Expertise
Capable of directional drilling through gravel, soil and rock formations.
2026-02-06
1. Gravel Strata
Drilling in sand, gravel, pebbles and fractured formations involves great difficulty in hole formation. Such formations are known as mechanically dispersive formations. Due to the lack of cementation between particles, the borehole wall is prone to collapse during drilling. For such formations, mud or high-viscosity slurry is used to stabilize the borehole wall. The key to solving the problem is to increase the cementation force between particles of the borehole wall.
Slurry with relatively high viscosity can properly penetrate into the formation of the borehole wall, which can significantly enhance the cementation force between sand and gravel particles, thereby improving the stability of the borehole wall.
The viscosity of the slurry can be increased mainly by using highly dispersed mud (finely dispersed mud), increasing the clay content in the mud, adding organic or inorganic viscosity-increasing agents, and adopting high-viscosity compound drilling powder.
Finely dispersed mud is a dispersed mud with a salt content of less than 1%, a calcium content of less than 0.012%, and no inhibitory high polymer. In addition to clay, Na₂CO₃ and water, viscosity enhancers, fluid loss reducers and deflocculants (thinners) are often added to meet hole-forming requirements.
Depending on the additives used, various types are available, such as sodium carboxymethyl cellulose mud, ferrochrome lignosulfonate mud, lignin sulfonate mud, and humic acid mud.
The main materials of high-viscosity compound drilling powder are derived from macromolecular natural plants, high-molecular mineral compounds or biopolymers. Through various chemical treatments, a wide variety of products are available. These powder products are soluble in water and can form highly viscous slurry.
2. Soil Strata
When drilling in clay and shale, one prominent problem is water swelling, diameter shrinkage, and even dispersion or collapse of the borehole wall. This is caused by abundant clay minerals in clay and shale, especially montmorillonite. When the clay on the borehole wall contacts water in the drilling fluid, it absorbs water, swells and disperses. Such formations are also called water-sensitive formations.
Obviously, for water-sensitive formations, the key issues are to minimize water infiltration into the formation, i.e., reduce the fluid loss of the drilling fluid, enhance the water sensitivity resistance of the borehole wall rock and soil, and inhibit dispersion.
3. Rock Strata
For drilling and reaming, hard rock has the following characteristics:
(1) Due to its hardness and high abrasiveness, the energy consumption for rock breaking is high, the penetration rate is slow, the bit wears severely, and overheating and bit burning are likely to occur.
(2) The borehole wall formed in hard rock is relatively stable. Except for large fractured zones, collapse similar to that in soil, shale and gravel strata seldom occurs under normal conditions.
(3) Since hard rock drilling mostly adopts grinding for rock breaking, the cuttings are fine, making cuttings suspension and removal relatively easy.
The design of drilling fluid for hard rock formations should focus on improving lubricity and cooling performance, reducing solid content to increase penetration rate; requirements for suspension, removal and wall stability are generally not high.
In terms of reducing pipe-laying resistance, trenchless pipe-laying in many formations often encounters high resistance in pipe jacking and pulling, leading to difficulties and even project failure. One key technology to solve this problem is to use drilling fluid with excellent lubricity, which significantly reduces the friction coefficient of the borehole wall.
Lubricating and drag-reducing drilling fluids can be divided into three types: emulsion, polyacrylamide slurry and emulsified mud.
4. Soluble Strata
Typical soluble strata include sodium chloride salt strata, as well as potassium salt, gypsum, mirabilite, trona, etc. Such formations are also called water-soluble formations. They dissolve when exposed to water in the drilling fluid, causing borehole wall erosion, which often results in over-sized boreholes and collapse.
Treatment of water-soluble formations mainly focuses on two aspects:
reducing fluid loss, and reducing the solubility of the drilling fluid to the formation.
A common method is to add the same dissolved substances as those in the formation to the base mud to saturate the solution. For example, using saltwater slurry in rock salt formations achieves good anti-collapse performance.
To provide the drilling fluid with proper viscosity and density, saltwater and clay powder are sometimes mixed to form salt mud. It is a stable coarse-dispersed mud system in which the sodium chloride content in the clay suspension exceeds 1%, or prepared with saltwater (seawater). The high sodium chloride content promotes moderate agglomeration of clay particles, stabilized by organic protective colloids.
According to salinity, it is classified into:
• salt mud (generally 3%–7% salt content),
• seawater mud (total salinity about 3.3%–3.7%),
• saturated salt mud (sodium chloride about 33%–36%).
Salt mud features low viscosity, low gel strength, good fluidity, salt corrosion resistance, inhibition of rock salt dissolution, strong resistance to clay contamination, and effective inhibition of hydration swelling, collapse and spalling of shale.
There are two preparation methods:
1. Prepare dispersed mud with fresh water first, then add salt to convert it into salt mud;
2. Directly prepare mud with saltwater or seawater.
The former is equivalent to treating mud after salt contamination and is easy to prepare. The latter is difficult because bentonite and ordinary clay disperse poorly in saltwater. Therefore, salt-tolerant clays such as attapulgite and sepiolite are recommended for direct mixing with saltwater or seawater.
In trenchless pipe-laying, severe lost-circulation formations may be encountered, where drilling fluid cannot circulate back to the hole, causing cuttings accumulation and sharp increase in fluid consumption. Low-density foam slurry can effectively solve this problem.
Foam slurry is a gas-liquid dispersion system containing a large number of bubbles. The liquid is the continuous phase (base fluid), and the gas is the dispersed phase. The total volume of gas at atmospheric pressure can account for 30%–90%. Thus, the weight of foam slurry is generally lighter than water of the same volume, with density as low as below 0.5 kg/L.
When used in low-pressure lost-circulation formations, its main effect is to significantly reduce the pressure of fluid loss into formation pores, ensuring little or no loss and maintaining normal circulation.
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