Before pre-expanding, an optimized reaming plan should be formulated, and the maximum reaming diameter and number of reaming should be determined according to the equipment capacity, stratum conditions and site conditions. (The final reaming diameter should be 1.25～1.5 times of the diameter of the pipeline to be laid). When pre-reaming, choose a reasonable reamer type and diameter, and determine the next-level reaming method according to the previous reaming situation.
① Selection points of rock hole reamer:
Choose rock reamer according to the type of traversing stratum: The type of traversing stratum is an important factor in the correct selection of the size and type of rock reamer hole opener. When reaming soft rock formations, larger diameter reamers and fewer reamers are allowed for the number of holes, the size difference between two adjacent reamers can also be larger, the level difference is 100-200mm; the reaming diameter required when reaming hard rock formations should not change too much, the level difference is 50-150mm and multiple reaming is adopted. In order to achieve the best reaming efficiency and smooth discharge of drill cuttings.
② Key points of reaming operation control:
Different diameters and different types of rock reamers have different requirements on WOB, especially during the reaming of long-distance rock formations, reasonable tension and torque must be controlled to avoid excessive wear of the reamer. Most rock reaming accidents are caused by excessive pursuit of reaming speed and excessive drilling pressure. An important principle that should be followed in rock reaming operations is: the harder the rock to be broken, the greater the drilling pressure required and the lower the drilling speed required. Under the same drilling speed conditions, the larger the diameter of the reamer, the greater the linear velocity of the outer edge and the more severe the wear.
③ Key points of hole reaming mud control:
When reaming a rock layer, mud is also a very important factor. At this time, the main function of mud is to cool the reamer and carry drill cuttings. The mud is generally made up of water, bentonite and mud additives. The mud should have proper viscosity and fluidity, and the ability to carry drill cuttings should be strong, so that the drill cuttings will be suspended at the bottom of the hole. The flow of the mud can make the drill cuttings return smoothly ground. The reflow in the hole has a certain relationship with the unobstructed condition in the hole. If there is no reflow phenomenon, it may cause pipe laying failure, and the actual reason should be analyzed. If the hole is not smooth and the water in the mud also leaks, the remaining mud skin will accumulate and thicker, and the pipeline will be easily “hugged”, causing great resistance to the pipe. In the case of normal pipe drawing, there will generally be a phenomenon of reflow. The actual mud displacement should be determined after calculation based on the amount of drill cuttings produced by each stage of reaming and the carrying capacity of the mud.
④ Control of key points of reaming:
The construction method of the reamer entering the rock layer: When the rock reamer enters the rock layer from the soft soil layer or enters the hard rock layer from the soft rock layer, the main point of operation is to first slow down the reaming speed, reduce the drilling pressure, and wait for the reamer body Increase the WOB after completely entering the rock layer or hard rock layer. If the drilling pressure applied during this process is too large, it is likely to cause jamming and damage the reamer. When pre-expanding the hole, slow down the speed of the hole in the place where the guide hole curve is not ideal and the boundary of the formation, and the curve should be adjusted appropriately.
⑤ In the process of hole cleaning, the mud displacement must be controlled to maximize the carrying out of drill cuttings. In addition, in order to clean up the drill cuttings in the hole, prevent mud hydration in the hole, drill cuttings settling, buy time for dragging the pipeline, and more importantly, check whether the tension and torque along the guide hole are normal.
2) Suitable for rock reamer hole opener of various diameters (more suitable for small and medium diameters), and suitable for various formations (diameter 220-1600mm).
3) According to the characteristics of the drilling rig and different stratum conditions, select the most suitable tooth type and bearing structure.
4) The back of the palm is inclined, and the anti-wear alloy is cold inlaid to reduce wear.
5) The unique anti-drop technology of the cone prevents the cone from falling into the hole after the cone bearing is damaged, causing an accident in the hole.
6) Hydraulic rock breaking, smooth slag discharge, reducing slag accumulation.
7) The welding process is guaranteed to prevent overheating of welding from damaging the roller bearing. The welding angle of the cone palm piece is well controlled, which increases the service life of the bearing.
8) The guiding spiral belt design of the drill body has good guiding and slag guiding effects, reducing repeated breakage of the drill bit.
2. Tool holder type replaceable rock reamer hole opener for medium and small drilling rigs
This kind of double fulcrum tool holder support structure can replace the hob rock reamer hole opener, suitable for small and medium drilling rigs, and has the following characteristics:
1) Solved the abrasion problem of the palm back of the reamer with cone structure.
