Impact rotary drilling technology in coal field geological exploration

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1. Drilling tool speed


When performing impact rotary drilling, the rotational speed of the drilling tool is generally determined according to factors such as the nature of the rock being drilled, the type of abrasive used, the impact energy of the impactor, and the impact frequency.

When drilling into hard rock or highly abrasive rock with a drill bit that uses block-shaped cylindrical carbide as the abrasive, the rotational speed of the drilling tool should be between 20-80./min since the dominant role in breaking the rock is impact load. . The harder the rock being drilled, the lower the rotational speed of the drilling tool.

Practice has shown that if the rotational speed of the drilling tool is too high, it will not only increase the impact distance, reduce the rock breaking effect and collapse the cemented carbide; but also as the drill bit’s peripheral speed increases, the friction mileage between the cemented carbide and the rock will increase, which will also It will quickly wear the cutting tool, and the result will be a significant reduction in the length of the footage. To increase the rotational speed of the drilling tool, the rotational speed should usually be between 80-300r/min.

In addition to rock properties, the performance of the impactor should also be considered when selecting the drilling tool speed. When the impact power of the impactor is large, the rotational speed of the drilling tool should be appropriately increased; otherwise, the rotational speed of the drilling tool should be reduced. Similarly, when the impact frequency of the impactor is high, the rotational speed of the drilling tool should be appropriately increased; conversely, the rotational speed should be appropriately reduced.

When the impact frequency is high, the rotational speed of the drilling tool should be increased, but when the impact frequency is low, the rotational speed should be reduced. The relationship between them can be seen in the following situation.

During impact rotary drilling, each cemented carbide on the drill bit is affected by the rotation at the same time. After completing the first impact to break the rock, it moves a certain distance to perform the second impact to break the rock. The distance between two impacts of cemented carbide is the impact spacing (S).

It can be seen from the calculation that the size of the impact distance (S) is closely related to the impact frequency and the speed of the drilling tool rotation speed.

The harder the rock, the greater its resistance to crushing, and the easier it is to chip and wear the carbide; therefore, the impact distance (S) should be smaller. When drilling into hard rock formations, a lower rotational speed should be used.

Various rocks have their optimal impact spacing values. According to practical production experience, generally rocks with drillability of level 6-7 have S=10~15mm, and rocks with drillability of level 7-8 have S=8 -10mm, rock with drillability level 9-10, S=5~8mm.

When drilling needle-shaped carbide, the rotation speed of the drilling tool should not be less than 200r/min, generally 200-300r/min.

When performing diamond impact rotary drilling, in order to give full play to the role of diamond grinding and breaking rocks and improve drilling efficiency, the rotational speed of the drilling tool should be above 600r/min.


2. Weight on bit


The axial pressure exerted on the hydraulic impact rotary drilling bit has two effects. The first is to cause a certain pre-stress in the rock and cut into the rock during rotation to improve the effect of breaking the rock; the second is to overcome the rebound force that occurs when the impactor is working to reduce the transmission loss of impact energy.

When drilling softer rock, due to the low anti-crushing strength of the rock, the rock is basically broken by rotational shearing. In order to give full play to the role of rotation in cutting rocks, greater axial pressure must be used. It can be controlled at around 800N.

When drilling into harder rock, due to the higher crushing strength of the rock, the rock is broken by the impact load. Therefore, the axial pressure used is smaller. Can be controlled within the range of 4000-6000N.

If the applied axial pressure is too large, it will not only cause premature wear of the cemented carbide but may also cause chipping or collapse, resulting in reduced drilling efficiency; and the pressure should not be too small, as too small a pressure cannot overcome the impact of the impactor. The rebound force that occurs will reduce the transmission efficiency of impact energy and also reduce the drilling efficiency.

When selecting the axial pressure, attention should be paid to both the instantaneous drilling speed and the average hourly efficiency during the entire drilling process.


3. Pump volume and pump pressure


Since the impactor is powered by high-pressure liquid flow to push the hammer to work; therefore, the flushing fluid not only cools the drill bit and flushes the rock powder, but also has a decisive impact on the working performance of the impactor. It directly affects the level of impact frequency and impact energy. That is, as the pump volume increases, the impact frequency and impact energy also increase accordingly.

As long as the formation allows it (the backflow velocity of the flushing fluid is not greater than the requirements of the formation being drilled) and the water pump works normally, the water volume when the impactor is working must be met as much as possible. It is also necessary to increase the pump capacity to make up for the leakage loss at each interface of the pipeline.

Judging from the two factors of currently commonly used impactor structure and calibre, positive-acting impactors and double-acting valve impactors usually require a pump volume of 1.34-2.5L/s, and double-acting jet impactors generally require 1.34-2.5L/s. The pump volume is approximately 3.34L/s.

Judging from the structure of impactors currently used, positive-acting impactors and double-acting valve-type impactors require a pump pressure of 1.47-2.45MPa. Double-acting jet impactor requires a pump pressure of 2.45-3.92MPa. At the same time, as the drilling depth increases and the loss along the pipeline increases, the pressure of the water pump must be increased accordingly (generally, the pump pressure needs to be increased by 0.294MPa per 100 meters).


4. Purification of flushing fluid


The flushing fluid used in hydraulic impact rotary drilling should be selected according to the formation conditions. When drilling into complete or relatively complete formations, clean water can be used; when drilling into complex formations, in order to protect the hole wall, reduce the loss of flushing fluid and increase the powder discharge capacity, dispersible mud, non-dispersible low solid phase mud or Solid-phase flushing fluid is used for drilling as flushing fluid. When performing diamond impact rotary drilling, lubricating flushing fluid should be used for drilling.

No matter which flushing fluid is used, in order to prevent large-grained rock powder and other debris from clogging the waterway of the impactor or jamming the moving parts of the impact frame, causing work failure, in order to extend the service life of the impactor, when performing impact rotary drilling, In particular, strengthen the management and purification treatment of flushing fluid. Commonly used methods for purifying flushing fluid mainly include the ground circulation system gravity sedimentation method, filtration method and hydro cyclone sand removal method.

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