The structure and rock breaking mechanism of PDC drill bits
PDC drill bits are made of polycrystalline diamond cutting blocks, and a certain number of cutting blocks are then embedded (or sintered) onto the drill body.
The cutting block is composed of a thin layer of artificial polycrystalline diamond (about 0.6-2.5mm) and a bottom layer of tungsten carbide sintered into a whole, forming a flat cylindrical shape, and is called polycrystalline diamond composite sheet (referred to as composite sheet). Weld the composite sheet and cylindrical body together to form a cutting element.
The structure of PDC drill bits is divided into two series based on different materials and manufacturing processes: steel body and matrix body.
Parabolic type: It has a larger drill surface and can be fitted with more cutting blocks to improve the wear resistance and footage of the drill. Its shape can concentrate the lateral thrust towards the center, reduce the deviation of wellbore direction, and is conducive to overcoming wellbore deviation.
Double cone type: It has an inner cone and an outer cone, which can maintain the stability of the drill bit. The outer cone surface is longer and can be embedded with more cutting blocks. When drilling hard interlayer or hard formation, the top cutting block is easily damaged under impact.
Short cone: It is beneficial for drilling through hard interlayers, and the surface area of the drill bit is small, so the hydraulic power is relatively concentrated and the cleaning is better.
Rock breaking mechanism of PDC drill bits
The PDC drill bit cutting block is used to break rocks by scraping and shearing. The self sharpening cutting block can cut into the formation under the action of drilling pressure, and then move forward to shear the rock under the action of torque. Fully utilize the low shear strength of rocks. When PDC drill bits break rock, there is no repeated cutting caused by the "pressure difference" caused by the cone bit teeth, similar to the pressure difference when breaking rock.
How cone drills break rocks
Impact and crushing effects of cone drills
During drilling, the drill bit and its cone rotate around the axis of the drill bit. The teeth of the cone rotate around their own axis under the resistance of the formation rock, and the teeth alternately contact the bottom of the well with a single or double tooth.
When the tooth wheel contacts the bottom of the well from a single tooth to a double tooth, the center of the tooth wheel drops from the highest position to the lowest position; When the tooth wheel contacts the bottom of the well again from double teeth to single teeth, the center of the tooth wheel rises from the lowest position to the highest position. This repeated movement periodically raises and lowers the axis of the cone, causing the drill bit to vibrate longitudinally, with the amplitude being the vertical displacement of the wheel center. During each longitudinal vibration process, the upward movement of the axis compresses the accumulated elastic performance of the lower drill string, while the downward movement of the axis causes the elastic elongation of the lower drill string to release the elastic performance. Therefore, when a cone drill bit crushes rocks at the bottom of the well, the force exerted by the teeth on the rock not only includes the static load generated by the drilling pressure, but also the dynamic load generated by the teeth rushing towards the rock at a high speed due to longitudinal vibration. The former causes the teeth to crush the rock, which is called crushing effect; The latter causes teeth to impact and break rocks, known as impact action.
The impact load of the drill bit is beneficial for breaking rocks, but it can also cause premature failure of the drill bit bearings, causing teeth, especially hard alloy teeth, to collapse and cause fatigue failure of the drill string. Therefore, in drilling wells, especially in hard formations, shock absorbers should be used.
The shear effect of cone bit
The shear effect of a cone bit is achieved by the rolling of the cone at the bottom of the well and the sliding of the teeth against the rock at the bottom of the well. There are three factors that cause sliding: overtop, complex cone, and axis shift.
Sliding caused by overtopping (sliding direction perpendicular to the tooth axis, i.e. along the tangent direction)
Sliding caused by axis shift (sliding direction along the axial direction of the tooth)
Sliding caused by complex cones
The compound cone cone includes the main cone and the auxiliary cone. If the main cone top coincides with the center of the drill bit, the auxiliary cone top must be the top of the super top, causing sliding in the tangential direction.
The axial sliding caused by shifting the axis can shear off the rock between the gear rings, while the tangential sliding caused by the super top and complex cone can shear off the rock between the adjacent tooth fracture pits of the same gear ring. Sliding increases tooth wear.
For drill bits in extremely soft to medium hard formations, there are generally both axial displacement, superelevation, and compound cone; Drill bits in medium or hard formations have super top and double cone. In extremely hard formations, the selected drill bit is pure rolling (single cone, no overtop, no axis shift).
