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Analysis of Why the Bit Fails in the Drilling Process
Release time:
2019-06-18
Author:
Source:
Shandong yi mining drilling technology co., ltd. production of drilling engineering dedicated to all kinds of geological drill pipe, spiral drill pipe, triangular drill pipe, coal drill pipe, drill bit, drill pipe, complete specifications! Strength manufacturers, trustworthy! company set research and development-production-sales-sales as one, welcome customers to purchase, tel: 0635-2311078
The drilling process generally refers to the three controllable process parameters of bit pressure, rotation speed and mud displacement in the drilling process. In practical application, the drilling process should be formulated according to the formation conditions, bit type, drilling equipment and the technical level of operators. According to the different requirements and conditions, the drilling process is divided:
1) Optimize the drilling process: under certain conditions, the drilling process parameters that can achieve the best economic indicators.
2) Strengthen the drilling process: In order to achieve a higher drilling speed, the drilling parameters are higher than the general drilling parameters.
3) Special drilling process: special measures or restricted drilling parameters are adopted for special purposes.
Different drilling parameters require the selection of different specifications and models of drill bits, and the failure mode of drill bits in drilling also has its own characteristics, which should be treated differently.
1. Influence of weight on drilling
Weight on bit is a necessary condition for rock breaking at the bottom of the well. The size of the weight determines the way and characteristics of rock breaking, and directly affects the drilling speed and the failure form of the drill bit. In drilling, the drill bit is affected by axial pressure and rotary force, and the cutting teeth are worn, blunt or damaged in the process of pressing and shearing the rock, which will inevitably affect the drilling speed. With the increase of weight on bit, the drilling speed will continue to increase, and the bearings and cutting teeth of the drill bit will also accelerate wear, affecting the drilling speed. There are three different stages in the change of the relationship between weight on bit and drilling speed.
Surface crushing stage: when the weight on bit is less than the hardness of rock indentation, the cutting teeth can not cut into the rock, but can only crush the rock in the form of friction on the rock surface, which will greatly wear the cutting teeth. Although the drilling rate increases in proportion with the increase of the weight on bit, the drilling rate is very low;
Fatigue crushing stage: when the bit pressure is close to the hardness of the rock, although the cutting teeth do not cut into the rock, many cracks are produced on the rock surface, and the repeated action of the cutting teeth also produces volume crushing;
Volume crushing stage: when the bit pressure is greater than the hardness of the rock, the cutting tooth cuts into the rock to produce volume crushing, and the drilling effect is obvious, which is normal drilling. Therefore, the weight on bit applied to the roller bit must be sufficient for the cutting teeth to press into the rock, causing the rock to be broken in volume.
By doubling the weight on bit, the test cone bit is drilled into different levels of rock, and the results show that the drilling speed obtained by increasing the drilling pressure is different for different rocks. Among them, the medium-hard rock formation (rock grade6 ~7grade) has a higher growth rate of drilling speed, while the soft (rock grade4～5grade) and harder (rock grade8～9level) of rock formation is relatively small growth. When drilling into cohesive soft rock, it is easy to produce water plugging and mud drilling, so the WOB should be selected to be smaller. When drilling into a large abrasive rock formation, insufficient weight on bit will easily cause early wear of the bit, so the weight on bit should be increased appropriately. When drilling the fractured rock formation, it is easy to cause drilling jump, so the WOB should be properly reduced to avoid chipping and breaking of cutting teeth. The drilling pressure is an important parameter, which is beneficial to give full play to the ability of cutting teeth into the rock, but also to minimize the wear of cutting teeth.
2. Influence of rotating speed on drilling
The rotating speed indicates the speed of rotation of a drill bit with a certain diameter, and is an index used to measure the rotating speed in the drilling process. When drilling, the rock breaking state of rocks with different hardness is different, and the influence of bit pressure on it is also different. Therefore, the influence of bit speed on rock breaking process and mechanical drilling speed should consider the factors of lithology and rock breaking time.
(1) Rotation speed during drilling in soft formation
When drilling in soft, plastic and abrasive rock (such as clay rock), the thickness of the cuttings cut by the cutting teeth is equal to the depth of the cutting teeth into the rock, and the wear of the cutting teeth during drilling is very small. Therefore, when drilling in soft formations, when the weight on bit is constant, the speed is proportional to the rate of penetration.
