TECHNICAL FIELD

The present disclosure relates to a unit for treating drilling mud, and to a corresponding treatment method.

BACKGROUND

The treatment unit and the treatment method of the disclosure are particularly adapted to treating drilling muds of the type used in deep foundation works such as diaphragm walls, rods, or piles, and also in the fields of tunnels and directional horizontal drilling.

During a drilling operation, the stability of the excavation or of the working face is obtained by filling the excavation with drilling mud. The mud forms a leakproof deposit on the walls of the excavation, which may be referred to as “cake”, and which serves to limit percolation into the terrain and prevent the walls from caving in. When the depth of the excavation has reached the desired level, the installation is filled progressively with concrete, beginning under the mud in the bottom of the excavation.

While it is in use, drilling mud sees its qualities deteriorate on contact with the ground, potentially making it unsuitable for use. Its density increases inevitably during digging, because of the added cuttings. Unfortunately, excessive density limits the efficiency of excavation, prevents satisfactory transport of drilling cuttings, and degrades the effectiveness of the drilling tool.

In order to conserve a drilling mud that continues to comply with the requirements for drilling, it is essential to treat it continuously in order to extract the largest particles, generally having a diameter greater than 80 micrometers (μm). This is referred to as “de-sanding” the drilling mud. Nevertheless, after two or three treatment cycles, its qualities are too degraded for it to continue to be used. It is then necessary to work with new mud and to manage stocks of drilling mud that cannot be reused.

For reasons of protecting the environment, regulations in an increasing number of territories across the world no longer accept spent drilling muds being disposed of in liquid form. Consequently, it has been necessary to develop methods of separating and dehydrating muds in order to make them pelletable (i.e. transportable without being liquefiable under the effect of vibration or shear stresses), prior to sending them to landfill. Unfortunately, drilling muds are typically characterized by very great stability and uniformity. The combination of particles of very small size, having a median diameter of less than 100 μm, or even 80 μm, or indeed 20 μm in most situations, associated with the presence of stabilizing agents such as certain clays or hydrosoluble organic polymers, gives them very great specific resistance against filtering and practically zero suitability for solid-liquid separation. In particular, drilling muds based on bentonite present stability and great resistance to settling/sedimentation because of their thixotropic nature.

Thus, in order to perform such separation on drilling muds so as to obtain a treatment product that is solid and pelletable, complex solid-liquid separation methods have been proposed, e.g. making use of rotary decanters comprising a rotary casing and, inside the casing, an extraction screw rotating in the same direction but at a different speed. Under the effect of centrifugal force, the dense solid particles of the mud are pushed towards the wall of the casing and then taken towards a first outlet of the decanter, while the liquid remains in the central portion, from which it is taken to a second outlet of the decanter. Such rotary decanters present the drawback of being very expensive and also very bulky.

In the publication of French patent application FR 2 875 495 A1, a mud treatment unit and method are described making use of a hydrocyclone for solid-liquid separation. Nevertheless, that disclosure relates to treating red muds coming from the Bayer method of fabricating alumina trihydrate, and does not relate to treating drilling muds. Although, like drilling muds, red muds do indeed present very fine grain size, they do not incorporate stabilizing agents such as those that make solid-liquid separation particularly difficult with drilling muds.

OBJECT AND SUMMARY

An object of the disclosure is to propose a drilling mud treatment unit and a corresponding method that remedy the above-mentioned drawbacks of the prior art.

In particular, an object of the disclosure is to propose a drilling mud treatment unit that is inexpensive and compact, and also a drilling mud treatment method that is inexpensive and easy to implement.

This object is achieved with a drilling mud treatment unit comprising:a primary duct for feeding coagulated drilling mud;an in-line flocculation system for flocculating the coagulated drilling mud flowing in the primary duct; andat least one hydrocyclone fed by the primary duct and arranged downstream from the flocculation system, the hydrocyclone having an overflow orifice for receiving a liquid product resulting from treatment of the drilling mud and an underflow orifice for receiving a solid product resulting from treatment of the drilling mud, the overflow orifice presenting an overflow diameter and the underflow orifice presenting an underflow diameter, and the underflow diameter being greater than the overflow diameter, with a ratio greater than 1.1 between the underflow diameter and the overflow diameter.

