DTH drilling is widely used in water well construction due to its high efficiency and speed. However, construction workers are often troubled by the problem of hole deflection, which can affect the quality and efficiency of water well construction. This article will explore the causes of the hole deflection problem in DTH drilling in water well construction, and propose corresponding solutions to help construction workers better deal with this problem.
Causes of DTH drilling hole deflection problems
The causes of DTH drilling hole deflection problems are varied, mainly including the following aspects:
Objective geological reasons
Anisotropy of rocks
Geological factors exist objectively, and some rocks with structural characteristics such as bedding and foliation have significant anisotropy in their drillability. The rock-crushing efficiency of the drill bit drilling in the direction perpendicular to the rock layer is the highest, while the efficiency in the direction parallel to the bedding is the lowest, and the rock-crushing efficiency in the inclined direction is in the middle. Therefore, when drilling in inclined rock layers, the borehole is prone to bending in a direction perpendicular to the layer.
Interlayer of soft and hard rocks
When drilling from soft rock into hard rock, or from hard rock into soft rock, or complete rock layer into broken rock layer, due to the difference of resistance to breakage of soft and hard parts, the drilling hole bends in the direction perpendicular to the level. When entering soft rock from hard rock, the axis of the drill tends to deviate from the direction of the layer. However, due to the hardness of the hole wall above, the deviation of the drilling tool is limited, and the original direction is maintained; when the drill hole passes through hard rock into soft rock and then from soft rock into hard rock, it eventually extends along the direction of the layer.
Reasons for soft, loose, fault, tectonic fracture zone, and cavernous strata
When encountering this type of strata, the aperture is easy to expand, and the force on the bottom of the drill bit is uneven, which is easy to cause deflection.
Reasons for process technology
The process factors that affect the bending of the borehole include drilling methods, drilling procedure parameters, and operating methods. Different drilling methods have various characteristics of rock crushing, resulting in different hole wall gaps. Drilling procedure parameters are significant factors affecting hole deflection.
Drilling pressure
The efficiency of DTH drilling primarily relies on the magnitude of the impact force and the frequency of impacts. While the drilling pressure ensures that the impact power gives full play to the auxiliary force, for a certain level of matching and bit diameter of the DTH hammer, the drilling pressure has a reasonable range, more than a specific limit value, which will cause the hole slant and even the drilling efficiency decline.
Rotation speed
DTH drilling is a drilling method that uses impact as the main way to crush rocks. The rotation is only to change the impact rock breaking position of the drill bit carbide to avoid repeated crushing, so too fast linear speed is not required. If the speed is too high, the centrifugal force of the drill rod rotation increases, thereby aggravating the lateral vibration and inclination of the drilling tool, causing the hole to deflect; if the speed is too slow, the impact of the column teeth will repeat the existing impact crushing point, resulting in a decrease in drilling speed and a larger hole diameter.
Wind pressure, wind speed, and wind supply
The impact frequency and impact work of the DTH hammer is related to air pressure, and air pressure is an essential factor in determining the impact work. The higher the pressure of compressed air, the higher the efficiency of DTH drilling and the longer the service life of the drill bit.
The air cleaning consumption in DTH drilling is determined based on the performance parameters of the DTH hammer and the minimum return speed required for removing cuttings from the hole. The air supply volume must ensure a specific annular return wind speed to facilitate the removal and transportation of cuttings at the bottom of the hole. The wind speed is related to the size of the annular gap between the drill rod and the hole wall. The wind pressure, wind speed, and air volume do not affect the bending of the borehole.
Drilling rig equipment installation
Uneven site foundations and improper installation of the drilling rig can lead to instability and erratic movement of the machine parts.
If the drilling tool is not vertical, the joints are not concentric, or the orifice pipe is not lowered securely, it can lead to deviation in the drilling trajectory. These factors primarily affect the initial stage of hole opening. However, once the hole starts to deviate, it can significantly impede the continued extension of the hole in the future.
If a diameter change occurs without a guide or if the guide is not concentric with the lower thick-diameter drilling tools, the original hole axis will deviate following the diameter change.
Thick-diameter drilling tools
The drill bit’s diameter is significantly larger than that of the thick-diameter drilling tools, requiring axial pressure to be applied to the drill bit during the drilling process. However, due to the gap between the hole wall and the slender, flexible drill rod, the drill rod may bend and deform under pressure. The axial pressure will generate a horizontal component along the bent drill rod. When the thick-diameter drilling tools have a large diameter and a short length, the drilling tools behave as rigid bodies. It can cause the thick-diameter drilling tools to tilt, leading to hole deviation.
Measures to prevent hole slanting
- When installing the drilling rig, it must be stable, round, and horizontal. The power head drilling rig requires the power head to slide vertically.
- It is the first step and the key to prevent the hole from slanting by putting a good opening switch to avoid the rig swinging and controlling the opening angle to prevent the hole from being skewed. After drilling 3~5m, put out the drill pipe and set out the center of the hole bottom by inverted plumbing method, and then the position of the drilling rig should be adjusted so that the center of the drill pipe and the center of the bottom of the hole are on a plumb line.
- We should stick to using vertical and anti-deviation drill pipes, extend the length of thick-diameter drilling tools, utilize drill pipe straightening devices, adhere strictly to regulations during operation, and strictly prohibit excessive pursuit of drilling depth and increasing drilling pressure without proper consideration.
- When drilling into the strong weathering layer, loose accumulation layer, or broken zone, must reduce drilling pressure and slow down the drilling speed to ensure the verticality of the hole.
- During drilling, if the drill pipe shakes violently or hysteresis occurs periodically, it indicates that there may be a broken zone, a large crack, or a collapse and falling block in the hole. We should promptly raise the drilling tools, perform repeated up-and-down movements to dislodge obstructions, and apply forceful air blows to expel any blockages from the hole. Reducing drilling pressure is essential to prevent hole deviation and mitigate the risk of accidents such as broken drill pipes.
- After completing the drilling with a single drill pipe, we should lift the drilling tools 0.3~0.5m for blowing air, and should be repeatedly moved up and down before adding rods to repair the hole wall, keep the bottom of the hole clean, and prevent the hole from deflecting, and then blowing and adding a rod when the hole does not return to the rock chips.
Conclusion
The hole deflection problem of DTH drilling in water well construction is a complex and severe challenge. To solve this problem, by further analyzing the causes of the hole deflection problem and taking corresponding measures, we can effectively reduce the probability of hole-slope problems and improve the water well’s quality and construction efficiency. In the future, with the continuous development and improvement of drilling technology and equipment, we have reason to believe that the application of DTH drilling in water well construction will be more extensive and in-depth. At the same time, we should continue to pay attention to and study new situations and problems of hole deviation to provide more comprehensive and effective solutions for water well construction.
