Bentonite-Polymer drilling fluids consist of water as the base medium to which bentonite and polymers are added. They are preferred in coarse particle soils with open structures, such as even coarse gravels, to prevent the loss of the drilling fluid into the subsoil. They possess a friction-reducing and viscosity-enhancing effect by means of which an improvement of boring is achieved and lower flow velocities for the directional drilling, among others, …

Reduction of filtrate in drilling fluid is understood to be the reduction of the fluid quantity filtering into the subsoil - the reduction of filtrate being due to the formation of a very thin, dense ductile and elastic sealing filter cake (see (Stability)) [HDD-14].

In order to achieve this reduction of filtrate, so-called filtrate reducers based on polymers are used. They consist of macromolecules that are sheathed with a fixed adhesive hydrate …

(Image: Model sketch of the protection of clay particles through polymer sheathing against salts by means of which protective colloidial effect is achieved)The polymer molecules of the filtrate reducers that are intensively adsorbed at the clay particles further form a protection against the coagulating influence of the salt ions (Na⁺, K⁺, Ca⁺⁺, Mg⁺⁺) at the clay surface.

In this function, they act as so-called protective colloids or inhibitors as …

The regulation of viscosity in HDD essentially means a reduction of the viscosity. This is primarily necessary when the drilling fluid begins to thicken due to the absorption of fine solids from the spoil or an increase in alkalinity from the groundwater (e.g. drilling fluids thickened by clay spoil).

The simplest method for reducing the viscosity of the drilling fluid is to add water. If the increased viscosity is not intended to or cannot be reduced …

In drilling technology, the flocculation or coagulating of clay particles (clay mud or bentonite) is known as an undesired phenomenon when, for instance, drilling through formations containing electrolytes such as salt with an unprotected bentonite suspension without inhibitors. In the presence of electrolytes usually the repulsive potential between the clay particles is disturbed, so the attractive potential dominates and they ball together into …

Bentonite-Polymer drilling fluids consist of water as the base medium to which bentonite and polymers are added. They are preferred in coarse particle soils with open structures, such as even coarse gravels, to prevent the loss of the drilling fluid into the subsoil. They possess a friction-reducing and viscosity-enhancing effect by means of which an improvement of boring is achieved and lower flow velocities for the directional drilling, among others, …

In the following, the demands, properties and application areas of water-based fluids with or without solids, which are used in HDD, as well as the additives used for an aimed influence of certain fluid properties will be presented.

When choosing the drilling fluid for the respective case of application, the geological and hydrogeological conditions described in the geotechnical report (see (Geotechnical investigations) and Volume 1, Section 4.5 […

Chemically active as well as chemically inactive (inert) additives can be added for targeted influencing of the drilling fluid or of the water as the fluid’s base.

According to DIN V 4126-100 [HDD-222], chemically active additives also called mixing media, are added to the drilling fluid with the aim of changing its mechanical properties, especially their flow, penetration and stabilising behaviour. Among the additives are filtrate reducers (see (…

Inert solids are referred to as chemically inert substances. They do not participate, or only partly, in the chemical processes in the drilling fluid and are grouped according to their origin, as well as their utility, into:

• lost circulation material, and
• ballasting substances.

Lost circulation material is used for tackling drilling fluid loss so that its quantity, sizes and the form of its particles can close the pore space, cracks and fissures, through which the used drilling fluid is led off (Figure 136). So the fluid circuit is recovered or maintained.

(Image: Loss of drilling fluid – flowing through the borehole walls, e.g. in soil with large open structures or rock with fissures)

The shape and size or the distribution …

Ballasting substances are used for increasing the density and thus the static pressure of drilling fluid especially with the occurrence of water under artesian pressure. They can be used together with polymers. Here care must be taken of the fact that these substances – added individually or in combination – do not influence the primary properties of the drilling fluid in a negative way [HDD-14] [HDD-35]. The following points are decisive in their …

Inert solids are referred to as chemically inert substances. They do not participate, or only partly, in the chemical processes in the drilling fluid and are grouped according to their origin, as well as their utility, into:

• lost circulation material, and
• ballasting substances.