Common tooth shapes of cone drills
The basic requirements for teeth are high rock breaking efficiency and long service life. To meet this requirement, firstly, the geometric shape of the teeth should be reasonable; The second is that the material of the teeth should be wear-resistant and have sufficient strength.
At present, there are two types of teeth for cone drills: milling teeth (also known as steel teeth) and hard alloy teeth (referred to as insert teeth).
L Milling tooth profile
The teeth of the milling cone bit are integrated with the cone shell, which is formed by milling the rough cone. In order to improve the wear resistance of the teeth, hard alloy powder is applied to the tooth surface.
The tooth profile of milling teeth is mainly wedge-shaped teeth. The outer diameter retaining teeth can be made into P-shaped, T-shaped, or L-shaped shapes.
L Insert tooth shape
a. Wedge shaped teeth: suitable for crushing soft and medium hard formations with high plasticity, with teeth sharp angles ranging from 65-90. Those with small teeth sharp angles are suitable for soft formations, while those with large teeth sharp angles are suitable for harder formations.
b. Scoop Chisel: There is also a type of wedge-shaped tooth that is asymmetric on both sides and concave on one side, called Scoop Chisel, suitable for drilling soft formations.
c. Conical teeth: There are two types: single cone and double cone, which break rocks by crushing. The 60 ° -70 ° medium conical teeth are used to drill medium hard formations, such as limestone and dolomite. 90 ° cone and 120 ° double cone teeth are suitable for hard formations with high abrasiveness, such as hard sandstone and quartzite.
d. Triple edged teeth: formed by cutting a plane every 120 ° on the conical surface of double conical teeth, suitable for intersecting layers of hard and brittle formations and soft and plastic formations.
e. Spherical teeth: With high strength and wear resistance, they can crush and impact hard formations with high abrasiveness, such as flint, quartzite, basalt, and granite.
f. Projectile shaped teeth: It is a deformation of spherical teeth, with higher teeth but certain strength, suitable for hard formations with high abrasiveness.
g. Flat top shaped teeth: cylindrical teeth with chamfers at the ends, used only on the back cone of the tooth to prevent wear and maintain the gauge diameter.
h. Oblique wedge-shaped teeth: Basically wedge-shaped, but the tooth edge is narrow at one end and wide at the other end. Used on the outer teeth of drill bits in medium to soft formations. The wide side has high wear resistance and is installed on the outer edge of the drill bit to cut the wellbore wall and prevent the drill bit from becoming smaller.
The Origin and Development History of Roller Bits
The Origin of Roller Bits
The cone bit originated in 1909. In the following decades, cone drills have made great progress, with significant improvements in the material, cutting parts, drill bearings, cleaning devices, and more.
In 1925, a self-cleaning roller bit was developed. It solves the problem of rock debris accumulation between the teeth of soft formation drill bits, which is prone to mud pockets.
In 1933, a three cone drill bit with rolling bearings was developed.
In 1935, after further improvement, a cone offset tricone bit was developed. In the salt layer, red layer, gypsum layer, limestone layer, and shale, the drill bit index has increased.
Three Major Revolutions of Roller Bits
In 1949, jet cone drills were introduced. At that time, the mechanical drilling speed could be increased by 50%, and the usage of the drill bit increased from 33% to 65%.
In 1951, a hard alloy ball tooth roller bit was developed. In the extremely hard abrasive flint layer, the drill bit's footage has increased from 1 meter to over 10 meters.
In 1960, a sealed lubricated bearing cone bit was successfully developed. Make the working time of the drill bit reach 40-60 hours, and increase the drill bit footage by 50%.
In 1968, a sealed lubrication sliding bearing roller bit was successfully developed. It has more than doubled the working time of the drill bit, reaching 80-120 hours, thereby accelerating the drilling speed and reducing costs.
Nowadays, the superiority of jet sealed lubrication (rolling or sliding) bearings embedded with hard alloy tooth drill bits (referred to as three-in-one or four-in-one drill bits) has been demonstrated in practice, and its technical and economic indicators greatly exceed those of ordinary cone drills.
Recommendations for the use and maintenance of non excavation tooth wheel reamers
The use of non excavation reamers requires attention to the following aspects:
1> Before assembling the drilling tool onto the drill pipe, it is necessary to check the condition of the drill bit or reamer. If it is a used drill bit or reamer, it is even more important to check:
① Do the wheels frame?
② Is the rotation very loose?
③ Is there excessive wear on the tip of the dental wheel palm (the area where the back meets the wheel)? If the rubber ring is exposed, it cannot be used again.
