September 9, 2022
Penetration Characteristics of Gravel Layer, Soil Layer, and Rock Layer
1. Gravel Stratum
Drilling in a sand layer, gravel, pebble, and fracture zone is very difficult. Such strata are called mechanically dispersed strata. Due to the lack of cementation between particles, the hole wall is easy to collapse when drilling. For such formations with mud or high viscosity slurry wall, the key to solving the problem is to increase the cementing force between the wellbore particles. The slurry with high porosity can be appropriately infiltrated into the formation, which can obviously enhance the cementation force between sand and gravel, so as to enhance the stability of the hole wall. Improve the slurry viscosity, mainly through the use of high dispersion mud (fine dispersion mud), increasing the clay content of the mud towel, adding organic or inorganic tackifier, and the use of high viscosity compound drilling powder and other measures. The finely dispersed slurry is a dispersed slurry with a salt content of less than 1 %, a calcium content of less than 0.012 %, and no inhibitory polymer. In addition to dry soil, Na2CO3 and water, in order to meet the needs of pore-forming, it is often added with a tackifier, fluid loss agent, and flocculant (diluent). Depending on the treatment agent, there are different types, such as sodium cunning methyl cellulose mud, iron chrome mud, lignin, sulfonate mud, and humic acid mud. The main material of high viscosity compound drilling powder is derived from natural plants, high molecular mineral compounds, or biopolymers. Through different chemical processing, its variety is numerous. These powder products are soluble in water and can form a highly viscous slurry.
2. Soil Layer
One of the outstanding problems in drilling in clay and shale is the expansion and shrinkage of the hole wall in water, and even the dispersion and collapse of the hole. The reason is that there are a large number of clay minerals in clay and shale, especially the presence of montmorillonite dry soil minerals, so that when the pore wall clay comes into contact with the water in the pore-forming fluid, the water absorption, expansion, and dispersion of clay occur. This stratum is also called water sensitive stratum. Obviously, for water-sensitive strata, to minimize the pore-forming fluid on the formation of water seepage, that is to reduce the pore-forming fluid loss and enhance the pore wall rock and soil water sensitivity, inhibiting dispersion is the most critical issue.
3. Rock Layer
For drilling and reaming, hard rock has the following characteristics:
(1) Due to the hard rock, strong friction, drilling broken rock required for large consumption, slow footage, severe bit wear, and easy-to-heat drill.
(2) The borehole wall formed by drilling hard rock is relatively stable. In addition to the large formation fracture zone, it is not easy to collapse and collapse holes like the soil layer, mud shale, and gravel layer.
(3) Because hard rock drilling mostly uses grinding methods to crush rock, the cuttings particles are small, so it is easy to suspend the cuttings.
The design of pore-forming fluid for hard rock formation should focus on enhancing lubricity and cooling, and reducing solid content to improve drilling speed; the suspension capacity and wall protection of pore-forming fluid are often not high.
In terms of reducing the resistance of pipe laying, trenchless pipe laying in many strata often encounters large resistance of pipe jacking and pipe pulling, resulting in difficulties in pipe laying, and sometimes even halfway. One of the key technologies to solve this problem is to use a hole-forming fluid with good lubrication performance to significantly reduce the friction coefficient of the rock and soil on the hole wall.
Lubrication drag reduction pore-forming fluid can be divided into three types: emulsion, polypropylene phthalimide slurry, and emulsified slurry.