In contrast to inert additives, there are other, chemically active additives that can be added to drilling fluids. With them, the following aims can be achieved [HDD-14] [HDD-35]:

• reduction of viscosity (thinner),
• increase of viscosity (viscosifier),
• controlled flocculation (flocculants),
• reductions of the surface tension,
• chemically active lost circulation material to seal cavities and cracks in case of fluid loss,
• regulation of the pH-value,

The following can be used as inorganic admixtures to drilling fluids [HDD-14]:

• alkalis,
• bactericides.

Alkalis such as soda ash (Na₂CO₃), potash (K₂CO₃) or slaked lime (Ca(OH)₂) have the following purposes in specific cases [HDD-14]:

• adjusting the optimal pH value of bentonite drilling fluid to its optimal value of 8.5 up to 9.5 even when using filtrate reducers,
• increasing the viscosity of clay drilling fluids,
• minimising the corrosion of steel.

Chemically active as well as chemically inactive (inert) additives can be added for targeted influencing of the drilling fluid or of the water as the fluid’s base.

According to DIN V 4126-100 [HDD-222], chemically active additives also called mixing media, are added to the drilling fluid with the aim of changing its mechanical properties, especially their flow, penetration and stabilising behaviour. Among the additives are filtrate reducers (see (…

For several years now self-hardening drilling fluids have increasingly been offered for HDD. They fulfil the tasks of a drilling fluid during the drilling process and harden after the pipeline is pulled-in. With these so-called self-hardening drilling fluids, the time-consuming and cost-intensive work of annular space grouting can be saved after pulling in the pipe string (see (Annular space grouting)).

An example of such a self-hardening drilling …

(Image: Functional principle of a simple mixing plant for homogeneous, collodial open suspensions)The production and preparation of drilling fluid is carried out manually, semi-automatically or fully automatically (computer controlled) in a mixing plant (Figure 168). Here the raw material for the drilling fluid (water) is mixed with the additives (bentonite, polymers, etc.). The composition, i.e. the materials and their proportions for the drilling …

(Image: Removal quantities of drilling fluids of different consistencies for model spoil (6.4 x 4.3 x 3.2 mm) in the annular space with drill string at rest and vertical drilling path)Besides the specific properties of the conveying medium such as density (mass per unit volume) ρ, viscosity η and yield point τ_F, the flow velocity also has a particular influence on the removal of the spoil to the surface. Figure 172 shows the removal capability of …

Due to the numerous potential site conditions, the drilling fluid quantities needed for maintaining the drilling fluid circuit in HDD vary. This is why the drilling fluid quantity should be pre-calculated as accurately as possible or should be determined based on practical experience during the planning and preparation of a drilling procedure by taking into account the length and diameter of the borehole or the soil type, for example. If the processing …

Important aspects that also determine the cost effectiveness of a hydraulic conveying installation are the type and scope of the separation that is required and the preparation of the pumped solids-fluid mixture. The purpose of separation is the removal of the solid components from the fluid, on the one hand, to return the drilling fluid medium again to the conveying circuit and, on the other hand, to make the separated solids (spoil) fit for disposal. …

(Image: Disposal of the solid-fluid mixture as fertilizer on agricultural land)The pumping out and transporting of used drilling fluids, i.e. fluids that are loaded with solids and their cheap discharge into the sewer system or natural bodies of water to waste dumps or agricultural land (Figure 173) was often used in the past.

The growing environmental awareness of the public and especially the discussions in 1999 about the environmental friendliness …

Compared to deep drilling technology, which often uses oil-based fluids, usually the drilling fluids for HDD, which are exclusively produced on a water basis, do not contain pollutants a priori.

Over 95% of these drilling fluids are based on bentonite, a phyllosilicate found in nature (see (Bentonite drilling mud)), that is primarily unhazardous and is furthermore used as an food additive.

However, virtually all drilling fluid additives (even if they …

In all cases where liquid drilling fluid is used, it is necessary to utilize a separation plant (also called recycling plant) for cleaning and reclaiming the drilling fluid and dewatering the spoil in order to make it ready for disposal. In drilling technology, this process is described with the term “solids control”.

An optimally working solids control is a fundamental prerequisite for the technical and economic success of an HDD project and furthermore …

Solids control means that the solids content in a drilling fluid is kept on a necessary minimum level, so all excess solids must be removed from the fluid before the fluid is again pumped into the borehole [HDD-263].

With regards to uncharged fluids, solids control means that all solids have to be removed except the flushing clays (bentonite) that were added on purpose. A much greater proportion of these flushing clays is available in grain size ranges < …