DIRECTIONAL DRILLING - THE GROWTH AREA
Ian Gray - Director, Sigra Pty Ltd
21 Lombank Street
Acacia Ridge
Queensland 4110.
ABSTRACT: This paper reviews the use and forms of directionally controlled drilling in the oilfield, civil and mining industries and points to future developments.
1.0 REASONS FOR DIRECTIONALLY CONTROLLED DRILLING
Directionally controlled drilling is required for a wide variety of reasons some of which are described below.
1.1 Drilling Straight Boreholes
Surprisingly one of the most common needs for directionally controlled drilling is the need to produce a straight borehole. Straight holes are required as pilot holes for raise bores. They are also needed for the installation of parallel pipe supports used to form temporary tunnel roofs ahead of an excavation. In some cases there is also a need for foundation boreholes to have a straight trajectory.
1.2 Exploration Within a Target Zone
Frequently the proposed trajectories of mine roadways or tunnels need to be explored using pilot boreholes, some of which must provide core for examination. In these cases a borehole must be provided that stays within the proposed alignment of the excavation.
1.3 Exploration of an Ore Body
The ability to deviate a vertical borehole to intercept an ore body can be of considerable use especially where the orebody is sub-vertical. There are also advantages in deviating non vertical boreholes to intercept orebodies from underground workings.
1.4 Drilling in an Oil or Gas Bearing Horizon
One of the most frequent uses of directional drilling is to stay within a producing portion of a petroleum reservoir. This may be in a thin reservoir or in a section of a reservoir. An example of the latter would be to remain below an oil water interface.
1.5 Coal Mine Gas Drainage
The greatest use of directional drilling in Australia is in drilling from underground for the purpose of draining gas from coal seams. Some 300 km per annum of directionally controlled in-seam gas drainage boreholes are drilled from underground mines. The length of borehole drilled typically ranges from 250 to 1500 m.
1.6 Boreholes for Services
The major growth area in directionally controlled drilling is to create boreholes for civil engineering purposes. These techniques form part of what is known as trenchless technology. These directionally controlled boreholes are typically used for pipelines or cables. The size of the holes that might be drilled is increasing, thus enabling the system to be used for a wider range of services.
1.7 Drainage Works
The use of directional drilling drainage boreholes is also increasing. These have been used to intercept leachate draining from landfill waste dumps.
2.0 METHODS OF DIRECTIONAL DRILLING
The methods of directional drilling are varied, however with one exception, they all contain some means for positively changing the borehole trajectory. That exception is the cable tool drilling system which under most circumstances enables a borehole to be drilled straight down in the direction of gravity.
2.1 Wedges
Wedges are one of oldest techniques for changing the direction of a borehole. They are simply a wedge which is set in a borehole such that it deflects the drill string in a direction when drilling recommences.
2.2 Stabilizer Load Combinations
The petroleum industry used to deviate boreholes by applying substantial load to the bottom hole assembly (BHA) thus causing the drill rods to buckle in the borehole and drill off vertical. Usually the BHA is fitted with one or two stabilizers to improve control. The procedure deviates the borehole from the vertical but generally in an uncontrolled direction. Once the hole starts to deviate it tends to maintain that direction.
2.3 Jetting Bits
The petroleum industry has long used jetting bits to change the direction of boreholes. These have typically been a roller bit with a large jet on one side. To change direction the drill rods were simply held in one position and the face of the borehole preferentially eroded in one direction.
The civil engineering industry is now using preferentially orientated jets to steer boreholes as a part of the push to trenchless technology. In these situations the jet is orientated out of the line of the borehole and the rods pushed to follow the eroded path of the borehole.
Over the past fifteen years substantial research has been conducted into drilling by water jets alone. Much of this research has been focussed on drilling in coal using water pressures of 20 to 100 MPa. These water jet drilling operations have utilized rotating nozzles which contain the jets. The rotating nozzles are driven by jet reaction and serve to spread the area over which the jet acts thus opening up a borehole as opposed to a discrete hole in front of the jet. Such jetting systems have achieved penetration rates of 9 metres/minute in coal. Directional control of such systems can be achieved by re-directing the orientation of the jets.
