How does directional drilling smoothly pass through hard rock and gravel layers?

In the construction of trenchless directional drilling, sand, pebble and rock layers are often encountered, which brings great difficulties to the construction, so how can we successfully cross them?


directional drilling, Hard rock formation


1. Soil layer


When drilling in clay and mud shale, one of the prominent problems is the swelling of the hole wall, diameter shrinkage, or even flow and collapse of the hole wall. The reason is that there are a large number of clay minerals in clay and shale, and when the clay on the pore wall contacts the water in the pore-forming liquid, the clay absorbs water, expands, and disperses. For this water-sensitive formation, the water seepage of the pore-forming fluid to the formation should be minimized.


2. Sand layer


The sand layer can be roughly divided into silt sand, fine sand, medium sand, coarse sand and gravel sand according to its particle size distribution. Such formations are called mechanical dispersion formations. Due to the lack of cementation between the particles, the hole walls are prone to collapse during drilling. When carrying out this type of formation, it is necessary to choose a reasonable drilling process and select a high-quality drilling fluid to protect the wall.


Drilling process


1) For construction in the sand layer, conventional directional drilling can be used, and the drilling trajectory should minimize the incident angle and exit angle to avoid excessive drilling curvature in the sand layer. The drill bit is made of a hard alloy pilot drill bit;

2) Increase the contact area between the inclined palm of the pilot bit and the quicksand, and improve the deflection building and correction functions of the inclined palm, so that the pilot bit can extend along the expected drilling trajectory and ensure the guiding accuracy.

3) After the construction of the pilot hole is completed, reaming should be started as soon as possible. The cohesion of the sand layer is small. When reaming, the number of reaming stages should be reduced as much as possible, and the number of vibrations of the drilling tool in the hole should be reduced to effectively prevent the collapse of the hole wall.


Drilling fluid


Drilling fluid is mainly used to stabilize the hole wall, reduce the rotary torque and pipe resistance, cool the drilling tool, and remove drilling slag. Drilling in the sand layer, combined with the characteristics of the sand layer, appropriately increases the density of the drilling fluid, increase the viscosity and reduces water loss, and considers its pump ability. In the ratio of the mud system, the mud of sodium bentonite and a small amount of lubricant are generally selected.


3. Pebble layer


The structure of the pebble layer is extremely unstable, which belongs to the mechanical dispersion formation, and there is no cementation between the particles. During the construction of the pilot hole, there will be situations such as hole collapse, difficulty in controlling the drilling trajectory, difficulty in removing drilling slag, difficulty in breaking pebbles, and loss of drilling fluid. During the reaming construction, there will also be the phenomenon that the reamer is stuck, or even the drill is buried. Therefore, when carrying out trenchless construction in this stratum, it is necessary to understand the compactness, particle size distribution, layer thickness, etc. of the pebble layer in detail, reasonably design the drilling trajectory, select reasonable drilling tools, high-quality mud and reamers, to improve drilling efficiency and directional control.

1) In the construction of a pebble layer, the jacking method of cemented carbide drill bit rotary extrusion can be used to squeeze the pebble to form a dense hole wall, and the drill bit with the function of gathering, crushing and splitting large-diameter pebbles and gravels can be selected.

2) After the construction of the guide hole is completed, the hole should be reamed as soon as possible to avoid the phenomenon of hole collapse due to the long time of the bare hole. The hole can be reamed by using a conical drill bit with cemented carbide teeth to ream the hole in stages. The diameter of the drill bit should be determined according to the actual situation, and the difference in the diameter of the drill bit between the stages is generally controlled within 50mm.


Mud retaining wall


For the pebble layer, the mud retaining wall is a great difficulty. The structure of the pebble layer is generally loose, easy to collapse, with large pores, and consumes mud. Therefore, when constructing the pebble layer, the key problem is to solve the problem of optimizing the mud ratio and choosing high-viscosity mud. However, due to the mud, The disadvantage that the viscosity is too high will affect the drilling efficiency. While utilizing the mud retaining wall, preparations should also be made to prevent leakage, and when necessary, the plugging slurry should be poured into the hole regularly.


4. Hard rock formation


Unweathered granite, diorite, diabase, basalt, andesite, gneiss, quartzite, quartz sandstone, siliceous conglomerate, siliceous limestone, etc., all belong to hard rock.

1) Due to the hard rock and strong friction, it requires a lot of consumption to break the rock when drilling, and the drill bit is severely worn and easy to burn the drill;

2) The well wall formed by drilling into hard rock is relatively stable. Except for large stratum fracture zones, it is generally not easy to collapse holes like soil layers, mud shale and gravel layers;

3) Since hard rock drilling mostly adopts grinding methods to crush rock, the cuttings particles are fine, so it is easier to hang and discharge cuttings. The design of pore-forming fluid in hard rock formation should focus on enhancing lubricity and cooling and reducing solid phase The content is beneficial to increase the drilling speed; while the suspension and drainage ability and wall protection of the pore-forming fluid are often not highly required.


Drilling process


Drilling in rock formations requires the principle of “high speed, low pressure” to achieve higher efficiency. We can learn from the experience of core drilling and oil drilling, and use the percussion rotary drilling method to efficiently break rocks. The percussion rotary drilling process The percussion rotary steering method has the characteristics of high efficiency, easy operation, and wide adaptability to strata and urban construction.

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