2) The cost is almost the same as that of the rubber-sealed cone reamer, which reduces the cost of use.
3) Can completely solve the problem of falling rounds.
Theoretically, the drilling pressure depends on factors such as formation rock hardness, diameter, drilling speed, drilling rig, drilling tool capacity and drill bit structural strength.
The harder the formation, the larger the diameter, and the greater the WOB required. The higher the weight on bit, the faster the drilling speed. However, if the drilling pressure is too large and the hob is pressed into the rock too deeply, the drill bit will be broken and cannot be rotated, causing damage to the drill tool and the drill bit and causing an accident under the hole. At the same time, the bearing life of the cutting tool is directly related to the weight on bit. Therefore, the drilling pressure should be limited.
During the specific operation, the drilling pressure shall be matched with 1/2-3/4 of the normal torque value of the drill rod, and the value shall not exceed the recommended value of the under-reamers-hole openers manufacturer.
2. Speed
The rotation speed of the drill changes with the diameter of the drill bit. Generally, the linear speed of the outermost cutting tool of the drill bit is not more than 3m/s. Under normal circumstances, the rotation speed of the drill pipe is controlled within 40RPM.
3. Mud displacement
The role of mud is to carry cuttings and cool the bit. In theory, the larger the mud displacement, the better. In order to reduce drill bit wear in sandstone formations, pump capacity should be increased as much as possible.
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The control of the pilot hole curve is very important. Choose the best combination of drilling tools according to the type of strata crossing. In the rock construction process, for the high strength of the rock layer, you can use: (tri-cone bit + power drilling tool + probe chamber + non-magnetic drill collar + weighted drill pipe + ordinary drill pipe) drill tool assembly summarized the following experiences during the construction process:
① When the directional drill crosses a long distance, the frictional resistance increases, and the thrust of the drilling rig is not easy to reach the drill bit. The casing technology and the use of weighted drill pipes can be used to coordinate drilling measures, which can effectively improve the drilling efficiency and prevent the that the parts of the wellhead and the clay layer are deformed and unstable because of the long distance of the drill pipe.
② Because the rock formation is relatively hard and difficult to drill when drilling pilot holes, we use a mud motor as a power drilling tool, and its operation is completely driven by mud.
Due to the high mud pressure and fast flow rate, the insulating shrink sleeve of the joint of the control line is constantly scoured, causing it to repeatedly rub against the inner wall of the drill pipe and rupture, forming a short circuit or open circuit, causing signal interruption. When drilling the pilot hole, a retaining ring is added to the mud drill pipe cavity to relieve the impact of the mud on the control line to protect the control line, which can effectively prevent the loss of signal due to the damage of the control line. This measure has a good effect.
③ Analyze the operation of ground equipment when drilling pilot holes.
The long pilot hole distance is complicated and the formation is complicated. Cuttings, weight on bit, torque, ROP, pressure and other surface instruments can directly reflect the downhole conditions. Pay close attention to when drilling pilot holes, especially torque, pump pressure, and ROP. And rock cuttings, record important data in the steering process, analyze and judge the changes in the formation.
④ Preparation of mud.
In addition to the functions of cleaning and cooling the drill bit, carrying cuttings and assisting rock breaking, the main purpose of the mud is to protect the well wall. The clean bottom of the well can not only increase the drilling speed, but also reduce the wear of the drill bit. We prepare the mud suitable for the formation according to different formations. When drilling the pilot hole, we strictly control the viscosity, specific gravity, displacement, and pressure of the mud. In order to assist the system, adjust the ratio of mud preparation, play the role of mud in direction control, and make the crossing curve “flat, smooth, round and gentle”.
⑤ It is difficult to find the rules when the drilling pilot holes. During construction, adjust the drilling speed of each drill pipe in time within the range allowed by the construction specifications, control the direction control parameters of each drill pipe, and ensure that the deviation of each rod is the minimum value. Because if the deviation of the pilot hole is too large, it will be difficult for the drill to break away from the original hole to drill a new hole when part or all of the drill rod is pulled out to correct the deviation. A curve with a large angle is formed at the intersection of the original hole and the new hole, which cannot meet the requirements of the radius of curvature of the pipeline.
In oil drilling and geological drilling, the cone drill bit is the most widely used. When the bit rotates, it can impact, crush and shear the stratum rock. Therefore, cone drill bit can adapt to soft, medium and hard strata. Especially after the emergence of jet type cone drill bit and long nozzle cone drill bit, the drilling speed of the bit has been greatly improved, which is a major revolution in the development history of the bit.
