Important operational considerations such as distance from the bit to the steering tool, bend angle, fluid volume, off-bottom pressure, stall pressure, operating pressure (about 100 psi less than stall pressure) and differential pressure (operating pressure minus off-bottom pressure) are important for maximizing tracking accuracy, penetration rates and tooling life. Manufacturers should be consulted for advice on these matters. An operational test report should be provided with each mud motor upon delivery from the manufacturer, and a torque/flow chart and a build rate chart also should be requested to determine the optimal operating parameters of the mud motor. It is important to have functional fluid flow and pressure gauges when drilling with any tooling, but it is essential when drilling with mud motors.
Directional control with mud motors is attained by using a maximum 3-degree bend (typically less than 2 degrees) about 5 feet behind the bit, which serves the same function as the slant on the face of a slanted-face bit. Steering is more aggressive with a shorter distance from the bit to the bend and a greater bend angle. As with slant-faced drilling techniques, the bend is oriented in the desired direction and the entire assembly is pushed to attain the steering corrections while only the bit rotates. The drill string and bit are rotated and pushed for drilling straight. One significant difference in drilling with a mud motor is that the drill string should rotate at less than 50 rpm (typically around 30 rpm) as the assembly oscillates in the bore when rotated and may be severely damaged or prematurely worn if rotated at excessive speeds. Additionally, rotation must be started slowly and carefully after steering. This allows the operator to assess any restriction that may prevent the bent assembly from freely rotating in the bore and, thereby, prevent expensive damage to the mud motor and the ensuing delays.
To successfully drill very hard rock, minimum requirements considered appropriate are:
- 50,000-pound-thrust rig
- 27⁄8 inch-diameter drill pipe
- 43⁄4-inch pilot bore, using tricone roller bit
- 33⁄8-inch diameter mud motor
- 135-gpm pump
However, penetration rates will be slow and costs will be high if the unconfined compressive strengths exceed 40,000 psi. Hard rock can be drilled if it is expected and properly tested, allowing selection of proper equipment before construction begins. Rock may become an obstruction when it is unexpected and tooling has been selected for soils, or if rock is expected but the rock encountered is much harder than anticipated. In these cases, the bit may be changed to one capable of cutting the harder rock. Higher capacity mud motor and pumps also may be required. These modifications can be time consuming and expensive.
Mechanically Driven AssemblyA specialty drill rig, with dual top drives for a dual-member drill string, mechanically powers the steerable downhole rotary drilling assembly. The downhole assembly is much like that of a mud motor, but without its fluid-driven power section. The rotary bit (usually a tricone roller bit) is continuously rotated by the inner member of the dual member drill string. The outer drill string member is used to control steering, by orienting the bent housing (also the bearing support for the rotating bit and home for the tracking transmitter) in the proper direction. Straight drilling is accomplished much like a mud motor, by slowly rotating the outer pipe. The tracking transmitter is closely positioned behind the bit, providing more timely indication of steering response than possible with most mud motors. Presence of the inner drive, however, prohibits use of non-walkover tracking systems.
The inefficiencies of drilling fluid pumps and the downhole mud motor are not factors with this mechanically driven system. Greater torque can be applied to the bit than with comparable size mud motors. Only the drilling fluid flow rate needed to clear cuttings from the hole is pumped. Thus, fluid requirements are substantially lower, and the cleaning system can be much smaller or possibly replaced with a vacuum system. The mechanically driven system is effective for short bores in soft to medium-hard rock, where limited work space may preclude use of a large rig and pump needed for driving a mud motor. The mechanically driven downhole system is not as efficient as a mud motor system for large-diameter, long bores in hard rock.
Percussive DrillingSeveral downhole percussive drilling systems are available. Most are compressed air-powered systems adapted from vertical downhole hammer drilling technology. Minor modification of a conventional drill rig will adapt it to an air compressor instead of one that pumps drilling fluid. However, the conversion requires a rig with properly sized drilling fluid circuit and drill string tool joint inside diameters as the starting point. Otherwise, flow path restrictions to high volume compressed air will create pressure losses that degrade penetration rates. The thrust requirements are low, as percussion and rotation fracture the rock. Excessive thrust actually may decrease penetration rates and increase transmission of impact shock up the drill string. Steering of percussive downhole systems is accomplished with an eccentric, flat-faced bit with tungsten carbide inserts. The bit has a high spot that is preferentially oriented to follow the desired path as it penetrates without rotation. Straight drilling is accomplished by slowly rotating under moderate thrust.
