Correct use of roller drill bits

(—) The influence of different formation lithology on drill bit failure

The influence of formation lithology on drill bit failure is reflected in drilling skills: affecting drilling speed and drill bit footage; causing complex situations such as well leakage, blowout, well collapse and drill stuck during drilling; causing changes in mud properties; affecting wellbore quality, such as well inclination and well diameter irregularity, which in turn affects cementing quality. By analyzing the formation lithology and its influence on drilling skills, the rationality of drill bit selection and use can be judged.

The influence of clay, mudstone and shale layers: It is easy to absorb free water in the mud and swell, which reduces the well diameter, causing drilling resistance and even drill stuck. As the soaking time increases, blocks will fall off, causing the well diameter to expand and causing well collapse. Clean water or low-density and low-viscosity mud should be used as much as possible for drilling. Carbonaceous shale has weak connection and is easy to collapse. Mudstone layers are soft, have fast drilling speeds, and are also easy to pack.

Sandstone: Its properties vary greatly depending on the size, composition and cement of the particles. The finer the particles, the more quartz particles, and the more siliceous and ferrous cements, the harder it is, and the greater the wear on the drill bit, such as quartz sandstone; the more mud cements, the more mica and feldspar components, the softer and easier to drill; the coarser the particles, the less cement, the better the permeability, and the more likely it is to cause mud permeability loss, and form a thicker mud cake on the well wall, resulting in complex situations such as adhesion and drilling, and abnormal use of the drill bit.

Conglomerate: Drilling in conglomerate layers is prone to drill jumping, stuck drilling and well 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 drill bit cone body and teeth is greater.

Limestone: It is usually hard, with slow drilling speed and less footage. Some have cracks and holes. When drilling into cracks and holes, it will cause drilling jams, emptying, mud loss, etc., and sometimes blowouts will occur after well leakage.

Limestone formations have a great influence on drill bit footage, mechanical drilling speed and drill bit failure. In addition, when the formation is soft and hard, such as mudstone and harder sandstone, well deviation is easy to occur; well deviation is easy to occur when the formation inclination is large. The drill bit is prone to damage when drilling in an inclined well. When the rock formation contains soluble salts, such as gypsum layer, rock salt layer, etc., it will damage the function of the mud and affect the normal use of the drill bit.

(II) Drilling skills

Usually refers to the three technical parameters that can be controlled during the drilling process: drilling pressure, rotation speed and mud displacement. In practical use, drilling skills should be formulated according to formation conditions, drill bit type, drilling equipment and operator skill level. According to different needs and conditions, drilling skills are divided into:

1) Optimized drilling skills: drilling skill parameters that can achieve the best economic goals under certain conditions.

2) Enhanced drilling skills: In order to achieve higher drilling speed, drilling parameters higher than normal drilling parameters are selected.

3) Special drilling skills: special methods or restricted drilling parameters are selected for special purposes.

Different drilling parameters require the use of drill bits of different standards and types. The failure methods of drill bits during drilling are also unique and should be treated differently.

1. The influence of drilling pressure on drilling

Drilling pressure is a necessary condition for rock breaking at the bottom of the well. The size of drilling pressure determines the method and characteristics of rock breaking, and directly affects the drilling speed and the damage method of the drill bit. During drilling, the drill bit is subjected to axial pressure and reverse force. The cutting teeth are worn, blunted or damaged in the process of pressing and shearing the rock, which will inevitably affect the drilling speed. As the drilling pressure increases, the drilling speed will continue to increase, and the bearings and cutting teeth of the drill bit will also wear faster, affecting the drilling speed. There are three different periods of changes in the relationship between drilling pressure and drilling speed.