According to the type of teeth, cone drill bit can be divided into milling (steel teeth) cone drill bits and insert (carbide teeth inlaid on the cone) cone drill bits. According to the number of cone, it can be divided into single cone drill bit, tricone drill bit and assembled multi cone drill bit. The tricone drill bit is the most widely and commonly used. The cone suppliers shall pay attention to whether the quality inspection is qualified when selling the cone drill bit.
Tricone drill bit is an important tool in oil drilling. Its performance will directly affect the quality, efficiency and cost of drilling. The effective way to reduce drilling cost is to increase the mechanical drilling speed and service life of bit. It has been proved by some data that the improvement of drilling rate has more influence on reducing drilling cost than the improvement of bit service life. The improvement of bit cutting structure is the most direct way to improve the mechanical drilling rate.
For a long time, the improvement of the cutting structure has focused on the main cutting tooth structure. For example, in the structure of cemented carbide teeth, people have invented wedge teeth, spoon-shaped teeth, partial top spoon-shaped teeth and equal wear tooth, etc. It is gradually recognized that the diameter maintaining structure also has a great impact on the bit mechanical drilling speed and service life. In particular, the bit used in highly deviated wells and horizontal wells requires a reasonable diameter maintaining structure. Many new types of diameter maintaining structures have been developed and applied to cone drill bits with good results.
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PDC bit is the abbreviation of Polycrystalline diamond compact bit, also known as Polycrystalline diamond cutting block bit or composite chip bit. Because of its advantages of fast drilling speed, long service life and high footage PDC bit is widely used in oil drilling.
1. Structure characteristics of PDC bit
PDC bit is composed of bit body, PDC cutting teeth and nozzle. According to different structure and manufacturing process, PDC bit is divided into two series: steel body and matrix body.
The whole bit body of rigid PDC bit is made of medium carbon steel and formed by machine manufacturing process. Drill holes on the working face of the bit, and fix PDC cutting teeth on the bit crown by pressing in and tight fitting. The crown of a drill bit is treated by surface hardening process (spraying tungsten carbide wear-resistant layer, carburizing, etc.) to enhance its erosion resistance. The main advantage of this kind of bit is simple manufacturing process, but the disadvantage is that the bit body is not erosion resistant and the cutting teeth are difficult to be fixed, so it is seldom used at present.
The upper part of PDC bit is made of steel and the lower part is made of WC wear-resistant alloy. The PDC bit is made by powder metallurgy sintering process. The PDC cutting teeth are welded on the preformed groove of the carcass with low-temperature solder. Tungsten carbide matrix has high hardness and erosion resistance, so the matrix PDC bit has long service life and high footage, which is widely used at present.
2. Working principle of PDC bit
PDC bit breaks rocks by cutting. The self-sharpening cutting teeth can easily cut into the formation under the action of WOB and move the shear rock forward under the action of torque. Many PDC cutting teeth work at the same time, there are many free faces of the bottom rock, and the rock is easy to be broken under the shear action, so the rock breaking efficiency is high and the drilling speed is fast.
3. Correct use of PDC bit
PDC bit has the best effect in large homogeneous soft to medium-hard formation. It is not suitable for drilling gravel layer and soft hard cross formation.
Drilling with low WOB, high rotation speed and large displacement has a good effect.
Before the bit is put into the well, the bottom of the well shall be clean to ensure no metal falling.
When the bit is just put into the well, it should adopt small bit pressure and low rotation speed to run and resume normal drilling after the bottom of the well is formed.
PDC bit is an integral bit, without any moving parts, suitable for high-speed turbo drilling.
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Drilling work is a complex and diversified system engineering. In order to ensure the successful completion, coordination and cooperation of multiple systems are needed, among which the reasonable selection of bit is the prerequisite to ensure the high quality and efficient completion of drilling work. Nowadays, the most widely used bits in drilling engineering are roller bits and diamond composite bits (PDC). This paper analyzes the application of roller bits and PDC bits in drilling engineering.
1. Definition of roller bit and PDC
1.1 The roller bit is the most widely used drilling bit. When the bit is working, the cutting teeth alternately contact the bottom of the well, the rock breaking torque is small, the contact area between the cutting teeth and the bottom of the well is small, the specific pressure is high, and it is easy to dig into the formation; the total length of the working edge is large, so the wear is relatively reduced. The cone bit can adapt to a variety of strata from soft to hard. According to the number of cone, it can be divided into single cone bit, three cone bit and assembled multi cone bit. According to the cutting material, it can be divided into steel teeth (milling teeth) and inserted teeth cone bit.
