A lot of work goes into creating error models which the client in mining generally never sees (we are required by the oil and gas industry to have these models).
These models allow us to model the borehole spatial location accuracy by creating error ellipses which will vary in size according to the many factors involved including direction, dip, surface location and length of borehole etc.
In practice, the client gives us a planned wellpath. We then plot our specific survey tool along this path with error ellipses. The client can then decide if they are happy with the final error uncertainty at the geological target. If not, we then find an alternative method to increase the accuracy of surveying to fit their needs.
Both magnetic tools and gyros have their own error model limitations.
If we speak about Southern Africa as an example, then declination can vary from –10 to -26 degrees – this large range requires accurate location data to get a good fix. With a latitude varying from 15 to 35 deg South, this is an area of strength for gyros.
If we then switch to Northern Finland or Sweden, at 67 deg North, we are approaching territory that is tricky for most gyros. Why – because the horizontal component of the Earth’s rotation is significantly reduced and therefore more difficult to measure accurately.
We believe that through our processing technology, we are uniquely capable of extremely accurate surveying with our gyro tools in this environment.
This is a relatively straightforward environment for magnetic tools – no great change in declination over time. However, depending on borehole orientation, a large amount of non-magnetic spacing might be required for the survey instrument to avoid magnetic interference from the drillstring.
The other factor though is the geology of Scandinavia – igneous rocks with very variable ferromagnetism which can cause havoc for magnetic instruments
The Voorspoed (Prosperity) test pipe was mentioned in a previous article in this magazine. This was a high inclination 370m long, PVC pipe near Kroonstad, SA, in which survey tools could be run. It had the advantage of entrance and exit at surface, with known surface locations at either end for absolute tool error evaluation.
Unfortunately, the mine closed and it is no longer available. There have been other test pipelines worldwide. In fact, Trenchless drilling pipelines give the same opportunity to definitively test a survey tool.
There are numerous other test boreholes worldwide, as used by the oil industry. However, these are closed-end; drilled from the surface and lack the ability to make a land (surface) survey from one end to the other. The operator then runs selected tools from various competitors (if they will participate!) and a comparison can be made.
At the end, the more surveys run, the better the statistical fit and the more comfort the operator has that he knows which tool is best but it is not the same as an open-ended test borehole.
All of this presumes the there are no other problems with the survey setup. A fundamental issue in survey accuracy is ensuring the survey tool is properly aligned with the borehole. There are three different causes of misalignment within surveying:
1. Internal misalignments within the sensor chassis and pressure housing itself. This is an area for the tool calibration to pick up and correct. In the Oil & Gas industry a lot of work went into moving from separate chassis to a monoblock chassis for gyros /magnetometers & accelerometers in the last few years. The act of pushing the electronics into the pressure housing was found to be putting a twist in the older separate chassis systems.
2. Running gear misalignment within the drillpipe or rods. We have various centraliser types in use to prevent this. Essentially we need centralisers above and below the stiff sensor pressure housing that keeps it centred in the rods.
3. The third is the deflection of the drillstring components within the borehole. If the drillstring components have connections of larger diameter than the rod or pipe body itself, then the pipe body can deflect or sag under gravity to the low side of the hole.
Again, if we know the drillstring setup, we can run post-processing to calculate and offset this deflection. This last misalignment highlights that surveying inside a cased hole is going to give a much more accurate bottomhole location than running a survey through drill rods.
We believe that this concept is not yet fully appreciated by those who have experienced mechanical gyros.
Reference vs North seeking gyros
An accurate tool misaligned is not going to reflect the true borehole geometry. In vertical boreholes and large cased boreholes, centralisation is critical.
When running a slick tool in a vertical well, the tool will tend to float towards vertical – the sum effect is a false bottom hole location.
In a highly deviated well it may be possible to run the tool slick. It will certainly sit on the low side of the borehole. However, in all cases, unless you run centralisation above and below the sensors you may not read all of the borehole “doglegs” or tortuosity.
