Comparative Analysis of Roller Bit and PDC Bit

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.