What Puts the ‘Direction’ in Directional Drilling?
In past columns we have discussed the use of horizontal directional drilling (HDD) for remediation well installations, soil sampling using HDD, and HDD drilling fluids and containment. In this article, we look at another of the unique aspects of the technology — locating and navigation. After all, with “directional” as a major part of the technology’s name, you might assume there are ways to figure out where you are and where you’re going when drilling an HDD well.
Terminology for HDD locating systems can be very confusing, even without going into the technology itself. Walkover vs. wireline, GST, grids, coils, and sondes — what do they all mean?
One of most misconstrued locating terms is “wireline.” Ask five different consultants or contractors what a wireline is, and you might get five different answers depending on their experience in the field. But it’s really pretty simple — a “wireline” is just a strand of insulated copper wire that is threaded through the inside of the drill string down to the locating tools in the downhole assembly (right behind the drill bit). At the surface, the wireline connects to a power supply and/or a computer at the rig. This wire may just supply power to a locating unit or it may also carry a data stream back to instrumentation at the rig. Seems simple, right? Unfortunately, different contractors may confuse “wireline” with a specific type of locating system, which we’ll discuss in a moment. Don’t be confused — a wireline just carries power or a signal; the source and type of signal is irrelevant.
So just how many signal sources are there? The confusion starts to dissipate at this point. There are currently two ways commonly used to determine the location of a drill bit in three-dimensional space:
- Magnetic guidance systems that use some form of sensing device to determine the location of the drill bit within either the earth’s or an artificially generated electromagnetic field.
- Inertial or gyroscopic systems that utilize sensitive accelerometers and fiber optic gyroscopes to track a drill bit’s motion against a fixed coordinate — often true geographic north.
Magnetic systems can be subdivided into several categories. The most commonly used is the “walkover” system. In this system, a downhole probe, commonly referred to as the sonde, generates an electromagnetic field sensed at the ground surface by a receiver carried by a locating technician.
Note that the sonde is located in a housing directly behind the drill bit and the locating technician physically follows the path of the sonde as it advances along the bore path, actually “walking over” the path. Physical access to the surface expression of the bore path is required for a walkover system. However, the access doesn’t necessarily have to be continuous: Think about trying to follow the bore while walking across a busy four-lane highway. The guidance technician can locate up to the edge of the road on one side and then pick up the signal on the other side of the road. While being very simple to use, walkover systems can be challenging to utilize in crowded industrial settings, steep or brushy terrain, or on other sites where access is challenging (railyards, crossing under a freeway or airport runway). However, depending on depth, walkover systems are surprisingly capable. You can even use them on water crossings by putting the technician and receiver in a boat!
Usually, walkover systems are battery powered, with the sonde running off Ccells or expensive lithium cells. Due to power and antenna length limitations, walkover systems are limited to use at less than 80 feet of depth and in areas with limited electrical interference.
Interference — the bane of any magnetic locating system — can be either passive or active. Passive interference is distortion of the generated electromagnetic field in the presence of concentrations of ferrous or other magnetic material (e.g., heavily reinforced concrete, buried pipelines or waste drums along the bore path, or inside a metal building). Active interference occurs where power lines, cathodic protection systems, industrial motors (diesel electric locomotives throw off a huge amount of interference) or other electrical devices generate electromagnetic fields of their own, which interfere with the field that the sonde emits. Either type of interference can make it difficult to accurately locate the sonde position.
Where more power is needed to generate a stronger electromagnetic field, either to provide deeper locates or to overcome interference, the sonde may also be powered through a wireline. Remember the wireline runs inside of the drill pipe, so every time a rod is added to the drill string, another length of wire is spliced into the wireline connection. The connection is crimped in place and covered in heat-shrink tubing to prevent short circuits. Wirelinepowered systems can extend walkover system usability to perhaps 100 feet in depth, depending on conditions. Deeper than 100 feet, or in areas of significant interference, it’s necessary to turn to different technologies.
More advanced magnetic guidance systems reverse the locations of sensors and magnetic fields. Instead of generating a magnetic field downhole with the sonde and sensing it at the surface, the sensor is put down the hole and is linked to the surface via a wireline that conducts both power and data. The sensor then can determine an azimuth direction based on the earth’s magnetic field. However, the earth’s geomagnetic field is variable from place to place and can be distorted by strong electrical fields associated with power lines, industrial applications or even by large masses of metal, like pipelines, ore bodies or slag piles, landfill debris or reinforced concrete — the types of things often found around environmental sites.
To counteract this variability, most non-walkover magnetic systems employ an artificial magnetic field to make the locating more precise. This is generated by laying a temporary coil on the ground surface with a loop of insulated wire that encircles the bore path. For a well that’s 700 feet long, the coil will be longer than 700 feet, and will usually extend to either side of the bore path by 20-50 feet. Depending on the site, several shorter, overlapping loops may be used, which are energized in sequence as the drilling advances, to always provide coverage in the current area of operation. Many contractors or consultants are referring to this type of system when they speak of “wireline” systems.
The layout of these surface coils must be precisely surveyed so that the electromagnetic field can be modeled and input to the system software to calculate the exact position of the sensor. In addition to the magnetometers that sense the magnetic field, accelerometers are used to measure the vector component of the drill head’s passage through the subsurface. The electromagnetic field that is generated is usually powerful enough to override any interference that exists from local power lines or metallic masses. These systems may be used to depths approaching 200 feet with great precision — locating the position of the drill head within a few tenths of a foot.
There are a few other variations worth mentioning regarding these surface-coil systems. The most accurate and precise systems tend to be based on magnetic fields generated by direct electrical current (DC) with positive and negative ends on a closed-loop coil. There are also alternating current (AC) systems, some of which do not require a closed loop. These socalled “single wire” systems use a single conductor to generate the electromagnetic field; the return path of electricity is through an earth ground. The single-wire systems are not that popular for environmental drilling, with DC systems predominating.
The primary disadvantage of coilbased magnetic guidance systems is the placement of the coil itself. As with walkover systems, you must have access to the surface expression of the bore path. In many cases, you simply have no place to construct a coil — like in a crowded refinery, or on a steep, brushy slope.
Most contractors use some variation of magnetic guidance systems, although inertial systems, more frequently known by their tooling — Gyroscopic Steering Tools or GST — are used in situations where magnetic systems are impractical or ineffective. In a future column we’ll discuss the other major guidance system, the GST, and how it overcomes some of the disadvantages of other systems.