As rock hardness increases, the penetration rate of percussive drilling surpasses that of rotary drilling. Thus, these systems may have an advantage over mud motors in harder rock. However, they may not be suited to completing a bore in as wide a range of geological formations. Cuttings are cleared from the hole with high velocity (up to 6,000 ft./min.) air. The cuttings and dust must be contained and controlled at the surface. The high-pressure air may erode the drilled hole or follow a crack or fissure and blowout to the surface in non-competent formations. Small flow rates (1-3 gpm) of water, or water and drilling foam mixture, generally must be injected along with the air to cool the transmitter. This fluid also aids in cleaning the hole of cuttings (which can be substantially larger than those from rotary drilling) and suppresses dust “fines” in the spoil returns to the entry pit.
Reamers and Hole OpenersReamers are used to enlarge the bore sufficiently to facilitate installation of the product. The reamer must be capable of displacing the native material or reducing it to manageable cuttings, mixing those cuttings with the drilling fluid, and preparing the bore for installation. Reamers generally are classified as barrel or compaction reamers, mixing and all-purpose. Within each category there exists several reamer types. Proper selection is based on soil conditions, hole size and pump capacity. Application guidelines are presented in the table above.
Recommended practice is to select a reamer that is the smaller of 1.5 times the outside diameter or 12 inches larger than the diameter of the product pipe to allow for an annular void for the return of drilling fluids and cuttings, to reduce frictional pullback forces and to allow for the bend radius of the pipe. It may be advisable to use a reamed diameter less than 1.5 times the diameter of the product in collapsing soil formations. However, reamed diameter values may need to be increased by up to 25 percent if substantial swelling of the soil is expected to occur. The larger the hole, the more important it is to mix the cuttings into a slurry with the drilling fluid before and during product installation. This ensures that cuttings remaining in the bore will not restrict displacement of the slurry by the product pipe. The slurry must be displaced from the hole in a quantity equivalent to the product pipe outer volume. If not, pressure can build in the hole, stalling the pullback operation or creating surface heave.
When formations cannot be penetrated with reamers, or if stalling or over-torquing of the reamer occurs, hole openers should be used. Hole openers that use rolling cutters are used primarily for reaming or pre-reaming the borehole in rock formations and come in many sizes and configurations. Most hole openers provide excellent flow characteristics, cutting capability and low torque due to rolling cutters. It should be noted that the selection of cutters depends on the formations encountered as presented in the table.
Percussive reamers are another option for up-sizing the hole in rock. These systems use a special carbide-button-faced bit powered by a downhole air hammer. Some are configured for conventional pullback reaming, where (a) air is supplied through the drill string and reaming bit to a reverse-circulation hammer, or (b) air is supplied through a hose trailing from the exit side to a conventional downhole hammer driving the reaming bit. Another technique, suited for upsizing a hole in competent (continuous) rock, is a conventional downhole hammer with a piloting reamer bit. The “piloting nose” on the bit guides the system along the pilot bore as it reams the hole in the forward direction. Thus, the drill string must be withdrawn from the completed pilot bore to install the hammer and piloting reamer bit for the forward ream. Risk of the piloting nose breaking out of the existing borehole is lessened by noting the nature of the rock being drilled over the length of the bore during the pilot bore, so as not to use this reaming technique in unfavorable conditions.
Specialized ToolsThere are a number of specialized tools that are used in special circumstances, or that can be used to solve problems encountered during drilling:
- Fishing tools including over-shots, spears and baskets are used primarily for retrieving lost drill pipe, bits and reamers from the borehole.
- Washover pipe is used to stabilize smaller diameter pilot drill string and to recover stuck drill pipe that cannot be rotated or pulled. Washover pipe may be used to save the borehole if the pilot string must be retracted for bit changes or re-tooling. Washover pipe also may be appropriate to stabilize the bore behind the drill bit and to help ensure circulation in unstable formations. Using washover pipe requires modification of the drill rig to enable removal of breakout wrenches and fabrication of a crossover to connect the washover pipe.
- Crossover subs are used to facilitate connection of different sizes and types of tool joints.
- Remote breakout wrenches may be either manual or hydraulic and are used to connect or break tool joints at access points forward of the drill rig. Drilling machine power should never be used with remote wrenches to make or break tool joints.
- End load and side load transmitter housings are used for wireline tracking, mud motor applications and tracking backreams. Tracking of the backream is not a standard practice, but can be accomplished, if required by the owner or regulatory agency or if the bore is through loose soils that may allow the reamer and drill string to migrate upward from the pilot bore.
- Wireline systems for navigating a bore where surface access necessary for walkover tracking is restricted. When using wireline systems, numerous supplies are required including:
- Compression fittings for sealing wire entering drill string
- Insulated wire (typically 7-strand, 10-gauge with oil and abrasion resistant insulation)
- Butt splices
- Heat shrink
- Splicing tools