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Characteristics and precautions of directional drilling through various strata
Characteristics and precautions of directional drilling through various strata
Characteristics and precautions of directional drilling through various strata
2025-10-20
1. Gravel-Sand Layers
Drilling in sand layers, gravel, pebbles, and fractured zone formations makes hole formation very difficult. Such formations are called mechanically dispersed formations. Due to the lack of cementation between particles, the hole walls are prone to collapse during drilling.
For these formations, mud or high-viscosity slurries are used for hole wall protection. The key to solving the problem is to enhance the cementation force between particles on the wellbore wall. High-viscosity slurries that properly penetrate into the hole wall formation can significantly strengthen the cementation between sand and gravel, thereby improving the stability of the hole wall.
Measures to increase slurry viscosity mainly include:
Using high-dispersion mud (finely dispersed mud).
Increasing the clay content in the mud.
Adding organic or inorganic thickeners.
Using 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 polymers. Its composition includes clay, Na₂CO₃, and water. To meet hole formation requirements, it often contains viscosity enhancers, fluid loss reducers, and anti-flocculants (thinners). Different types of finely dispersed mud are available based on the additives used, such as sodium carboxymethyl cellulose mud, ferrochrome salt mud, lignin sulfonate mud, and humic acid mud.
The main materials of high-viscosity compound drilling powder are derived from macromolecular natural plants, macromolecular mineral compounds, or biopolymers. After different chemical treatments, it has a wide range of varieties. This type of powder is easily soluble in water and can form high-viscosity slurries.
2. Soil Layers
When drilling in clay and mud shale, one of the prominent problems is that the hole walls swell, shrink in diameter when in contact with water, and may even disperse or collapse.
The reason is that clay and mud shale contain a large amount of clay minerals—especially montmorillonite. When the clay on the hole wall comes into contact with water in the hole-forming fluid, the clay absorbs water, swells, and disperses. Such formations are also called water-sensitive formations.
Obviously, for water-sensitive formations, the most critical issues are:
Minimizing water seepage from the hole-forming fluid into the formation (i.e., reducing the fluid loss of the hole-forming fluid).
Enhancing the water sensitivity resistance of the hole wall rock and soil.
Inhibiting dispersion.
3. Rock Layers
For drilling and reaming, hard rock has the following characteristics:
Due to the hardness and high friction of the rock, drilling requires high energy consumption to break the rock, resulting in slow penetration rate, severe bit wear, and easy overheating (bit burning).
The wellbore formed by drilling in hard rock is relatively stable. Except for encountering large formation fractured zones, collapse (as in soil layers, mud shale, and gravel-sand layers) rarely occurs under normal conditions.
Since hard rock drilling mostly uses grinding to break the rock, the cuttings are fine, making it easier to suspend and remove the cuttings.
The design of hole-forming fluid for hard rock formations should focus on:
Enhancing lubricity and cooling performance.
Reducing solid content to improve drilling speed.
Generally, there are no high requirements for the cutting suspension and hole wall protection capabilities of the hole-forming fluid.
In terms of reducing pipe-laying resistance: During trenchless pipe laying in many formations, high resistance in pipe jacking and pipe pulling is often encountered, leading to difficulties in pipe laying and sometimes even abandonment halfway. One of the key technologies to solve this problem is to use hole-forming fluid with good lubrication performance, which significantly reduces the friction coefficient of the hole wall rock and soil.
Lubricating and drag-reducing hole-forming fluids can be divided into three types: emulsions, polyacrylamide slurries, and emulsified muds.
The most typical dissolvable formation is the sodium chloride salt layer; others include potassium salt, gypsum, mirabilite, and natural alkali layers. Such formations are also called water-soluble formations. When they come into contact with water in the hole-forming fluid, they dissolve, eroding the hole walls. This often results in excessive hole diameter and collapse.
To address water-soluble formations, solutions mainly focus on two aspects:
Reducing fluid loss.
Decreasing the corrosiveness of the hole-forming fluid to the formation.
A common method to control corrosion is to add substances identical to the dissolved components of the formation to the base slurry, making the solubility tend to saturation. For example, when forming holes in rock salt, saltwater slurry is used as the hole-forming fluid, which provides good anti-collapse effects.
To ensure the hole-forming fluid has a certain viscosity and density, saltwater and clay powder are sometimes mixed to form saltwater mud. It is a stable coarse-dispersed mud system:
It is a clay suspension with a sodium chloride content of more than 1%, or a mud prepared with saltwater (seawater).
It relies on a high sodium chloride content to promote moderate agglomeration of clay particles, and uses organic protective colloids to maintain this moderate agglomeration.
Based on the salt content, saltwater mud is classified into three types:
Saltwater mud (general salt content: 3%–7%).
Seawater mud (total salinity: generally 3.3%–3.7%).
Saturated saltwater mud (sodium chloride content: approximately 33%–36%).
Saltwater mud has the following advantages: low viscosity, low shear force, good fluidity, resistance to salt invasion, inhibition of rock salt formation dissolution, strong resistance to clay invasion, and effective inhibition of hydration swelling, collapse, and spalling of mud shale.
Vanessa
Sales Manager
Jiangsu Goodeng Heavy Machinery Technology Co.,Ltd
Tel: +8618052028038 Whatsapp:8613323768447
Email: lixinyu@goodeng.com.cn
Website: www.goodeng.com.cn