④ Are the threads on both ends excessively worn?
⑤ Is the size of the tooth wheel nozzle suitable for the construction layer? Is it unobstructed?
⑥ Is the centralizer excessively worn? The centralizer is generally 20-30mm smaller than the front hole (the centralizer in broken rock or pebble layers can be smaller than the front hole). The centralizer can stabilize the reamer and reduce wear on the reamer and tooth wheel matrix. If the centralizer is worn too much, it should be repaired in a timely manner.
⑦ Check if there are any open welding areas on the base body of the reamer?
⑧ If it is a used drill bit or reamer, it is necessary to determine whether the reamer can be completed in one go based on wear, crossing length, rock hardness, etc?
Wait a minute.
After completing the above inspection and confirming that there are no problems, the lower hole can be assembled.
2> During the construction process, as a manufacturer of rotary drilling tools, we only provide the following suggestions for speed, drilling pressure, and mud displacement:
① Rotational speed: According to the instruction manual provided with the drilling tool, carry out the construction at the rotational speed.
According to market research, most drilling rig manufacturers currently do not have instruments to display the RPM of the drilling tool, which is the number of revolutions per minute.
For example, the RPM of an 8 1/2 inch rubber sealed roller bit is 60-100 revolutions per minute. It can be calculated that the range of revolutions per minute for a 330mm reamer assembled with the roller palm of this 8 1/2 inch rubber sealed roller bit should be 39-65; It can also be calculated that the RPM range of the 500mm reamer assembled with the 8 1/2-inch rubber sealed cone bit's cone palm should be 26-43.
After market research, it has been found that the RPM of most tooth wheel reamers has not received the necessary attention during use, resulting in the rubber sealing ring losing its sealing effect on the bearing prematurely due to exceeding the usage range of revolutions per minute.
Metal seals belong to high speed drill bits and can withstand approximately twice the rotational speed of rubber seals. At higher rotational speeds, metal seals still have a sealing effect on bearings.
This also has to make us think, can we improve the service life of rubber sealing teeth by increasing the tension and controlling the number of revolutions?
Although there may be slight differences in overall physical properties between rubber sealed teeth and metal sealed teeth, the main difference is still in the speed and lifespan of the bearings. Can we save a lot of drilling costs by using rubber sealed teeth well? This is just a reflection, you can study it.
② Drilling pressure. In oil drilling, we generally refer to it as drilling pressure, and in non excavation, it is called tensile force.
When using a tooth wheel reamer, we should pay attention to the tension and not exceed the range of tension specified in the product's accompanying instructions.
Especially when assembling smaller diameter reamers such as those below 300mm, it is inevitable to use smaller dental teeth, and it is even more important to note that the tension should not exceed the range of use.
③ Regarding mud, as a manufacturer of rotary drilling tools, I have limited construction experience and can only provide limited suggestions as follows.
Pay attention to the mud ratio, observe the instrument during use, and determine the sand carrying situation of the mud. As a drilling tool manufacturer, we hope that the large pieces of rock that have been broken during the operation of the drilling tool can be discharged as soon as possible, rather than repeatedly breaking in the hole, causing excessive wear and tear to the teeth and parent body of the tooth wheel and rinsing. If the sand carrying performance of the mud is poor, it will greatly reduce the construction progress.
After stopping midway and starting the construction again, it is necessary to increase the displacement for sand removal. According to our experience, restarting the drilling rig in the morning is the time when the most jamming occurs.
3> Maintenance and storage of drilling tools.
① Whether it is a roller drill or a reamer, it is necessary to rinse it thoroughly with clean water after use and keep a record of the use of the drilling tool.
The record content includes: project name, location, drilling time, drilling time, effective drilling time (minus rest time), crossing length, average tension, rotational speed, rock type hardness, drilling tool wear, etc.
If the drilling tool is excessively worn, it should be repaired in a timely manner for the convenience of next use.
② When flushing the drilling tool with clean water, it is important to pay attention to flushing the threads and cleaning the thread end faces. After wiping them dry, apply thread oil and assemble thread protectors. Those without wire protection caps can be wrapped in ordinary packaging such as bubble paper to avoid collisions.
③ Even metal sealed roller drills have rubber sealing rings inside, and prolonged exposure to sunlight can affect the sealing performance of the rubber. Whether it is a metal sealed roller bit or a rubber sealed roller bit, it must be placed in a cool and dry place. Composite sealing is a type of rubber sealing that also requires the same treatment.
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