1.2 PDC bit is a polycrystalline diamond composite chip drill, and refers to the abbreviation of the head. The shape of the composite chip is a circular cutting tool which is inlaid and welded on the cylinder, and the cutting tool is inlaid on the drill body to become a PDC drill. PDC bit is much cheaper than natural diamond bit, but it is only suitable for soft to medium hard formation. PDC bit developed rapidly after it was manufactured and used successfully in the early 1970s. It can be used with low WOB and high rotation speed, high bit footage and low unit footage cost. In some oilfields of China, diamond polycrystalline composite chips are used for inlaying and welding on scraper, and great effects are obtained.
At present, PDC bits are designed with cutting wings. With the increase of the number of cutting wings, the impact load acting on the cutting teeth decreases, the controllability of the bit increases, and the life of the bit extends. On the contrary, with the decrease of the number of cutting wings, the drilling performance of the bit improves, and the bit has a higher drilling speed. Generally speaking, the larger the formation strength is, the more the number of cutting teeth is, and the more the number of cutting wings is in general. The number of PDC bit wings is no less than 3, but there are few more than 10.
2. Comparative analysis of rock breaking mechanism between roller bit and PDC bit
2.1 The mechanism of rock breaking by roller bit. Under the action of bit pressure and drill string rotation, the teeth crush and dig into the rock, and at the same time produce certain sliding and shearing rock. When the cone rolls at the bottom of the well, the teeth on the cone successively impact and press into the formation. This effect can crush part of the bottom rock, and cut off the remaining part of the rock between teeth by the shear effect brought by the roller sliding, so that the bottom rock is completely broken and the well can be extended.
2.2 Rock breaking mechanism of PDC bit. The working principle of PDC bit is basically the same as that of scraper bit.
(1) When PDC bit is drilling into some hard formation, it presses the rock under the action of WOB, which makes the rock in contact with diamond in a very high stress state and makes the rock appear plasticity.
(2) In the plastic (or rock is plastic under stress) formation, diamond digs into the formation and causes the front rock to break or plastic flow under the action of bit torque, which separates from the rock matrix and forms rock debris.
(3) In the rock with high brittleness, the stress produced under the action of WOB and torque makes the rock show brittle fracture, that is to say, the rock is destroyed by shear and tension. In this case, the rock breaking speed of diamond bit is higher, and the volume of rock breaking is much larger than that of displacement after diamond digging.
(4) In hard rock, due to the limitation of the strength of diamond itself, the bit pressure on the larger size diamond is not enough to cause the plastic deformation in the rock, so the diamond with fine particles is generally used to make impregnated diamond bit for drilling, which is characterized by micro cutting, carving and other ways to break the rock depending on the angle of diamond. The cuttings thus separated are basically fine-grained powder, and the working efficiency and service life of the bit are very low.
3. Applicable conditions of rotary rock drill bits and PDC bits
3.1 The application scope of rotary rock drill bit is the most widely used in petroleum drilling and geological drilling. The rotary rock drill bit has the effect of impacting, crushing, and shearing the formation rock when it rotates. Therefore, rotary rock drill bit can adapt to various formations, soft, medium, and hard. Especially after the emergence of jet cone roller bits and long nozzle cone roller bits. The drilling speed of roller cone bits is greatly increased. It is a major revolution in the development history of roller cone bits. According to the type of teeth, roller cone bits can be divided into milled teeth (steel teeth) roller cone bits and inserts (hard alloy tooth roller cone bits are installed on the roller cone; according to the number of roller cones, they can be divided into single rotary rock drill bit and tricone drill bit and assembled multi-cone bits. The tricone drill bits are the most widely used at home and abroad.
3.2 Application scope of PDC bit at present, often uses well section, oil-based mud, or water-based mud used in non reducing well section where bit needs to work for a long time at the bottom of the well. PDC bit also has advantages in high speed drilling tools, such as turbodrill, screw drill or deviation section.
PDC bit is often used in the formation with low compressive strength, poor cementation, low abrasiveness or thin interlayer, such as salt rock, gypsum rock, marl, limestone, etc., and the formation with medium compressive strength, certain abrasiveness and brittleness, such as sandstone, siliceous shale, porous carbonate rock and gypsum rock. But PDC bit is in high compressive strength, tight cementation like the abrasive sandstone, chert and granite, corundum, pyrite and grave, the effects are not good.