2.4 Deflecting Bits
In soils use can be made of bits which can be pushed through the formation. These bits have the form of a shoe with a cutting edge. The shoe is usually set at an angle to the drill string and when pushed into the soil tends to change the angle of the hole. Some of these bits can be rotated to drill ahead whilst others are fitted with teeth to cut rock if needed, their action is often augmented by water jets.
2.5 Downhole Motors
Probably the most widely used system for controlling the direction of drilling is by downhole motor attached to a bent sub. These motors are powered by the drilling fluid. They are predominantly of a positive displacement progressive helical cavity type, though turbines and vane motors have also been used. The mode of operation to change direction is to hold the drill rods in a fixed orientation (tool face angle) and to pump fluid through the motor which rotates the drill bit whilst advancing the drill rods. Because the motor is set at an angle to the line of the borehole it preferentially drills in that direction. In some systems the entire BHA can be rotated so as to drill a straight hole when angular build is not required. This mode has the added advantage that the rotation of the drill rod stirs up the cuttings bed in the borehole.
2.6 Directional Coring
In Australia it is reasonably common to drill a cored hole for such applications as a pilot hole for a raise bore. If the hole gets out of line then a correction is made by replacing the core barrel with a downhole motor and bent sub to drill a correction. This procedure is slow and means that sections of core are missed.
An alternative is the Vic drill head which is equally slow but avoids missing core sections. This device uses a wide kerf core bit and small diameter barrel deviated by eccentric bushes which are locked in a particular orientation by a packer.
2.7 Directional Hammers
Several attempts have been made at producing directionally controllable hammers (Johns et Al US Patent No 5305837). This has a rather similar form to a conventional mud motor and bent sub combination except that the mud motor is replaced by a down hole hammer with an mechanism to index the bit. Another system is rumoured to exist which uses a bit which tends to drill preferentially to one side of the borehole. By increasing the amount of time that the bit is held in a particular tool face angle the hole is developed in that direction.
The advantage of a hammer is to enhance penetration rates especially in hard rocks.
2.8 Directional Drilling Tools without Drill Rods
Several forms of directional drilling systems exist that do not use drill rods.
The petroleum industry has made use of coiled tubing. In this the drill rod is replaced by a coil of steel tube. The drill head is normally formed by a downhole motor the orientation of which can be changed by raising the mud pressure so permitting the system to rotate and index. This is necessary as it is not possible to rotate the drill string to change the tool face angle.
Some drilling systems make use of hoses as their connection between the drill head and the hole collar. A non directional drilling method uses a compressed air powered hammer within the head to advance it through the soil trailing the hose behind. In another form the drilling method has been used with water jets. In this case a drilling head has been attached to the end of a hydraulic hose. This head contains rotating nozzles to drill ahead and reactive jets for orientation and to provide thrust. The hose is in fact self propelled through the borehole by the net hydraulic shear acting on it. This means that the traction of the fluid on the inner wall of the hose exceeds that of the returning fluid on the outer wall of the hose. These systems have significant potential in drilling tight radius boreholes, say from a vertical hole to horizontally in a coal seam.
3. BOREHOLE SURVEY AND GUIDANCE SYSTEMS
Borehole guidance has traditionally been based on geometric criteria. This has been augmented in the petroleum industry over the last decade by the addition of geological data derived from geophysical tools held within the BHA.
3.1 Mechanical Survey Systems
Early mechanical survey systems seemed to universally involve locking in position some magnetic eccentrically weighted ball.
This could be accomplished by floating the ball in a setting jelly or holding the ball in gimbals and locking it in place mechanically (Tropari/Pajari). In these tools the survey device would be run with or within the drill string and withdrawn to be read. A spot reading of borehole inclination and azimuth would be the result.
This process was developed in the form of the Eastman single shot survey tool in which the ball was photographed and the orientations determined by examining the film once the tool had been withdrawn and the film developed.
