What is the flow limit of fresh water

Self-hardening drilling fluids for the HDD process

Furthermore, it may be imperative to seal the drilling channel so that it is erosion-proof. In coastal regions, for example, artesian saline groundwater must be prevented from reaching the surface of the terrain through the borehole and leading to the salinisation of agricultural areas.
Non-positive stabilization and erosion-proof sealing of the borehole can neither be achieved by adding commercially available cement to a conventional drilling fluid nor by subsequent cement injection.
If commercial cement is added to the drilling fluid containing bentonite in the mixer, the drilling fluid is destabilized. The sedimentation of the solid particles suspended in the rinse forms up to 20% by volume of free water.
When cement suspensions are subsequently injected into the drilling channel filled with bentonite fluid, there is only a punctual entry of binding agent and also a high level of water settling.
Self-hardening drilling fluids are resistant to sedimentation despite the binder content and have good rheological properties (Tab. 1). The drilling process leads to an even distribution of the binding agent contained in the mud in the entire annulus. After the processing time has expired (approx. 2 days as standard), the rinse slowly hardens and thus creates a force-fit stabilization and an erosion-proof seal of the drill channel.
parameterMeasured value
Marsh time [s]43
Plastic viscosity [cP]9
Yield point [lb / 100ft2]25
Apparent viscosity [cP]22
Gel strength 10 seconds [lb / 100ft2]25
Gel strength 10 minutes [lb / 100ft2]29
Tab. 1: Typical rheological parameters of a self-hardening drilling fluid (solids content 160 kg / m3), Viscometer measurement.
Composition of self-hardening drilling fluids

Factory-made self-hardening drilling fluids contain a cement-stable sodium bentonite and a cement-based special binder.

Binder content
The binding agent used in self-hardening drilling fluids is specially tailored to the long processing times required in HDD projects. As a standard, the self-hardening drilling fluid does not harden until after approx. 2 days.
The formation of needle-shaped crystals, the so-called CSH phases, on the surface of the binder particles is decisive for the subsequent development of strength. The CSH crystals result from the reaction of the mineral phases contained in the binder (e.g. clinker phase C3S) with the mixing water. As a result of the dense network of CSH phases that forms, the material increasingly hardens. The standard final strength is around 0.3 N / mm2. When the binding agent reacts with the mixing water, calcium hydroxide (Ca (OH)2), which leads to an increased pH value.

Bentonite content
In conventional HDD rinses, natural sodium bentonites or sodium bentonites obtained through soda activation are used as standard. These are characterized by a high swelling capacity, which leads to stable, structurally viscous suspensions even with comparatively low solids contents. The main component of bentonite is the swellable clay mineral montmorillonite. If commercial cement is added to a conventional drilling fluid, the calcium ions released in the course of cement hydration are deposited in the interlayers of the montmorillonite crystals in exchange for sodium ions. There is a drastic reduction in the ability to swell. This results in a destabilization of the rinse and a settling of large amounts of free water (up to 20% by volume). In the case of the cement-stable sodium bentonite used in self-hardening drilling fluids, the presence of the calcium ions released by the binder content leads to a comparatively small decrease in the swelling capacity. There is no destabilization; the mud remains stable to sedimentation and after the processing time has elapsed there is a homogeneous strength development in the entire bore channel.

Quality assurance in the production of self-hardening drilling fluids

In order to ensure that the delivered product achieves the rheological and mechanical material parameters defined in the data sheet, extensive quality assurance is carried out at the factory. This is divided into:
  • Incoming inspection of raw materials
  • Production control
  • Production control

Incoming inspection of raw materials
Quality agreements have been concluded with the bentonite suppliers, which stipulate narrow limit values ​​for all relevant parameters. Every delivery of bentonite is sampled. Using a precisely defined test recipe - consisting of a bentonite sample and a calibration binder - both the relevant rheological parameters (Marsh time, flow limit, filtrate water, water settling) and the storage stability are tested. The bentonite is only released for injection into the silo if the specified limit values ​​are adhered to. By using the cement-containing test formulation, it can be checked whether the bentonite is suitable for use in products containing binding agents. The quality of the binding agent raw materials is checked using chemical analyzes. In the event of possible quality fluctuations, control measures can be taken in the run-up to drying or grinding. The grinding process itself is monitored online. Finally, the binder is examined for its strength development using specific test formulas.

Production control
In addition to the incoming inspection of the raw materials, the finished mixture is checked during the production of the self-hardening drilling fluid.
After the mixer driver calls up the recipe stored in the mixing computer, the first batch is mixed, the mixer is stopped and a sample is taken. This sample is checked for compliance with the target values ​​for flowability (Marsh time). If the setpoints are adhered to, production is released.
If the test of the sample from the first mixed batch should deviate from the target values, the dosage will be increased and the recipe stored in the mixing computer will be adjusted accordingly. The batch in the mixer is then sampled and checked again. Further production with the slightly corrected recipe is only released when it matches the setpoints.

