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Mud technology for horizontal directional drilling in sand layer
Author: Qin Lei
Introduction to 0
Horizontal directional drilling (HDD) is a technology that uses drilling equipment installed on the ground to drill into the stratum at a small incident angle relative to the ground to form a pilot hole, and then expands the pilot hole to the required size to lay a pipeline (line). It has tracking and guiding functions in construction. This technology solves the problem that underground pipelines cannot be laid by excavation in urban areas or large natural obstacles.
The drilling tool passes through the formation to form a hole, which is almost horizontal in space. When the hole is surrounded by loose strata such as sand, accidents or quality problems are easy to occur in the hole.
1 Adverse influence of sand layer on crossing construction
Because of the weak or no cohesive force between sand particles, the original stress state of the stratum is broken after drilling, and the hole collapse accident is most likely to occur in the qualitative crossing construction of sand layer. If hole collapse occurs during drilling, the direct consequence is that drilling tools are buried, resulting in buried drilling and stuck drilling construction; If the hole collapses in the process of drawing back, it may lead to excessive pipe drawing resistance or even failure. In the sand layer, the main causes of well collapse are as follows:
(1) The stratum has little cementation. Once exposed, it will lose its original balance and collapse.
(2) Mud erosion, in the process of drilling, mud is ejected from the nozzle at high speed, which has a strong impact on the sand layer and causes collapse;
(3) drilling (pipe laying) too fast, too hard to open the pump, resulting in a large pressure in the hole;
(4) The drilling tool continuously collides with the wall of the extrusion hole;
⑤ Excessive mud pressure will fracture the formation and cause well collapse.
2 Selection of mud system
2. 1 the basic function of mud mud technology is an important part of HDD engineering. With the increasing difficulty of HDD construction, this technology plays an increasingly important role in ensuring safe, high-quality and rapid construction. The most basic functions of mud are as follows:
① Carrying and suspending drilling cuttings.
The primary and most basic function of mud is to carry the drilling cuttings in the hole to the ground through its own circulation, so as to keep the hole clean, make the feeding and dragging unimpeded, ensure that the drill bit always contacts and breaks the new stratum in the hole, avoid repeated cutting and maintain safe and rapid drilling. When the drill pipe is unloaded or the pump is stopped for some reason, the mud can keep the cuttings in it in stable suspension and prevent them from being deposited in the borehole in large quantities.
② Stabilize the hole wall and balance the formation pressure.
The stability of hole wall and drilling law are the basic conditions to realize safe, high-quality and fast construction. Mud with good performance can cement loose formation to a certain extent, prevent mud from leaking into formation continuously, weaken hydration expansion and dispersion of shale, and increase the stability of pore wall. At the same time, the liquid column pressure of mud can balance part of formation pressure and reduce the risk of wellbore collapse.
③ Cool and lubricate the drill bit and drilling tool.
In the process of drilling, the drill bit rotates and breaks the rock layer, generating a lot of heat. At the same time, the drilling tool also constantly rubs against the borehole wall, generating heat. It is through the continuous circulation of mud that heat is absorbed in time, then brought to the ground and released into the atmosphere, thus playing a role in cooling the drill bit and drilling tool and prolonging its service life. Because of the existence of mud, both the drill bit and the drilling tool rotate in the liquid, which greatly reduces the friction resistance and plays a good role in lubrication.
(4) transfer hydrodynamic force.
Mud impacts the drilling at a very high speed at the nozzle of the drill bit, thus improving the drilling speed and rock crushing efficiency. In addition, the mud in the circulation process can provide driving force for power drilling tools in the hole, such as screws and hydraulic impactors, and meet the needs of compound drilling or motor drilling in the hole.
2.2 Main performance indicators of mud
For HDD mud, mud indexes that should be paid attention to mainly include: specific gravity, funnel viscosity, plastic viscosity, dynamic shear force, static shear force, API filtration, PH value, sand content and solid content.
2.3 Selection of mud performance indicators
① Density: The mud should have a certain density. The first density is to give the formation a back pressure to keep the formation stable and not collapse.
If the borehole is about 20m below the surface and the formation water has a pressure of 2kg/cm2, then the density of mud is 1.02g/cm3, which can generate a pressure of 2.04kg/cm2 at the same depth, that is to say, it increases the pressure of the formation by 0.04kg/cm2, which in a sense reduces the probability of formation collapse. Of course, the density should not be too high, otherwise it will fracture the formation and cause lost circulation. Therefore, when the crossing depth is 20m-30m, the mud density is1.02g/cm2-1.04g/cm2. At this time, the above density can be achieved by adding 6% ~ 8% bentonite into the mud, so a certain amount of bentonite must be maintained.
② Viscosity: Mud should have proper viscosity. The essence of viscosity is the internal friction between liquid, liquid and solid, which can adjust the rheology and help to carry cuttings and clean the borehole.
If the clay layer is drilled, the viscosity can be lower, 30s-50s. If the gravel layer is drilled, the viscosity can be higher, 70s-80s. In order to improve the viscosity, the dosage of bentonite can be increased, and tackifiers such as high viscosity CMC, polyanionic cellulose and modified starch can also be added.