(2) Rotation speed during drilling in medium-hard and hard formations
Medium-hard and hard formations have high indentation hardness and high abrasiveness. The cutting teeth are constantly being blunt during drilling, and the contact area between the teeth and the rock is also increasing, which prolongs the deformation and crack development time when the rock is broken, increases the difficulty, slows the drilling speed, and requires greater drilling pressure. With the increase of the hardness of the formation, the rock breaking time of the drill bit is prolonged, and the drilling speed is increased, which will make the rock crushing process develop incompletely, and the cutting teeth are separated from the rock before they fully break the rock, causing the rock breaking depth to decrease. Therefore, due to the limitation of rock breaking time, in order to avoid faster wear of cutting teeth, the speed cannot be increased excessively when drilling in medium hard and hard formations.
(3) Rotation speed of drilling in different rocks
For different rocks, the drilling speed has a certain change curve and limit speed with the increase of speed. When drilling in clay rocks, the drilling speed increases in proportion to the rotation speed; when drilling in hard and highly abrasive rocks, the drilling speed increases relatively slowly with the increase of the rotation speed. Due to the extension of the rock breaking time, the limit rotation speed is smaller than that of other rocks. When the rotation speed exceeds the limit rotation speed, the drilling speed will decrease.
By doubling the rotation speed, the test cone drill bit is drilled into different levels of rock, and the results show that the rock level is4 marble, drilling rate growth rate93%The rock level is9grade porphyritic granite, the growth rate of drilling rate is only28%, from4Level9level, the growth rate of drilling speed is decreasing curve. Therefore, for the soft abrasive formation, it is beneficial to increase the speed, but for the hard high abrasive formation, it is of little significance.
In practice, the increase in speed is limited by the strength of the drill string, the length and the performance of the drill bit and the capacity of the drilling equipment. At this stage, with the development of slim hole drilling technology, as well as the improvement of the working state of the drill string in the well and the use of advanced lubricants to reduce the rotary wear resistance, high speed will be fully applied.
3. Influence of drilling fluid on drilling
In addition to the functions of cleaning, cooling the bit, carrying rock cuttings and assisting in rock breaking, the main purpose of drilling fluid is to protect the borehole wall. The displacement of drilling fluid, the size of the pressure, and the choice of water, mud or emulsion have a great impact on the effect of well flushing. With the increase of displacement, the ability of cleaning cuttings and cooling the drill bit at the bottom of the well increases. The clean bottom of the well can not only improve the drilling speed, but also reduce the wear of the drill bit. Drilling fluid at a certain injection rate can play the role of jet breaking rock when drilling in a relatively soft formation. Even in the hard formation can also play the role of auxiliary rock breaking. The general use of large displacement is beneficial to drilling. Therefore, there is a problem of optimization of drilling fluid parameters. The research results show that:
The larger the well diameter, the lower the rock drillability level, the higher the drilling rate, and the larger the selected displacement;
The increase of drilling fluid specific gravity will increase the pressure acting on the rock surface at the bottom of the well, which will increase the difficulty of rock breaking and reduce the drilling speed;
When the viscosity of drilling fluid increases, the viscous resistance of the bottom hole flow will increase, which is not good for the cooling of the cutting teeth of the drill bit, and will also cause the drilling speed to decrease. Generally, it is easier to obtain higher drilling speed by using clean water than by using mud.
Viscosity is one of the most important properties of drilling fluid. If the viscosity is too high, the circulating pressure consumption will be too high. In addition, the viscosity will continue to rise with the influence of rock cuttings dispersing during drilling, and finally it will not move, so it needs to be flushed (I. e., part of mud will be drained and glue solution will be added for dilution). If the viscosity is too high, it is not conducive to the gas in the gas leaching, in addition, high viscosity also has a very adverse effect on improving the drilling speed. However, too low viscosity will lead to difficulty in carrying cuttings, resulting in sand sinking and sticking when receiving a single root. For high density mud, too low viscosity will also lead to settlement of weighting materials, thus bringing more problems. Matching with viscosity, shear is also critical.
Generally increase the viscosity of the clay, water loss agent, etc., and reduce the viscosity of the viscosity of the viscosity of the viscosity of the agent.
Key words:
Drill pipe, geological drill pipe, rib drill pipe, spiral drill pipe