A hydrocyclone is a device that uses centrifugal force to separate solid particles in liquid suspension depending on their size. The hydrocyclone normally presents a main casing of cylindrical-conical shape that is fed tangentially under pressure and that has two outlets: an overflow orifice through which the finest particles and the liquid usually exit, and an underflow orifice through which the larger particles and a small fraction of liquid exit. Known hydrocyclones are typically used as classifiers for de-sanding drilling muds, or as thickeners in the field of water treatments, where they serve to thicken organic muds.

The inventors have shown that an appropriate ratio between the diameter of the overflow orifice and the diameter of the underflow orifice, associated with a physicochemical coagulation/flocculation treatment of the drilling mud enables the action of the hydrocyclone to be modified so as to obtain, in underflow, a solid treatment product from drilling muds that is pelletable.

The term, “a solid treatment product that is pelletable” is used herein to mean a product of solid consistency suitable for preventing flow, thus making it suitable for pelleting. For example, such a product should not present any measurable flow in a Marsh cone flow test, as defined for example in API Recommended Practice 13B-2. In underflow from the hydrocyclone, the solid treatment product may be accompanied by free water that can easily be separated therefrom, given the solid consistency of the solid treatment product, merely by gravity on a grid.

A liquid treatment product of drilling mud is on the contrary a product presenting liquid flow that does not enable it to be pelleted, this flow being measurable by the Marsh cone flow test.

In an example, the primary feed duct may be provided with a pump, and the in-line flocculation system may include a secondary duct for feeding flocculating agent, said secondary duct then being connected to the primary duct at a junction situated downstream from the pump.

In an example, the in-line flocculation system may comprise in-line mixer means for mixing the coagulated drilling mud and the flocculating agent, said means being provided in the primary duct downstream from the junction. These means facilitate mixing the flocculating agent with the coagulated drilling mud in order to form the flocculated drilling mud that is to be fed to the hydrocyclone.

For example, the mixer means may comprise a static mixer. Such a mixer guarantees good flocculation by ensuring good mixing of the coagulated drilling mud with the flocculating agent, while preserving the flocs formed by the mixing.

In a particular provision, in order to enable the mixture of coagulated drilling mud and flocculating agent to reach the hydrocyclone rapidly, the mixer means are arranged at a distance from the hydrocyclone in the primary duct that is shorter than 5 meters (m), for example, shorter than 2 m.

The ratio between the underflow diameter and the overflow diameter may lie in the range 1.1 to 1.6.

The present disclosure also provides a treatment method for treating drilling mud, the method comprising at least the following succession of steps:feeding a primary feed duct with coagulated drilling mud;flocculating said coagulated drilling mud in-line while it is flowing in said primary duct;introducing the coagulated and flocculated drilling mud into a hydrocyclone having an overflow orifice presenting an overflow diameter, and an underflow orifice presenting an underflow diameter, the underflow diameter being greater than the overflow diameter, with a ratio greater than 1.1 between the underflow diameter and the overflow diameter; anddischarging a solid product resulting from treatment of the drilling mud via the underflow orifice and a liquid product resulting from treatment of the drilling mud via an overflow orifice.

Furthermore, free water may be discharged, together with the solid treatment product, via the underflow orifice.

In particular, the drilling mud may include a stabilizing agent such as, for example, smectic clay and/or a hydrosoluble organic polymer. Smectic clays suitable for use as stabilizing agents in such drilling muds, include in particular various bentonites (natural sodium bentonite, activated sodium bentonite, sodium-calcium bentonite, or calcium bentonite), sepiolite, and attapulgite. For their use as a stabilizing agent for drilling mud, they may be incorporated in the drilling mud in the form of fine powders with particles having a mean size of less than 100 μm and a water content that may lie in the range 5% to 20%. Hydrosoluble organic polymers that are also suitable for use as stabilizing agents in such drilling muds, include in particular modified celluloses and their derivatives (carboxymethyl cellulose, polyanionic cellulose, hydroxyethyl cellulose), xanthan gum, and guar gum. Furthermore, in most circumstances, the drilling mud may be formed by an aqueous suspension of particles having a mean size smaller than 100 μm, or indeed 80 μm, or even 20 μm.

In an example, the solid treatment product may present dryness—i.e. percentage by weight of solid matter—that is greater than 30%.

In an example, the liquid treatment product may be water having less than 600 milligrams per liter of matter in suspension.

In a particular provision, the primary duct may be fed under turbulent conditions.

