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         (1) The effect of different formation lithology on bit failure
     
         The influence of formation lithology on bit failure is now on the drilling skill: affecting the drilling speed and the drill footage; making the drilling process complex such as lost circulation, blowout, well collapse and stuck drill; changing the function of the mud; affecting the quality of the wellbore If the well is inclined and the diameter of the well is irregular, it will affect the cementing quality. After analyzing the lithology of the formation and its influence on drilling skills, the rationality of the selection and application of the drill bit can be discriminated.
     
        The influence of clay, mudstone and shale layer: it is very simple to absorb the free water in the mud and expand, so that the diameter of the well is reduced, which constitutes the resistance of the drill, and even the stuck drill, followed by the extension of the soaking moment, and the block will fall off. The caliper expands to form a well collapse. Try to use clear water or low specific gravity and low viscosity mud. Carbon shale has weak coupling and collapses easily. The argillaceous rock is soft, with a fast drilling speed and simple mud pack.
     
        Sandstone: Its properties vary greatly depending on the size of the particles, the composition, and the cement. The finer the particles, the more quartz particles, the harder the siliceous and iron cements, the harder the bit wear, such as quartz sandstone; the more muddy cement, the more components of mica and feldspar are softer. Easy to drill; the coarser the particles, the less the cement, the better the permeability, the leakage of the mud which is easy to break out, and the thick mud cake on the well wall, resulting in complicated conditions such as sticking and drilling, which constitutes the bit of the drill. Normal use.
     
        Conglomerate: Drilling in the conglomerate layer is easy to break, boring and wall collapse; when the pump displacement is small or the mud viscosity is low, the gravel particles are not easy to return, and the damage to the bit cone body and teeth is large.
     
        Limestone: usually hard, slow drilling speed and less footage. Some have seams and caves. When drilling holes, it will cause boring, venting, mud loss, etc. Sometimes a blowout will occur after a leak.
     
        Limestone formations have a significant impact on drill footage, ROP and bit failure. In addition, the local layer is soft and hard intertwined, such as mudstone and hard sandstone, easy to attack well; when the dip angle is large, it is easy to occur. Drilling in a deviated well can easily cause damage. When the rock layer contains soluble salts, such as gypsum layer and rock salt layer, it will damage the function of the mud and affect the normal operation of the drill bit.
     
        (2) Drilling skills
       
        Usually refers to the drillable, rotational speed and mud displacement three skill parameters that can be manipulated during the drilling process. In practice, drilling skills should be based on formation conditions, drill bit type, drilling equipment, and operator skill levels. According to their different needs and conditions, drilling skills are divided into:
     
        1) Optimize drilling skills: Drilling skill parameters that can reach the best economic goals under certain conditions.
     
        2) Intensive drilling skills: In order to reach higher drilling speeds, drilling parameters higher than the usual drilling parameters are selected.
     
        3) Special drilling skills: Special methods or constrained drilling parameters are selected for special purposes.
     
        Different drilling parameters require different standards and types of drill bits. The method of failure of drill bits in drilling also has its own characteristics and should be treated differently.
     
        1. Influence of drilling pressure on drilling
       
        Drilling pressure is a necessary condition for rock breaking at the bottom of the well. The size of the drilling pressure determines the method and characteristics of breaking the rock, which directly affects the drilling speed and the damage method of the drill bit. During drilling, the drill bit is subjected to axial pressure and reversal force. The cutting teeth are worn, dull or damaged during the process of pressing in and shearing the rock, which must affect the drilling speed. With the addition of the drilling pressure, the drilling speed will continue to improve, and the bearings and cutting teeth of the drill bit will also accelerate the wear and affect the drilling speed. There are three different periods in which the connection between the weight-on-bit and the drilling speed changes.
     