Many service companies and drilling contractors use rock compressive strength analysis software to determine the hardness of the formation. These computer analysis models of rock compressive strength use logging analysis technology to analyze and determine the triaxial compressive strength of the formation. Rock compressive strength analysis software can help the drilling contractor to determine the drillable well section of PDC bit, optimize bit selection and determine the optimal drilling parameters. The development and application of this software can promote the use of PDC bit.
4. Advantages and disadvantages of 4 cone bit and PDC bit
4.1 Advantages and disadvantages of roller bit
(1) The cone bit is suitable for various strata, so it is the most widely used bit.
(2) The rock cuttings produced by the broken rock of the cone bit are larger, which is helpful to distinguish the rock cuttings and judge the geological conditions more accurately.
(3) The service life of cone bit is not long enough and the footage is small, which increases the tripping times and affects the whole construction progress. Moreover, frequent tripping prolongs the immersion time of mud to oil and gas reservoir, causes serious damage to oil and gas reservoir and affects the exploitation of oil and gas.
(4) The core of cone bit can not be reused, which increases the drilling cost to a certain extent.
(5) Due to the limitation of its structural characteristics, the cone bit is easy to fall to the bottom of the well and cause downhole accidents.
4.2 Advantages and disadvantages of PDC bit
(1) PDC bit is suitable for high speed, high displacement and low WOB, so the requirement of equipment is relatively high.
(2) PDC bit’s working principle is basically the same as scraper bit, so it generally requires lower WOB and higher rotation speed. The bit pressure of PDC bit is generally 20-40% of that of the same size bit when it is used on site.
(3) PDC bit is composed of bit matrix and diamond composite sheet. When cutting rock, the composite sheet mainly contacts with the rock. The matrix will not be damaged basically, so the matrix can be reused. To a certain extent, the drilling cost is saved.
(4) PDC bit is not well used in abrasive sandstone, chert and granite, corundum, pyrite and gravel with high compressive strength and tight cementation.
(5) When PDC bit is drilling, micro cutting is realized by the edge angle of diamond. To break rocks by carving, etc. The cuttings separated in this way are basically fine-grained powder. It is not easy to accurately judge the downhole situation when cuttings logging is used in the field.
(6) Because of some characteristics of PDC bit itself, it causes the problems of deviation and azimuth control in directional well, so it is often used in field directional well.
5. Development prospect of 5 cone bit and PDC bit
The advantages of roller bit and PDC bit are very obvious in the process of drilling. The roller PDC mixed bit came out and was used in the field drilling, and achieved remarkable results. With the development of science and technology, there will be more and more advanced bit structure and drilling technology.
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In the process of drilling, drill bit is the main tool to break rock, and borehole is formed by a drill bit breaking rock. The formation of a well is good or bad, and the length of time it takes is not only related to the rock characteristics of the drilled formation and the performance of the bit itself, but also related to the matching degree between the bit and the formation. The reasonable selection of bit plays an important role in improving the drilling speed and reducing the comprehensive cost of drilling.
The bit is one of the important tools for oil drilling. Whether the bit adapts to the rock property and its quality plays a very important role in the selection of drilling technology, especially in the aspects of drilling quality, drilling speed and drilling cost. PDC bit is a rock breaking tool widely used in the oil and gas exploration and development industry today which effectively improves the mechanical drilling tools and shortens the drilling cycle.
Bit is one of the main drilling equipment. According to the different working environment and regional environment, the specification and shape of bit should also be different. When drilling oil, the bit should be selected reasonably and scientifically based on the specific needs and design scheme.
At present, there are many kinds of bits used in the petroleum industry. According to different drilling methods, the bits can be classified into diamond bits, roller bits and scraper bits. These three kinds of bits are the most basic ones.
Because of the excellent cutting performance of diamond drill bits rock, the diamond bit can drill at high speed and enlarge the drilling depth to a certain extent when being selected as oil drilling tool. In the process of drilling oil with diamond bit, the following aspects need to be paid attention to:
First, the price of diamond bit is relatively high, so we should operate it carefully to reduce the damage degree;
Secondly, the diamond bit has some defects in thermal stability, so the cooling performance and cleaning condition of the bit should be guaranteed when it is used;
Thirdly, its texture is relatively brittle, so the impact resistance of the diamond bit will be relatively poor, so we should strictly follow the relevant rules of the diamond bit to carry out strict and standardized operation.