Control of the finished product
The rheological parameters are determined on a sample of the finished mixture by means of a viscometer as well as the Marsh time, flow limit, filtrate water release and water settling after 1 day and after 5 days. In order to ensure that the self-hardening drilling fluid has sufficient strength development, a so-called rapid test is carried out. The shear strength of the sample is determined after storage at 40 ° C. for 2 days.
Then it is determined which shear strength development takes place at the temperatures prevailing in the soil (10 ° C). For this purpose, the drilling fluid samples are stored in a climatic chamber at 10 ° C. and tested after 7 days, 14 days, 28 days and 56 days.

Product properties and instructions for use

Preliminary tests
As a rule, the pilot drilling is carried out with a non-hardening mud, which is then displaced by the hardening drilling mud used in the expansion / retraction step. Here it can be useful to carry out preliminary tests to check which flow properties arise on the contact surface or the mixing zone between the two rinses. In these laboratory mixing tests, only 2 separately prepared rinses may be mixed with one another (no addition of dry powder!). It is important that the suspensions produced in the laboratory are prepared at high speed, as this is the only way to ensure that the bentonite components contained in the rinses are properly broken down. In order to simulate the mixing process of the two muds in the drilling channel, the conventional drilling mud is introduced and the self-hardening mud is slowly stirred in (e.g. with a whisk).
The mixing zone that forms in front of the expander head as well as the increasing displacement of the conventional by the self-hardening drilling fluid can be simulated by suspension mixtures with the following mixing ratios:
  • 70 vol.% Conventional irrigation / 30 vol.%
    Self-hardening mud - phase 1: The self-hardening mud flows into the drilling channel filled with conventional mud
  • 50 vol.% Conventional flush / 50 vol.% Self-hardening flush - phase 2: partial displacement of the conventional flush
  • 30% by volume conventional rinse / 70% by volume self-hardening rinse - Phase 3: Extensive displacement of the conventional rinse

On the basis of these rinse mixtures, it can first be determined whether there is an incompatibility of the two rinses, e.g. whether flocculation effects or immediate stiffening occur.
If a homogeneous and stable mud mixture is formed, viscometer measurements should be carried out to determine the flow properties. The Marsh funnel is only suitable to a very limited extent for assessing the flow properties.

Procedure on site

Mixing process
The preparation of self-hardening drilling fluids takes place with the supply of fresh water in the mixing units used as standard in HDD projects. If water is used for which it is not certain whether it is free of salt loads (e.g. river water), a preliminary investigation is useful. The solids content of the self-hardening drilling fluid is usually approx. 160 kg / m3. A good discharge capacity is achieved even with comparatively low marsh times. Self-hardening drilling fluids should be set at the construction site by varying the solids content to a Marsh time of 40–50 seconds.
The easiest way to set the Marsh time is by initially approx. 150 kg / m3 Solid be mixed. If the marsh time is less than 40 seconds at this solids content, solid is added until a marsh time. 40 seconds is reached. It should be noted that after each dosing step, a sufficiently long mixing time (. 2 minutes) is awaited before the Marsh time is determined.

Expansion / retraction step
If a self-hardening drilling fluid is used during the expansion / retraction step, the conventional drilling fluid is increasingly displaced in the mixing zone that forms in front of the expansion head. The retraction speed and the pumping rate are to be selected so that in the areas that the expansion head has already passed, at least 85% of the annulus filling consists of self-hardening drilling fluid.

Strength development
After the processing time of approx. 2 days has expired, the self-hardening drilling fluid initially stiffens and then slowly hardens. The final strength of the material corresponds to that of a well-compacted clay soil (approx. 0.3 N / mm2). Since the strength of the annular space filling corresponds to a good approximation of the strength of the surrounding soil, an optimal bedding of the drawn-in product pipe is achieved.
For projects in which very high strength and rapid strength development are required, special flushes with around 330 kg / m are available3 to disposal. However, these rinses have comparatively short processing times (approx. 8 hours).
It should be noted that the hardening process can be disturbed by organic substances present in the soil. Since the self-hardening drilling fluid hardens after the processing time has expired, the drilling rig must be cleaned after the drilling work has been completed.

Environmental sustainability
During the drilling process and after completion of the drilling work, there is direct contact between the groundwater and the drilling fluid in the drilling channel. It is therefore essential that the drilling fluid used is harmless in terms of water hygiene. Corresponding studies on self-hardening drilling fluids showed that the use of such fluids does not pose a risk to the groundwater [4].

When properly stored, self-hardening drilling fluids can be kept for approx. 6 months.

[1] Herrmann, G. (2006): personal communication.

[2] Stark, J .; Möser, B. & Eckart, A. (2001): New approaches to cement hydration.- ZKG, 2: 114-119.

[3] Märten, A. (2004): Cut-off wall pre-mixes - A “Black Box”? - Communication Inst. F. Grundb. and floor mech. Univ. Braunschweig, 74: 355-361.

[4] Hygiene Institute of the Ruhr Area (2001): Assessment of the drill-mix building material (groundwater - landfillability - application to agricultural areas - report C 2031/01 / hs.