         During the surface crushing: When the drilling pressure is less than the rock pressing hardness, the cutting teeth cannot cut into the rock, and the rock can only be broken by friction on the surface of the rock. The cutting teeth wear more, although the drilling speed is also proportional to the increase of the drilling pressure. Add, but the drilling rate is very low;
       
        During fatigue and crushing: When the drilling pressure is close to the rock pressing hardness, although the cutting teeth are not cut into the rock, many cracks occur in the rock surface, and the volume is broken by the repeated action of the cutting teeth;
       
        During the volumetric crushing: When the drilling pressure is increased above the rock pressing hardness, the cutting teeth are cut into the rock and the volume of the rock is broken, and the drilling function is remarkable, which is normal drilling. Therefore, the weight-on-bit applied to the roller bit has to be satisfied that the cutting teeth can be pressed into the rock to break the rock mass.
       
        After double the drilling pressure, the experimental roller bit drilled into different grades of rock. The results show that different rockes are different for increasing the drilling speed. In the meantime, the rate of penetration of the medium hard rock layer (rock grade 6-7) is higher, while the softer rock grade (grade 4-5) and the harder rock grade 8-9 grade are relatively less added. . When drilling into cohesive soft rock, it is easy to attack the water paste, and the drilling pressure should be selected to be smaller. When drilling into a rock layer with high abrasiveness, the lack of drilling pressure tends to constitute early wear of the drill bit, and the drilling pressure should be appropriately increased. When drilling a fractured rock formation, it is easy to break the drill, and the drilling pressure should be properly reduced to prevent the cutting and breaking of the cutting teeth. Drilling pressure is an important parameter for drilling, that is, it is necessary to fully exert the ability of the cutting teeth to cut into the rock formation, and to minimize the wear of the cutting teeth.
     
        2. The effect of speed on drilling
       
        The speed indicates the speed at which the diameter of the drill must rotate. It is the target used to measure the speed of the reversal during the drilling process. When drilling, the rock breaking condition of different hardness rock is different, and the influence of drilling pressure is different. Therefore, the influence of the bit speed on the rock breaking process and the mechanical drilling speed should be considered in the lithology and rock breaking moments.
     
        (1) Speed ​​in soft formation drilling
       
         When drilling in a soft, plastic, and abrasive rock formation (such as a clay-like rock formation), the cuttings cut by the cutting teeth are equal to the depth of the cutting teeth and the wear of the cutting teeth is small. Thus, drilling in a soft formation, when the drilling pressure is mandatory, the rotational speed is proportional to the mechanical drilling speed.
     
        (2) Speed ​​in medium and hard formation drilling
       
        Medium and hard lamination has high hardness and high abrasiveness. The cutting teeth are continuously blunt during drilling, and the tooth and rock touch area is also increasing, which makes the deformation and crack development of the rock break when the rock breaks. The drilling speed is slower, and the demanding drilling pressure is greater. Following the improvement of the hardness of the formation, the bite breaking moment and extending the drilling speed will make the rock crushing process not complete. The cutting teeth have not fully broken the rock, and they are separated from the rock, which reduces the rock breaking depth. Therefore, in order to prevent the cutting teeth from being worn out quickly, it is not allowed to over-fire and increase the rotational speed when drilling in medium-hard and hard formations.
     
       (3) Different speeds of drilling in the rock
       
        Different rock, the drilling speed has a certain change curve and limit speed with the speed. Drilling in clay-like rocks, the rate of penetration is proportional to the speed of rotation; drilling in sturdy, high-abrasive rock, the rate of penetration increases with the addition of speed, which is relatively slow, because the breaking moment extends, the limit speed is higher than other types of rock To be small, when the speed exceeds the limit speed, the drilling speed will decrease.
       
        After doubling the speed, the experimental roller bit drilled into a different grade of rock. The results showed that the marble grade 4 marble, the rate of penetration rate was 93%, the grade 9 grade of porphyry granite, the rate of penetration rate. Only 28%, from 4th to 9th, the rate of penetration rate is decreasing. Thus, for softer abrasive formations, improved speed is advantageous, while for strong, highly abrasive formations, it does not make much sense.
       
        In practice, the progressive speed is constrained by the strength, length and drill function of the drill string and the ability of the drilling equipment. During the current period, along with the development of small well drilling skills, as well as the improvement of the drill string in the well and the selection of leading lubricants to reduce the reverse wear resistance, high speed will be fully utilized.
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