Surface crushing period: When the drilling pressure is less than the rock pressure hardness, the cutting teeth cannot cut into the rock, and can only crush the rock by friction on the surface of the rock, which causes greater wear on the cutting teeth. Although the drilling speed also increases proportionally with the increase in drilling pressure, the drilling speed is very low;

Fatigue crushing period: When the drilling pressure is close to the rock pressure hardness, the cutting teeth do not cut into the rock, but many cracks occur on the rock surface. After repeated action of the cutting teeth, volume crushing also occurs;

Volume crushing period: When the drilling pressure is added to a value greater than the rock pressure hardness, the cutting teeth cut into the rock to cause volume crushing, and the drilling effect can be obvious, which is normal drilling. Therefore, the drilling pressure applied to the roller drill bit must be sufficient for the cutting teeth to press into the rock and cause volume crushing of the rock.

By doubling the drilling pressure, the test roller drill bit drilled rocks of different grades. The results showed that different rocks have different drilling speeds when the drilling pressure is increased. Among them, the increase rate of drilling speed in medium-hard rock formations (rock grade 6-7) is higher, while the increase in softer (rock grade 4-5) and harder (rock grade 8-9) rock formations is relatively small. When drilling cohesive soft rock, water plugging and drilling are prone to occur, and the drilling pressure should be selected to be smaller. When drilling in rock formations with high abrasiveness, insufficient drilling pressure is prone to cause early wear of the drill bit, and the drilling pressure should be appropriately increased. When drilling in fractured rock formations, drill jumping is prone to occur, and the drilling pressure should be appropriately reduced to prevent the collapse and breakage of cutting teeth. The drilling pressure is an important parameter for drilling, that is, it is conducive to fully exerting the ability of the cutting teeth to cut into the rock formation, and to minimize the wear of the cutting teeth.

2. The influence of rotation speed on drilling

The rotation speed indicates the speed of rotation of a drill bit with a certain diameter, and is an indicator used to measure the reverse speed during drilling. During drilling, the rock breaking conditions of rocks of different hardness are different, and the drilling pressure has different effects on them. Therefore, the influence of drill bit speed on the rock breaking process and mechanical drilling speed should take into account the factors of rock properties and rock breaking time.

(1) Speed in soft formation drilling

When drilling in soft, plastic, and low-abrasive rock formations (such as clay rock formations), the thickness of the rock cuttings cut by the cutter is equal to the depth of the cutter cutting into the rock, and the wear of the cutter during drilling is very small. Therefore, when drilling in soft formations, when the drilling pressure must be maintained, the speed increases in direct proportion to the mechanical drilling speed.

(2) Speed in medium-hard and hard formation drilling

The medium-hard and hard formations have higher hardness and higher abrasiveness. The cutter is continuously blunted during drilling, and the contact area between the cutter and the rock is continuously increased, which makes the deformation and crack development of the rock longer during breaking, increases the difficulty, slows the drilling speed, and requires greater drilling pressure. As the hardness of the formation increases, the rock breaking time of the drill bit increases. Increasing the drilling speed will make the rock breaking process incomplete. The cutting teeth will separate from the rock before they can fully break the rock, resulting in a decrease in the rock breaking depth. Therefore, due to the constraints of the rock breaking time, in order to prevent the cutting teeth from wearing out quickly, the speed should not be increased too much when drilling in medium-hard and hard formations.

(3) The speed of drilling in different rocks

For different rocks, the drilling speed has a certain change curve and limit speed as the speed increases. When drilling in clay rocks, the drilling speed increases in direct proportion to the speed; when drilling in hard, highly abrasive rocks, the drilling speed increases relatively slowly as the speed increases. Because the rock breaking time is extended, the limit speed is smaller than that of other types of rocks. When the speed exceeds the limit speed, the drilling speed will decrease.

By doubling the speed, the test roller drill bit drilled into different grades of rocks. The results showed that the drilling speed increased by 93% for marble with rock grade 4, and only 28% for porphyritic granite with rock grade 9. The drilling speed increased in a decreasing curve from grade 4 to grade 9. Therefore, increasing the speed is beneficial to softer abrasive formations, but it is not very meaningful for hard and highly abrasive formations.

In practical operations, increasing the speed is restricted by the strength and length of the drill string, the performance of the drill bit and the capacity of the drilling equipment. At present, with the development of small hole drilling technology, the improvement of the working conditions of the drill string in the well and the research on the use of leading lubricants to reduce reverse friction, high speed will be fully used.