With high-capacity pumps and HDD rigs with higher drilling-fluid-flow capacities, the down-hole mud motor has become the tool of choice for directionally drilling in rock. The drilling-fluid requirements to power mud motors are much higher than for mixing and transporting cuttings; however, the majority of the fluid can be recycled and reused. The important factors in classifying mud motors for HDD applications are the output torque, fluid volume requirements, and length. Length is important because the tracking equipment is located in the drill string, which may be as much as 26 feet behind the drill bit and thus difficult to track with the desired accuracy.
Mud motors achieve directional control by using a maximum 3-degree bend located approximately 5 feet behind the bit. The drill operator can obtain more aggressive steering if the distance from the bit to the bend is shortened and a greater bend angle is used. 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 because the assembly oscillates in the bore when rotated and may be severely damaged or prematurely worn if rotated at excessive speeds. The rotation should start slowly and carefully after steering. This allows the assessment of restrictions that may prevent the bent assembly from freely rotating in the bore and prevent expensive damage to the mud motor.
When drilling in formations with strengths exceeding 40,000 psi, the penetration rates will be slow and costs will be high. Hard-rock formations are routinely successfully drilled with today's technology and experienced HDD contractors. Proper site evaluation and project planning are important for successful drilling in
hard rock. If hard rock is expected and properly tested, proper selection of equipment can be done before construction begins. Rock may become an obstruction when it is unexpected and tooling has been selected for soils or if 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 motors and pumps may also be required. These modifications can be time-consuming and expensive.
A mud motor is no more effective than the volume of fluid, which generates power, that is pumped through it. If the mud flow is on the low side of specifications, the mud motor will have low bit speed and a slower penetration rate. As the pumping rate increases toward the maximum of the flow specification, the motor is more powerful and efficient. This is why it is essential to match the mud motor with the available deliverable mud flow from the pump. Most mud pumps used in the directional boring industry run at higher speeds to operate mud motors. However, mud pumps lose operating efficiency as the mud viscosity increases and the effect of entrained air in the mud increases. The pump does not completely fill during the suction phase, resulting in a drop in the volume of fluid being pumped.
Most pressure-fed piston or plunger pumps generally have a 96 to 97 percent volumetric efficiency. The loss in efficiency largely results from the valves not opening and closing instantly. The efficiency goes down as mud viscosity increases and drops significantly if air is in the mud. Air in the mud can reduce pump efficiency to 50 percent or less. The main cause of entrained air is the design of the mud-cleaning system. Mud is cleaned by agitation, which mixes air into it. Higher-viscosity mud results in more air entrapment. The cleaning system must provide enough time for the mud to expel this air before it enters the suction manifold of the pressure pump. In addition, small internal diameters of piping and hoses cause pressure losses as pumping rates increase. This reduces the pressure available for the motor, causing a decrease in motor torque and slower boring.
Small inside-diameter drill pipe also tends to require a higher surface mud-system pressure to offset piping losses. Mud-flow rates provided by manufacturers are usually a calculated volume based on pumping water at 100-percent efficiency. Air in the mud, the size of the restricted passages, mud viscosity, and the length of the various restrictions reduce the true working efficiency of the pumps.
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