All of these systems represent a measurement by gravity and magnetic field. The use of magnetic field for orientation requires that there is no magnetic interference from drill rods or magnetic rocks.
3.2 Electronic Survey Systems
Virtually all electronic survey systems utilise the earth's fields for survey measurements. The gravitational field is measured directly by accelerometers and from that its direction is determined. Alternatively the direction of the field is measured by inclinometers. Magnetic field data is determined using magnetometers and the direction of this field provides the additional information that is necessary to determine the orientation of the survey tool.
Electronic survey systems may store information on orientation on board to be subsequently downloaded. Alternatively they may transmit information to surface through various transmission systems to be discussed later in this paper.
3.3 Gyroscopic Surveying
Gyroscopes can be used to measure the orientation of the drill pipe. These tools are designed to hold their orientation despite changes of direction of the housings. Some gyroscopic survey tools also include accelerometers to define the vertical position. Because of their mechanical nature gyroscopes are easily damaged by shocks. They also suffer from problems associated with drift. This drift is a function of the rotation of the earth and the inaccuracies in the gyroscope causing the unit to rotate. Therefore great care must be made in using such tools to ensure that errors due to these causes are accounted for. All gyroscopes need to be checked for orientation before and preferably after a hole is drilled.
Light operated gyroscopes come in two forms, laser ring gyroscopes and fibre optic ring gyroscopes. In the case of the former light is reflected around the inside of a prism whilst in the latter it travels around a fibre optic coil.
Changes in orientation are determined by phase shifts between the incoming and outgoing light beams. Commercial ring lasers tend to be comparatively large and therefore have not been generally used in drilling. Fibre optic gyroscopes have been developed to form borehole survey tools, notably by Hitachi.
Neither of these types of gyroscope are free from drift problems.
3.4 Calculating Borehole Trajectory from Tangential Measurements
The gravitational/magnetic field devices and gyroscopes provide borehole orientation in the form of measurements of inclination and azimuth to the path of a borehole. Calculating the trajectory of the borehole from this information and the true length of the borehole is not trivial. The process is one of integration of the measured borehole angle with distance.
The model and equation type used in the integration is important. The model used should mirror the angular build characteristics of the BHA. Commonly the average angle between survey readings is calculated and the borehole position is
estimated on from this measurement. A more accurate approach is that of fitting a great circle or minimum curvature between survey points. Neither properly mirror the build character of the bottomhole assembly. The accuracy of the borehole position estimate can be improved by more frequent survey information.
3.5 Offset Survey Systems
Offset survey systems measure the incremental displacement of the borehole between measurements. These should be made sequentially with overlap to minimize error. The gravitational field direction is usually obtained to provide a reference from which the offset calculated. This does not work in a near vertical borehole and the system must rely on another reference which may be as simple as marked drill rods.
The original version of this survey system was the Fotobar which photographed rings spaced up a tube. The offset of the rings could be measured directly and the high side of the instrument measured by a bubble which was also photographed.
Offset survey systems have the potential to be extremely accurate provided adequate care is taken.
3.6 Geosteering Systems
The survey systems described previously provided only geometric information. The oil industry has had a need to drill within oil or gas bearing formations. These are not geometrically but rather geologically defined prior to drilling. If a borehole is defined in terms of geology then it needs to be steered by geological information. In open hole drilling this can only come from drilling information (torque, thrust and penetration) or from geophysical sensors. These sensors would typically include gamma and resistivity probes but may include any of the range of downhole geophysical tools. Information from these and geometric survey tools are transmitted to surface where an operator decides where to steer the borehole next.
3.7 Steering Systems for Civil Trenchless Technology
Typically the depth of buried civil engineering services is not very great. Civil cable and pipe laying operations make use of this by locating the position of the drill head from surface. A transmitter in the drill head sends to a hand held receiver. The strength of the transmission guides an operator over the drill head and the inclination of the drill head as measured internally is contained in the transmitted signal. In the event of the hole being deep significant attenuation of the signal through the ground occurs.
