The rock fragmentation process of the hole bottom of DTH Bit:
1. The process of crushing rock at the bottom of a hole with a surface-mounted diamond bit
Consider a diamond on the drill bit as a cutting tool that moves along the rock surface under the action of axial pressure Py and tangential force Px. When the rock is broken, the tangential force Px is consumed to overcome the reaction force Pg and friction force T against the rock. Upper T=Pyf.
In the formula: f--the friction coefficient between diamond and rock; Py--the load distributed on a diamond.
1-Crushed rock; 2-Tensile stress in the rock; 3-Crushed and sheared rock; Ⅰ-bit body; Ⅱ-surface-inlaid diamond edge; Ⅲ-rock
When the diamond moves along concentric circles, it transfers energy to the rock. The rock absorbs the energy and breaks and forms small grooves. In elastic and brittle rocks, the width of the grooves greatly exceeds the depth of the diamond into the rock due to the generation of large and small shear bodies. While breaking the rock, the diamond is gradually blunt. At this time, the blunt diamond strains the rock under the action of the axial load. The result of the strain is that some tiny cracks appear in the rock, which changes the dense structure of the rock. The number of cracks and their depth depend on the size of the axle load transferred to each diamond and the speed of the drill bit. After the diamond is blunt, repeated supplementary loads at a certain point at the bottom of the hole may cause the rock to break, that is, the rock breaking process at this time has the nature of fatigue fracture.
2. The process of rock crushing at the bottom of the hole of the impregnated diamond bit
The process of crushing the bottom of the hole with impregnated diamond bits is different from that of surface-mounted diamond bits. Because the diamond particles used are small and buried in the carcass, the impregnated bit maintains the self-sharpening performance (called "self-sharpening") during the drilling procedure to maintain a constant drilling speed without attenuation.
Generally speaking, the actual rock fragmentation mechanism of impregnated drill bits is similar to the grinding wheel grinding workpiece, that is, the workpiece (the rock at the bottom of the hole) is scored and ground with many and small hard spots (diamonds) on the lip surface. And with the gradual wear and disappearance of the hard spots and the continuous wear of the bonded carcass, the new hard spots are exposed to work. Of course, if the diamonds on the lip surface are well exposed, it is not ruled out that the micro-shearing and micro-crushing of the bottom of the hole will occur while grinding. In fact, the grinding wheel itself is worn during high-speed grinding, and is often in a sharp state due to its "self-sharpening" effect. For an impregnated diamond bit, if the matrix performance is not suitable for the drilled rock or sufficient drilling pressure is not guaranteed, the matrix cannot be worn in advance and the diamond particles that have lost the ability to break the rock can fall off by themselves, and "self-sharpening" cannot be achieved. . In the process of the hole bottom, the drill bit "slips" and the drilling speed drops rapidly.
The working process of the table setting or impregnated drill bit at the bottom of the hole is very complicated. The drill bit is pressed tightly against the rock surface at the bottom of the hole by the drill tool. The vibration and elastic deformation of the drill tool strengthen or weaken the drilling pressure at the bottom of the hole, and the pressure fluid sent by the reciprocating pump is also pulsating, which intensifies the impact vibration. Therefore, scholars believe that there is still a rock breaking process caused by micro-movement at the bottom of the hole.
Due to the brittleness of diamond, poor impact resistance and strong heat sensitivity, its use conditions are much stricter than other drill bits.
1. Requirements for the cleanliness of the hole bottom
Before entering the diamond drill bit, use a cemented carbide drill bit with a powder tube similar in shape to the diamond drill bit for workover. Use water to wash up the big rock debris, broken metal and other objects at the bottom of the hole and sink into the powder pipe.
2. The new drill bit is first grinding
The diameter of the new diamond drill bit selected is smaller than that of the non-diamond drill bit, and it is run-in drilling. The initial rotation speed should be 40-50r/min, the initial WOB is 1/8 of the normal WOB, and the drilling should be about 0.15-0.2m before the normal drilling.
3. Selection of parameters for normal drilling
(1) The weight on bit is determined by considering the lithology of the formation and the lifting force at the bottom of the hole caused by hydraulic washing. Generally, the maximum specific pressure of the bit lip surface in soft formations is 6.9 MPa. In hard formations, due to the small cutting depth of diamond and the small contact area with the rock, and in hard formations, the diamond is easily broken due to overload, so a smaller specific pressure is recommended, generally 5.1MPa. The WOB values of diamond bits of different diameters used in different formations can be selected by pressing. The WOB value for normal drilling should first select the lower limit of the curve, and gradually increase the WOB with the dullness of the diamond, but it should not exceed the upper limit of the curve. The WOB value obtained by estimation or checking the curve should also add the influence of the buoyancy and lift-off force in the hole to be the actual WOB value.
(2) After the drill bit has run-in, the ideal rotation speed should be as high as possible to give full play to the benefits of DTH Bit. It is generally considered that the rotation speed is 150r/min, and the rotation speed is increased to 300r/min under the conditions permitted by the formation conditions, drill pipe quality and equipment capacity. When the diamond bit is matched with the turbo drill, it has been successfully used at 600～1100r/min and achieved good results.
(3) Pump volume The pump volume DTH Hammer should first meet the requirements of low backflow velocity in the annular space, and at the same time ensure the need to clean the bottom of the hole and cool the drill bit. The recommended pumping range for drills of different diameters is given, the lower limit is used for high-density mud, and the upper limit is used for low-density mud.
Satisfaction Of The Working Conditions Of Dth Hammer
The rock fragmentation process of the hole bottom of DTH Bit: