Correct use of dental drills

The correct use of the tooth wheel drill bit:

(A) Influence of different formation lithology on drill bit failure

The impact of formation lithology on bit failure is now shown in drilling skills: affecting drilling speed and bit feed; making the drilling process present complex situations such as well leakage, blowout, well collapse and stuck drilling; changing mud function; affecting the quality of the borehole, such as well slope and diameter irregularities, which in turn affects the quality of cementing. After analysing the formation lithology and its influence on drilling skills, the reasonableness of drill bit selection and application can be judged.

Influence of clay, mudstone and shale layer: it is easy to absorb the free water in the mud and expand, so that the diameter of the well decreases, constituting obstruction to drilling, or even jamming, and with the extension of the soaking time, it will be attacked by falling off, expanding the diameter of the well and constituting the collapse of the well. The drilling should be carried out with water or low specific gravity and low viscosity mud as much as possible. Carbonaceous shale has a weak joint force and collapses easily. Mudstone layer is soft, drilling speed is fast, also simple mud pack.

Sandstone: The nature of the sandstone varies greatly according to the size of the particles, composition and cementation. The finer the particles, the more quartz particles, the more siliceous and ferruginous cement is, the harder it is, and the more abrasive it is to the drill bit, such as quartz sandstone; the more muddy cement is, the more mica and feldspar components are softer and easier to drill; the coarser the particles are, the less the cement is, and the better the permeability, the more prone to the osmotic leakage of the mud, and the formation of a thicker mud cake on the wall of the well, which leads to the complexity of the stuck card drilling situation, and causes the abnormal use of the drill bit.

Conglomerate: drilling in conglomerate layer is easy to attack skipping drilling, crappy drilling and well wall collapse; when the pump displacement is small or the viscosity of mud is low, the gravel particles are not easy to return upwards, which is more damaging to the bit's tooth wheel body and teeth.

Limestone: Usually hard, slow drilling speed, less feed. Some of them have slit hole development, when drilling encountered slit hole, it will lead to crappy drilling, emptying, mud leakage, etc., and sometimes blowout will occur after well leakage.

Limestone formation has a great influence on the drill feed, mechanical drilling speed and bit failure. In addition, when the formation is soft and hard intertwined, such as mudstone and hard sandstone, it is easy to attack the well inclination; when the inclination of the formation is large, it is easy to attack the well inclination. The drill bit is easy to be damaged when drilling in the inclined well. When the rock formation contains soluble salts, such as gypsum layer, rock salt layer, etc., it will damage the function of mud and affect the normal use of the drill bit.

(B) Drilling skills

Usually refers to the three manipulable skill parameters in the drilling process, namely drilling pressure, rotational speed and mud displacement. In practice, drilling skills should be formulated according to formation conditions, drill bit type, drilling equipment and operator skill level. According to the different needs and conditions, drilling skills are divided into:

(1) Optimisation of drilling skills: under certain conditions, the drilling skill parameters that can reach the best economic goals.

(2) Enhanced drilling skills: in order to achieve a higher drilling speed, the drilling parameters than the usual drilling parameters.

3）Special drilling techniques: special methods or constrained drilling parameters are used for special purposes.

Different drilling parameters require the use of different standards and types of drill bits, and the failure of the drill bit in drilling is also unique and should be treated differently.

1、The influence of drilling pressure on drilling

Drilling pressure is a necessary condition for breaking rock at the bottom of the well. The magnitude of drilling pressure determines the method and characteristics of rock breaking, which directly affects the drilling speed and the damage method of the drill bit. In drilling, the drill bit is subjected to axial pressure and counter-rotation force, and the cutting teeth are worn out, blunted or damaged in the process of pressing into and shearing the rock, which will definitely affect the drilling speed. With the increase of drilling pressure, the drilling speed will progress, and the bearing and cutting teeth of the drill bit and other parts will wear out faster, affecting the drilling speed. There are three different periods when the connection between drilling pressure and drilling speed changes.