4.0 INFORMATION TRANSMISSION
The information from geophysical, drilling or survey parameters measured in the BHA needs to be transmitted to the operator.
The oldest method is to store the information in the tool and to remove the tool to obtain the information. This is the case for single shot survey systems.
Wire transmission can also be used. The wire can run through or outside the drill rods. Wire running through the drill rods can include conductor rods contained within the drill rod and connected with the rod make up. It may include wire held on a spool or cassette which is drawn through the rods as they are made up. It may also be simply connected piece by piece as each rod joint is made up. Such a process is tedious and the wire breakage can lead to large time loss. Geophysical cable is sometime run outside the drill rods using a side entry sub to get the cable from the tool inside to the outside of the rods. Such a system precludes drill rod rotation.
The system which has gained the most favour in the petroleum industry is mud pulse communications. This process involves transmitting digital pulses in the fluid stream. This is achieved by opening and partially closing a valve in the BHA so as to cause pressure fluctuations in the flowing fluid. The pressure fluctuations are transmitted very rapidly to the borehole collar and decoded by a pressure transducer connected to a computer. The system is neat because it does not interfere with the drill string and can transit huge distances (10 km) but is not without problems. Low transmission rate (maximum 10 bits/second) is the prime problem. The second is difficulty sometimes experienced is extracting the signal from pump noise.
Alternatives to mud pulsing include tapping on the drill string. This works well but drill rod/borehole interaction seems to lead to signal attenuation at about 600 m. Another option available is radio communication. Transmitting radio waves through the earth however requires power, very low frequencies and favourable rock formations.
5.0 THE CONTROL LOOP
A requirement of any directional drilling system is that it is controlled. The control path is involves sending information from the bottom hole assembly to the hole collar where the operator makes a decision on where to steer the borehole. The steering is usually accomplished by turning the drill string so that the BHA has a new tool face angle. The two weak parts of this loop are the transmission up the drill string of information and the inaccuracy in changing the tool face angle by twisting a drill rod from surface. In long holes the friction may build up and the drill string have several hundred degrees of rod wind up making an adjustment of even 30 degrees at the tool face virtually impossible.
6.0 SELF STEERING SYSTEMS
The control loop can be shortened by building intelligence into the BHA. With this in place survey or geophysical sensors can provide their information directly to the on board computer that can make a decision on what correction needs to be made to the borehole trajectory. Usually this information needs to be augmented by information on borehole depth. This can be most effectively provided by a mud pulse from surface to the BHA.
Several variants of self steering systems have been proposed and exist. The simplest of these is the cable tool drilling system. Here if the chisel goes off vertical it gravity directly pulls it back into line.
Deutsche Montan Technolgie (DMT) developed a drilling system to cut vertical holes. It utilised a roller bit driven on the end of the drill string. The BHA above the bit contains sensors to measure inclination. The output from these is used to actuate rams to side load the drill bit and bring the tool back into line.
The next version of this type of equipment was developed by Christensen. It used the same principals but used a downhole motor instead of drill rod rotation. The further development of this is seen now in a form of geosteering equipment now marketed by Baker Hughes. This has similarities to the DMT tool but apparently can be programmed to respond to inputs from the geophysical sensors on board.
Other equipment is patented to work in this area. Jurgens and Roper have a patent (US Patent No 5156222) assigned to Baker Hughes Inc over a directional hammer utilizing this concept and Patton (US Patent No 5419405) has a number of patents covering the concepts of self steering systems. Leising also has a Patent (US Patent No 4637479) assigned to Schlumberger in this area. The directional control is in this case achieved by pulsed water jets and erosion. Sigra Pty Ltd is also active in this area with a system that automatically adjusts the tool face angle of downhole motor.
7.0 DRILLING TO A TARGET
It is useful to be able to drill boreholes that intersect a small target. Drilling relief wells into oil reservoirs became a necessity to allow pressures to be sufficiently lowered to enable well capping to be undertaken following the Kuwait war.
