1  Introduction

In China, the mud used in horizontal directional drilling construction is composed of a single component of bentonite, which is prone to problems such as collapse of the drilled hole and sticking, causing horizontal directional drilling failures. To address the causes of drilling failures, drawing on the drilling mud technology of the petroleum industry, and combining variable surface soil conditions and on-site construction experience, a mud formulation for horizontal directional drilling has been developed.

In horizontal directional drilling construction, mud is compared to the blood of drilling; when mud circulation stops, drilling work cannot continue. With the improvement of mud technology, people's understanding of the role and significance of mud has gradually increased. The role and status of mud have become more prominent, and mud has received more widespread attention. The main functions of mud:

1. Carry and suspend drill cuttings. This is a basic function of mud, which is to carry the rock (soil) cuttings broken by the drill bit (reaming bit) out of the hole, keeping the hole clean.

2. Stabilize the hole wall. A stable and regular hole wall is an important basic condition for high-quality and fast drilling, and is also the basic foothold of mud technical measures.

3. Cool and flush the drill bit, and clean the rock (soil) cuttings in the hole (bottom). In addition, the mud density can be adjusted within a relatively large range to establish a liquid column pressure balanced with the formation pressure, preventing underground complexities such as leakage, collapse, and sticking.

In short, according to the formation characteristics, structure, drilling measures and other specific conditions at each stage of drilling, the correct mud type should be selected, and corresponding reasonable mud measures should be determined to achieve high-quality and fast drilling.

2 About Mud Formulation

(1) Selection of Mud Type

At present, the basic types of mud are: water-based mud, oil-based mud, and gas-based mud.

1. Water-based mud uses water as the dispersion medium, and its basic components are clay, water, and chemical treatment agents;

2. Oil-based mud is made from diesel (or crude oil) and asphalt (or organic bentonite) and related treatment agents;

3. Gas-based mud: the gas comes from chemical generation, and the fluid is composed of gas-liquid two phases or gas-liquid-solid three phases.

Based on the performance of our existing drilling rigs and China's geological conditions, we choose water-based mud with water as the dispersion medium and clay, water, and chemical treatment agents as the basic components.

(2) Composition of Mud

① Clay (Bentonite)

The clay in horizontal directional drilling mud is mainly composed of very fine (below 2 microns) clay mineral particles. They mainly contain silicon oxide, aluminum oxide, and water, as well as small amounts of iron, alkali metals, and alkaline earth metals. The chemical composition of clay minerals is hydrous aluminum silicate. There are many types of clay minerals, but only four are common and important for mud technology: kaolinite, illite, montmorillonite, and sepiolite. The clay in horizontal directional drilling mud is bentonite (also called clay) mainly composed of montmorillonite.

② Chemical Treatment Agents

In order to ensure the stability of horizontal directional drilling mud and improve various process performances of the mud to adapt to drilling requirements under various conditions, various chemical treatment agents (mud additives) are used in the mud. Commonly used chemical treatment agents (mud additives) in horizontal directional drilling mud include: A. soda ash; B. anti-collapse agent; C. fluid loss reducer; D. flocculant; E. lubricant; F. viscosifier.

A.  Soda Ash

Sodium carbonate (Na2CO3), also called soda. Soda ash easily ionizes and hydrolyzes in water. Adding it during preparation effectively improves the hydration and dispersion performance of clay. Therefore, adding an appropriate amount of soda ash can reduce the fluid loss of the mud and increase viscosity and shear force.

B.  Anti-collapse Agent

Mainly include: potassium humate, asphalt powder, sulfonated asphalt powder, sulfonated phenolic resin, coal-alkali liquid. Among them, asphalt powder has dual properties and can be used in water-based drilling fluids. It can increase the lubricity of the mud cake, has an anti-sticking effect, can plug the pores of the mud cake and adjust the bonding force between solid particles in the mud cake, and has the effects of reducing filtration loss and preventing collapse.

C.  Fluid Loss Reducer

Fluid loss reducers include: sodium carboxymethyl cellulose (Na—CMC), polyacrylonitrile;

D.  Flocculant:

The polymer we use is: polyacrylamide (PAM); the polymer has three major effects: flocculation, anti-collapse, and shear.

F.  Lubricant:

In horizontal directional drilling, extreme pressure lubricants, stearates, sulfonated asphalt, etc. are used. Because the hydrophilic groups of the lubricant molecules adsorb on the surface of metals or clay, forming a firm chemical film, causing friction between contact surfaces to occur between oleophilic groups, it improves the lubricity of the mud, thereby reducing friction, reducing the friction between drilling tools and mud, and preventing adhesive sticking.

 ③ Water

The water in the mud: we choose fresh water with pH controlled at 7-8.