③ Filtration: The mud should have a low filtration, which means the amount of free water in the mud. The large filtration indicates that there is much free water in the mud and the colloid performance is poor; The small filtration shows that there is more bound water in the mud and the colloid performance is good. When the crossing depth is 20m-30m, the mud filtration rate can be controlled at 8ml- 10ml to meet the requirements. Low viscosity CMC, modified starch, hydrolyzed ammonium polyacrylate (NH4-HPAN) and other filtrate reducer can be added to reduce the filtrate.
④pH value: mud should have a proper pH value, which represents the pH value of mud.
Mud is a colloid in which bentonite is dispersed and suspended in water, and its pH value is generally above 8, that is, it is alkaline. The pH value of water-based mud is required to be 9, but considering that the clay in the formation will adsorb hydroxyl ions (OH-), and sometimes there are calcium salts and magnesium salts in the formation, the pH value of the mud should be controlled at 9- 10 when crossing the formation of 20m-30m or deeper. Generally, caustic soda is used to adjust the pH value in mud. Considering that there are calcium salts and magnesium salts in the formation, caustic soda and soda ash are used together in the mud during crossing, each accounting for 50%. Especially when reaming with large diameter, the pH value is adjusted to 10- 1 1, which is beneficial to formation stability and mud stability.
3 sand and mud system design
3. 1 Mud design principle is based on the characteristics of sand layer. In the process of horizontal directional crossing construction, the main difficulties are:
① There is no cementation between sand grains, and the structure is loose. Once the stratum balance is broken, well collapse will easily occur during construction;
② The sand ratio is large and the particles are coarse, so the slurry is difficult to move in suspension;
(3) The sand layer generally contains groundwater. During drilling, there is a risk that the mud will be diluted due to the infiltration of groundwater. Once the mud is diluted, it will not form a dense mud cake ring near the hole wall, and the pressure in the hole and the formation pressure communicate with each other, so the mud can not maintain the stability of the hole wall, and the risk of hole collapse is high.
Based on the above difficulties, mud design should pay attention to the following aspects:
① The polymer mud system is used as far as possible in the sand layer, and the performance is stable during use. Improving the viscosity of mud by polymer polymerization can greatly enhance the cementation of mud to sand particles, quickly form a mud cake ring with small thickness and good toughness around the hole wall, and quickly establish a positive pressure difference inside and outside the hole, thus forming a good supporting effect on the hole wall.
In addition, the polymer slurry is not easy to be diluted by groundwater intrusion, which better maintains the engineering safety;
② Try to use mud with high viscosity and high yield value.
Because of the coarse sand and heavy proportion, the suspension performance of mud itself must be enough to be discharged out of the well; In addition, because the sand layer is rich in groundwater, the cut cuttings also contain a lot of water, which will greatly dilute the mud.
Therefore, when designing mud, we should find out the water content of sand layer in detail, and estimate the proportion of water entering mud per unit time to the amount of mud pumped in this time period in advance in combination with the design parameters such as reaming diameter, reaming gradation, mud displacement and reaming speed. In the process of ground test, it is necessary to ensure that the designed mud still has sufficient suspension performance after mixing this proportion of clear water, that is, the dilution effect of water should be fully considered when designing mud indicators;
③ Reasonable control of mud displacement can not only ensure the timely removal of drilling cuttings, but also avoid excessive erosion of the hole wall; In addition, in the process of towing back, lubricant can be added, which is helpful to reduce the towing back resistance.
3.2 Preventive measures
In the construction of sand layer crossing, there is often a lack of real and detailed geological data. Therefore, the mud performance index cannot be completely implemented according to the index determined by the laboratory; In the process of construction, the cuttings content in the returned mud should be regarded as an important basis for whether the mud index is qualified. If the cuttings content in the returned mud is low, the main indexes such as viscosity and static shear force should be continuously improved until the cuttings content in the returned mud is equivalent to the theoretical value.
Secondly, due to the unfavorable influence of groundwater introduced earlier, the mud is most obviously affected by drilling speed when crossing the sand layer. The faster the drilling speed, the more cuttings are produced in the same time, and the more groundwater enters the mud, the more obvious the dilution of the mud is.
Therefore, when drilling in sand layer, the mud displacement and drilling speed of reaming at each stage should be designed; In the whole construction process, the reaming speed should not be higher than the design value, and it is best to keep the same drilling speed to keep the mud in the hole relatively uniform.
Table 1 is the mud parameter index used in the second crossing of the West-East Gas Pipeline.
Because of the abundant groundwater, high viscosity mud is used, and the mud index is improved to a certain extent at each reaming level, which ensures the smooth implementation of the project.
Among them, in the construction process, the most obvious point is that the viscosity of recycled mud is generally only 50 ~ 70s, which is less than 1/3 of the viscosity of new mud, which shows that how to effectively prevent mud from being diluted by groundwater is the key to the success of the whole project.
References:
[1] Ma Bao Song et al. Trenchless engineering [M]. Beijing: People's Communications Press, 2008.
Wu Xiaoming, Hu Yule, Li Lianggang, et al. Guided drilling and trenchless pipe laying technology [M]. Wuhan: China Geo University Press, 2004.
Yan, Jiang, et al. Trenchless underground pipeline engineering technology [M]. Shanghai: Shanghai Science and Technology Press, 2005.
Wu Xiaoming, wait. Drilling fluid and geotechnical mud [M]. Wuhan: China Geo University Press, 2002.