This has become a recognised issue in the oil and gas industry where operators now run tortuosity surveys to ensure they do not sit critical downhole pumps in extremely deviated sections of the well.
The tortuosity survey allows them to choose the best location. But tortuosity surveys have also highlighted the underlying issues – directional drilling creating unnecessarily sharp changes of direction or “doglegs” and insufficient surveys across these changes in direction, thus resulting in “smoothed” borehole surveys and cumulative errors until ending with a significantly poor bottom hole location at the end of the well.
In the trenchless technology (horizontal directional drilling) market post-installation surveys are run through the pipe in many countries where legislation requires it.
This is in order to ensure the “fall” or slope of the pipe has no dips which would hold back fluid flow through the pipe.
These surveys are run on roller type centralisers which will not collapse and give a true reflection of the inclination of the pipe at any point.
So in summary, the outcome of continuous surveying for tortuosity is enhanced bottom hole location accuracy. This is a trick that a lot of clients are not aware of.
When running a gyro survey, continuous mode gives constant survey updates, allowing a final survey at metre or even centimetre level for a significant improvement in bottom hole location accuracy for no time cost. All it takes is survey software that can calculate surveys on this basis.
Again, within the oil industry, continuous surveying has removed large discrepancies between surveys.
In particular, the accuracy of measurement of true vertical depth of horizontal boreholes, extended reach boreholes has been massively improved!
The cost of time
The goal is a highly accurate survey, run at a minimum loss of client’s productive drilling time, handed over to the client immediately after running with minimal processing.
Today, with the latest rig-proof handheld devices, with internal, automated QC checks, the survey can be accepted by the surveyor in seconds and uploaded immediately to the cloud and central office for timely decisions.
One of the research and development foci in bringing these tools to market is an extensive evaluation of time required to get the most accurate survey vs. time to acquire the survey. We could sit there acquiring data, increasing accuracy as long as we like but there is a breakeven point at which no further benefit is achieved.
All manufacturers must go through this process. A gyro survey consists of stationary referencing and continuous modes. The standard settings for each tool are designed to optimise this. Every gyro survey needs a reference or start direction – whether from an external reference or from an internal North seek.
The referencing gyro requires an input of borehole orientation at the start of the survey. This then is an area for possible systematic error.
The client will supply the surveyor with the North reference direction. One way to minimise this potential source of error is to use a rig aligner – a device that can measure the rig inclination and azimuth in 5 minutes to an accuracy equivalent to the gyro itself. One device can be shared amongst many rigs on a mine site if geography permits. Alternatively a standard optical line of sight survey will be used as the reference.
With a North seeking gyro, this error is eliminated. And there are many occasions when there is no rig to align wit
Different types of gyro have different temperature characteristics. For example, a MEMS gyro has a lower linear temperature response limit. For most mining applications this is irrelevant as it never reaches these temperatures. However, this is not always the case.
Tool calibration – and the ability to hold that calibration against temperature, shock, vibration and general ageing of components – is a key to reliable accuracy – but also to minimising downtime. Tools need to be not only calibrated, but calibrated at a full range of temperatures. This cannot practically be done in the field and requires a master calibration stand.
However, if you have a quality instrument and a quality calibration stand for local verification of calibration, it is possible to minimise the downtime and cost of return to the manufacturer for re-calibration. What is a quality calibration stand?
One that can realistically work to the accuracy levels of the latest gyros – that means in practice one that has micro-adjustment to all three axes and above all secure attachment to solid ground. The levels of accuracy of modern gyro tools is such that the days of an angle iron stand in the corner of a shed should be finished!
The solid state gyro has changed the maintenance strategy for survey companies completely. It is now possible to make regular calibration verifications and therefore not need to recalibrate the tool regularly. Even software updates can now be downloaded to the tool in remote locations, keeping the tool working 24/7.
Error models are just that. Models. The acid test of any gyro is consistency, repeatability, minimal real survey drift between in and outruns, ability to function in the more difficult environments of horizontal wellbores, high latitudes and other tough conditions.
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