(3) Mud Formulation

① Selection of Mud Formulation Research Scheme

According to the formation characteristics, structure, drilling measures and other specific conditions, the mud formulation research schemes are divided into:

A. Conduct mud formulation research according to formation characteristics, structure, and soil classification;

According to the actual situation of our current horizontal directional drilling, combined with the distribution of surface soil in China, and according to formation characteristics, structure, and soil classification, we select a mud formulation that is universal and widely applicable to various regions and construction needs for research. Soil types can be divided into:

a. Clay type; b. Sandy soil type; c. Lime soil type; d. Weathered shale type; e. Gravel type; f. Silt type;

The above six types of soil are relatively common and widely distributed on China's surface (within 30m depth from the ground surface), and are the ones we encounter most in construction and need to solve first for mud formulation;

② Mud Performance Requirements and Composition for Clay Layers

A. Geological characteristics of clay layers: coarse soil, loose geological structure, sand, alluvial sandy soil layers, planting soil, backfill, etc., silty sand and silt. Fine soil geological structure is relatively hard, with strong water absorption, among which yellow mudstone and brown mudstone are the most representative.

B. Main characteristics: good hole-forming ability, good drillability, drilling tools can rotate and advance in the soil, deflect and exit the ground with relatively good hole formation, not easy to collapse, and it is the most suitable formation for crossing.

c. Mud performance requirements for clay layers:

Density         Viscosity      Shear force      PH value    Fluid loss (medium pressure)   Mud cake

1.05-1.07   35-40″  0-2/2-6    8.5-9      4-7ml    1.0-1.2

d.  Mud composition:

Bentonite + soda ash + fluid loss reducer + lubricant

Mud mixing ratio (Kg/m3):

1:0.04 : 0.06 : 0.06

Under general conditions, the above mixing ratio is used; during construction, appropriate adjustments should be made according to the actual on-site conditions.

③ Mud Performance Requirements and Composition for Sand Layers and Fine Sand, Pebble, Gravel (small amount) Formations

A. Geological structure characteristics of sand layers: a small amount of clay or silty soil mixed with a large amount of yellow sand, which also contains a certain amount of pebbles and a small amount of gravel.

B. Main characteristics: average hole-forming ability, easy to collapse, easy to leak, strong water absorption. The pilot hole and reaming hole are very prone to collapse; high sand content in the mud causes repeated crushing of cuttings by the machinery, and serious wear of machinery and equipment.

c. Mud performance requirements for sand layers:

Density    Viscosity     Fluid loss (medium pressure)      PH value    Mud cake     Shear force

1.05   50-65″    4-5ml           8.5-9    1.5     2-4/4-8

K≥11 (Increasing the K value is beneficial for preventing leakage and collapse; the most direct way to increase the K value on site in directional drilling is to add bentonite or high molecular polymers (such as Wanyongwang or Wannengwang, etc.);

n≤0.55 (Reducing the n value is beneficial for carrying cuttings and cleaning the hole);

t0 dynamic shear force ≥15 (yield value, dyne/cm2)

d. Mud composition:

Bentonite (high content) + anti-collapse agent (polymer, appropriate concentration) + lubricant

e. Mud mixing ratio (Kg/m3):

1:0.05 :0.06

Under general conditions, the above mixing ratio can be used; during construction, appropriate adjustments should be made according to the actual on-site conditions.

④ Mud Performance Requirements and Composition for Sand Layers and Fine Sand, Pebble, Gravel (small amount) Formations

A. Geological structure characteristics: pure yellow sand, quicksand with high water content, including pebbles and gravel; such formations are generally relatively rare.

B. Main characteristics: extremely poor hole-forming ability, easy to collapse, easy to leak mud, easy to cause large-section borehole wall collapse that wraps or buries drilling tools or machinery, slow drilling speed, serious wear of drilling tools and machinery, high sand content in mud, and serious repeated crushing by drilling tools and machinery.

c. Mud performance requirements for pebble and gravel layers:

Density        Viscosity     Fluid loss (medium pressure)  PH value    Mud cake    Shear force

1.05-1.10   60-70″  4-5ml        8.5-9   1.5-2.0  2-6/6-10

K≥15        n =0.57-0.66       t0 =20

d. Mud composition:

Bentonite (high content) + anti-collapse agent (polymer) + fluid loss reducer + lubricant

e. Mud mixing ratio (Kg/m3):

     1:0.03:0.04:0.03

Under general conditions, the above mixing ratio can be used; during construction, appropriate adjustments should be made according to the actual on-site conditions.

Note: The above three formulations have obtained national invention patents;

 4. Examples

       Taking the six rivers crossed in the "West-East Gas Pipeline" project as an example, the economic benefits generated by the self-developed mud are explained. The calculation example below is the necessary quantity for the hole during crossing, without considering losses and circulation usage; generally, the total of losses and circulation usage is considered to be 2.2-2.5.

5. Conclusion

       The mud we developed for horizontal directional drilling has been successful through application in construction, meeting the requirements of horizontal directional drilling process design, fully satisfying the hole-forming technology of horizontal directional drilling under various geological conditions, and has reached the domestic advanced level, obtaining a national invention patent; its cost is very low, with significant economic benefits, which can save a large amount of investment and reduce construction costs; it can also be used as a product for external sales to generate revenue and achieve a virtuous cycle of offsetting expenditures; with the progress of science and technology, mud for horizontal directional drilling will continue to develop. Therefore, it is necessary to continuously track advanced technology and timely supplement the mud formulation for horizontal directional drilling, so that the mud formulation for horizontal directional drilling always maintains its advanced nature.