Outward crushing period: when the drilling pressure is less than the rock indentation hardness, the cutting teeth can not cut into the rock, but only in the rock exterior to break the rock by friction method, the cutting teeth wear more, although the drilling speed is also increased with the increase of drilling pressure and proportional to the increase in drilling speed, but the drilling speed is very low;

Exhaustion crushing period: when the drilling pressure is close to the rock indentation hardness, although the cutting teeth did not cut into the rock, but in the rock surface onset of a lot of cracks, by the repetitive action of the cutting teeth, but also onset of volume crushing;

Volume crushing period: when the drilling pressure is increased to more than the rock indentation hardness, the cutting teeth cut into the rock to attack the volume crushing, the drilling effect is only significant, and is normal drilling. Therefore, the drilling pressure exerted on the tooth wheel bit must be satisfied with the cutting teeth can be pressed into the rock, so that the rock on the volume of broken.

After doubling the drilling pressure, the experimental tooth wheel drill bit drilling into different levels of rock, the results show that: different rocks, to increase the drilling pressure when the drilling speed is not the same. The increase in drilling speed is higher in medium-hard rock (rock grade 6-7), while the increase in drilling speed in softer (rock grade 4-5) and harder (rock grade 8-9) rock is relatively small. When drilling cemented soft rock, it is easy to plug the water and paste the drill, so the drilling pressure should be selected smaller. When drilling abrasive rock layer, the lack of drilling pressure is easy to constitute early wear of the drill bit, and the drilling pressure should be increased appropriately. When drilling the fissure rock layer, it is easy to jump drilling, the drilling pressure should be appropriately reduced to prevent the collapse, broken cutting teeth. Drilling pressure is an important parameter of drilling, that is to say, it is necessary to give full play to the ability of cutting teeth to cut into the rock layer, but also to minimise the wear of cutting teeth.

2, rotational speed on the impact of drilling

The rotational speed indicates the speed of rotation of the drill bit of a certain diameter, which is the target for measuring the reversal speed in the drilling process. When drilling, different hardness rock breaking condition is not the same, the drilling pressure is not the same, so the impact of the rotational speed of the drill bit on the process of rock breaking and mechanical drilling speed, to think about the lithology and rock breaking time elements.

(1) Rotational speed in soft stratum drilling

When drilling in soft and plastic rock layer (such as clay rock layer), the thickness of rock chips cut down by cutting teeth is equal to the depth of cutting teeth cut into the rock, and the wear of cutting teeth is very small in drilling. Therefore, when drilling in soft strata, when the drilling pressure must be guarded, the rotational speed is increased in proportion to the mechanical drilling speed.

(2) Drilling speed in medium-hard and hard strata

Medium hard and hard strata indentation hardness, high abrasiveness, cutting teeth in the drilling is constantly being blunt, teeth and rock touching area is also increasing, making the deformation of rock crushing and fissure development time extension, the difficulty increases, the drilling speed slows down, and the need for greater drilling pressure. With the progress of the hardness of the stratum, the drill bit breaks the rock moment extension, increase drilling speed, will make the rock crushing process development can not be complete, cutting teeth are not yet fully broken rock, and rock separation, resulting in the depth of the rock breakage is reduced. Therefore, subject to the constraints of rock-breaking time, in order to prevent the cutting teeth from wearing out faster, the drilling speed cannot be increased too much when drilling in medium-hard and hard strata.

(3) Drilling speed in different rocks

Different rocks, drilling speed with the increase of rotational speed have a certain change curve and limit rotational speed. In clay rock drilling, drilling speed increases proportionally with the rotational speed; in solid, highly abrasive rock drilling, drilling speed increases with the rotational speed and increases relatively slowly, because of the extension of the time of rock breaking, the limit rotational speed is smaller than other types of rock, when the rotational speed exceeds the limit rotational speed, it will lead to a reduction in the drilling speed.

After doubling the rotational speed, the experimental tooth wheel drill bit drilling different grades of rock, the results show that: rock grade 4 marble, the rate of increase in drilling speed is 93%, rock grade 9 mottled granite, the rate of increase in drilling speed is only 28%, from 4 to 9, the rate of increase in drilling speed is a decreasing curve. Thus, for the softer abrasive stratum, it is favourable to improve the rotational speed, while for the solid high abrasive stratum, it is of little significance.

In practice, the progress of rotational speed is constrained by the strength and length of drill pipe column and the function of drill bit and the capability of drilling equipment. Nowadays, with the development of small borehole drilling technology, the improvement of the working condition of the drill pipe in the well and the research on the selection of leading lubricants to reduce the resistance to reverse grinding, the high rotational speed will be fully used.