Even tighter tolerances are required if one borehole is to intersect another. One specific use of such a system is the provision of a vertical pump well for producing from horizontal boreholes. No survey systems are accurate enough to drill to a target over a significant distance. Even a 0.1 degree accuracy survey tool has associated with it a 1.7 m error at 1000 m range. For longer boreholes the only way to intercept a small target is to place a transmitter into the target borehole and to aim for it. Because of transmission distance limitations in rock it may be necessary to miss, side branch and then return to hit a target having first established the relative positions of the two boreholes.
8.0 BOREHOLE TRAJECTORIES, FRICTION AND ROD LOADS
The trajectory of a borehole or well path strongly effects the distance that can be drilled and the ease of drilling.
Normally most drilling is conducted with tension in the drill string and weight on the bit being provided by the weight of the drill pipe. Where the inclination of the borehole is such that the drill pipe will not slide freely down the borehole then it is necessary to push the drill rods. Where long near horizontal sections exist then the amount of thrust required to slide the drill string may be considerable.
The thrust must overcome friction between the borehole wall and the drill rods. This frictional force is proportional to the normal force between the drill rods and borehole wall and the coefficient of friction between the two. Fluid additives can reduce the coefficient of friction. Reducing the normal force between the drill rods and the wall of the borehole can be achieved by lighter drill rods or by drilling a straighter
trajectory. Dog legs and particularly those caused by flip flopping the tool face of a bottom hole assembly lead to increased normal force between the rods and the borehole wall.
The thrust required to overcome this friction can cause the drill string to buckle within the borehole. When helical buckling occurs any additional force is not transmitted to the bit but is absorbed on the borehole wall.
One of the cases that can occur is during directional drilling is that where a drill string can be run into a hole containing dog legs. In this case compressive force and some measure of buckling helps reduce friction. However when an attempt is made to withdraw the drill string a much higher load is required to pull it out.
Designing a directional borehole path is an important aspect of any directional drilling project. Drill string torque and thrust simulators play an important part of any such design so that an estimate of drill rod loadings can be made. They are equally important to use during drilling with simulations based on the actual drilling loads and borehole paths. Their use provides a protection against stuck or broken drill rods.
An example of the importance of borehole path to being able to drill long distances is the case of NQ pipe in a horizontal borehole of 96 mm diameter in coal. Most holes can only be drilled to 1000 to 1500 m when either friction stops the drilling or the tool face angle cannot be adjusted accurately enough to maintain directional control. The use of a simulator rapidly demonstrates that this is the case. If, however, the hole could be drilled straight then the distance reached could theoretically be 4 km before bucking stopped the hole.
In all cases the difficulty of directional drilling increases exponentially with distance.
9.0 HOLE CLEANING
Directional boreholes have special cleaning needs. In a vertical hole there is only one way the cuttings can fall, namely down. Provided the mud lifts the particles and they do not fall back too far between rod changes then hole cleaning becomes relatively simple and either a laminar or turbulent flow regime can be used. A laminar regime is however preferred as it conserves power.
In a horizontal borehole the particles need to be transported along the hole against the gravitational tendency to lie on the floor of the borehole. Turbulence is required to stir up the particles off the base of the hole and keep them in suspension in the drilling fluid. The problem of particles on the borehole floor is aggravated by the fact that the drill string lies on the floor and the area between the two tend to form a protected cuttings bed. For this reason the practice of rotating the drill string to stir up the cuttings bed is favoured.
Inclined boreholes have their own problems. This occurs when the cuttings settle on to the base of the borehole and then may slump down the hole in the manner of a mudslide thus causing blockage of the borehole.
10.0 CONCLUSIONS
Directionally controlled drilling is growing and will continue to grow. As it grows so will the technology to accomplish it.
The main area of technology development has been the petroleum industry. This is likely to change as the demand in mining and particularly the civil engineering industries for directional drilling is likely to far outstrip that in the petroleum industry. It is logical then that the main areas of directionally controlled drilling developments will come from those areas rather than the petroleum industry.