When selecting a bit as an oil drilling tool, it is necessary to select the bit according to the actual situation of the drilling equipment, the actual conditions of the formation, the geological data of the adjacent oil wells and the formation data. During the selection, the following issues need to be considered:
First of all, we need consider whether there is soft and hard interlacing in the drilling formation;
Secondly, it should consider whether anti deviation drilling and curve operation are needed in the oil drilling work;
Thirdly, the actual depth of different drilling sections in the same well should be considered;
Finally, drilling geology, formation abradability and hardness should be considered.
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Horizontal Directional Drilling(HDD for short) is a modern trenchless construction technology that combines the directional drilling technology of the petroleum industry with the traditional highway pipe jacking technology.
In recent years, with the improvement of scientific and technological level, the horizontal directional drilling rig and its supporting technical tools have been continuously improved and perfected, the performance of the whole machine has been greatly improved, and the horizontal directional drilling crossing technology has also been widely used, especially in urban municipal pipeline construction and oil and gas pipeline crossing river projects have more unique advantages. The reamer is an important part of the horizontal directional drill bit to complete its reaming operation during the construction process, which will be directly related to the success or failure of the pipe laying.
Reamer is a kind of power equipment used for reaming pipes during reaming operation, including roller cone reamer and PDC reamer. The main function of reamer is to cut and expand the guide hole along the curve of the guide hole. Tungsten carbide alloy teeth and nozzles are installed on the reamer, the former is mainly used for cutting and squeezing rock and soil layers, and the latter is mainly used for mud jet.
The horizontal directional drilling crossing technology principle: through the computer-controlled guidance and detection, a small-diameter pilot hole with the same design curve will be drilled first, and then a reamer will be installed on the head of the drill rod at the outlet end of the pilot hole to pull back and expand the hole to guide. After the hole is expanded to the size that required, the rotary joint, the tube head and the pipeline will be connected to the rear of the reamer, and drag the product pipeline back to the enlarged guide hole to complete the construction process of pipeline crossing. The hole expansion operation is one of the key technical links in the horizontal directional crossing construction, and the key to the hole expansion operation is to select an efficient and safe reamer according to the actual conditions.
During the working procession, the forward movement of the reamer needs to overcome the positive pressure resistance and friction resistance of the rock layer or soil body, and the rotary movement of the reamer needs to overcome the resistance torque of the rock layer or soil body to the tool and the friction torque on the surface of the reamer.
The optimization of the shape and size of the reamer has a great influence on the resistance and torsion torque during the working procession, which is directly related to the working efficiency of the horizontal directional drilling machine. It can be seen that the analysis and research on the selection of reamer and engineering application are of great significance for the smooth progress of horizontal directional drilling construction.
The enlargement of horizontal directional drilling through the rock layer should focus on the following three aspects: the service life of the reamer, the torque value generated by the reamer at each stage of reaming, the efficiency and economics of reaming. In addition, the reaming process also plays a certain role in correcting the guide hole. If the guide hole is only bent at a small angle, after selecting the appropriate reamer, the ideal smooth hole can be achieved through the reaming operation. For example, in a hard and uneven formation, the reamer tends to deflect to the soft formation, that is, to the direction of least resistance, and long-term use will cause damage to the reamer. In practice, flexible choices should be made according to the conditions of the rock formation, the ability to traverse the equipment, the diameter of the hole expansion, and the condition of the traversing mud.
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During the drilling process of conventional PDC bit, based on the affection of the shearing action of PDC cutting teeth, most of rock debris obtained is mostly fine sand with very small size, which cannot be accurately geologically seized and evaluated.
The design concept of the core-while-drilling bit is to remove 1-2 teeth in the center of the normal PDC bit. During the normal drilling process, the core is continuously formed in the center area of the bit without affecting the cutting structure of the bit.
When the length of the small-sized core is continuously increased to the nominal length, it will contact a core destruction device and cut off the core using lateral force. There is a larger and deeper groove in the middle of the two front blades, and the small-sized core will move into the annulus through this hollow area. This hollow area is always kept open to prevent the risk of blockage of a drill bit.
Compared with the traditional PDC bits, the advantages of core-while-drilling bits are:
1. It can increase the mechanical drilling speed. The cutting surface of the common drill bit has the lowest drilling speed in the center position. The new drill bit avoids it.
2. It can improve the quality of cuttings and form micro-cores for geological analysis and evaluation without having to repeatedly drill, which saving costs.
3. The stability of the drill bit can be strengthened, and the central core column can effectively prevent the eddy and lateral vibration of the drill bit, so that the outer cone cutting teeth are prevented from being subjected to a large impact load, thereby improving the drill bit service life.
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