WO1999054104A1

WO1999054104A1 - Method of preparing cement mixtures and mixer to be used for the implementation of the method

Info

Publication number: WO1999054104A1
Application number: PCT/NO1999/000127
Authority: WO
Inventors: John Kenneth Jamth
Original assignee: Offshore & Marine As
Priority date: 1998-04-22
Filing date: 1999-04-19
Publication date: 1999-10-28
Also published as: NO981784D0; NO308346B1; AU3540799A; NO981784L

Abstract

A method and a mixing plant (10) for mixing together components comprised in a cement mixture. The components are added separately to a first chamber (12) equipped with an agitator (14, 16, 16', 18, 18') in which they are stirred and mixed together and forced downwards into an underlying chamber (22), in which the partially mixed mass performs a circulating flow. The circulating, partially mixed mass is displaced by mass supplied from above and is forced to flow upwards through an annular transition opening (42), into an abovelying, third chamber (24) forming the output chamber. Complete cement mixture flows out into a pipeline loop (46), into which there is mounted a pump (52) arranged to pump the cement mixture further to the place of use, for example into a subsea well in which casing is to be cemented.

Description

METHOD OF PREPARING CEMENT MIXTURES AND MIXER TO BE USED FOR THE IMPLEMENTATION OF THE METHOD

This invention relates to a method of preparing cement mixtures, in which cement, water and possible additives are fed separately into a mixing vessel in which they are stirred together continuously or batchwise for the formation of a cement mixture of the kind used in cementation operations in oil or gas wells. In such cementation operations it is very important that the cement, water and possible additives are intimately mixed, so as to ensure the desired compressive strength upon curing.

The invention also relates to a mixer to be used for the convenient implementation of this method.

Drawbacks of current methods of mixing and mixing plants are that the substances comprised in the cement mixture often are not mixed together well enough until the cement mixture is pumped into the well bore, and that bridging of the cement silo leads to too small an amount of cement being supplied per time unit compared to the amount of water added in a cor- responding period of time. Too wet a cement mixture leads to, among other things, the amount of cement mixture being fed to a point of cementation, resulting in a smaller volume of cured concrete than that achieved from a cement mixture with a correct water content. Cement is often fed from a silo to a mixing vessel by means of pressurised air. Such feeding of cement, based on pressurised air, has proved to be highly unreliable .

In accordance with the present invention a worm screw is used as the feeding device for cement from the silo to the mixer, while at the same time it is aimed at alleviating or reducing, to a considerable degree, defects and drawbacks associated with known and conventional cement mixing techniques also in other respects.

This has essentially been achieved by proceeding in accordance with the characterizing part of claim 1. A cement mixer according to the invention excels through the features appearing from the subsequent dependent claims .

Cement is fed to the mixer in a downward flow leaving said worm screw at the end portion adjacent to the upward opening of the underlying mixing chamber, while water and possible chemicals are injected right into the cement flow through nozzles having their mouths at a downward angle.

In the mixing vessel are arranged, as known in itself, one or more agitators, preferably in the form of vertical, rotatable shafts provided with transverse, stirring blades, similar to for example knife blades, and/or stirring organs of other circumferential shapes, for example shaped blades with the largest width at the outer end. According to the invention, below this mixing chamber, in which these propeller-like stirring elements carry out a forced stirring and mixing process, an underlying chamber is arranged. According to the rotational speed of the shafts and the dimensions and configuration of the blades, the stirring elements create a particular flow pattern and subject the matter being processed, to shear forces, and forces the matter, which then presents an initial degree of mixing, into the underlying chamber.

Preferably concentrically around said mixing chamber with the agitator, is a wider annular chamber.

The underlying chamber which has a larger width than the mixing chamber with the agitator, but preferably somewhat smaller width than said abovelying chamber concentrically surrounding the mixing chamber with the agitator, is communicating upwards with the abovelying chamber presenting an annular, horizontal cross-section, so that matter which has been further mixed in the underlying chamber and is displaced by matter supplied from above, is brought to flow in a direc- tion upwards from an annular area of the underlying chamber, located outside the outer lateral limits of the abovelying mixing chamber with the agitator, but within the outer lateral limits of the upper annular chamber.

Trials have shown that a cement mixture which has passed the three chambers successively, and has been mixed in each of them, will be a complete mixture when it lands in the upper, annular chamber, and that the degree of mingling is very high then and consequently highly satisfactory. From the annular, upper chamber which has the largest width and all in all the largest volume of the three chambers built together, a pipeline may extend to feed out ready cement mixture. This pipeline, which is advantageously taken in a closed circuit, thus forming a loop which has an outlet and an inlet in the annular output chamber, has an overcapacity pump arranged thereto, for the pumping of the complete cement mixture in said closed circuit.

Somewhat upstream of its inlet in the annular output chamber, the pipeline loop has a further pump unit arranged thereto, whose task it is, to pump complete cement mixture down to the place of use. When the place of use cannot accept so much ready cement mixture as is produced by the stirring and mixing plant, and thus cannot keep up with said plant, the plant may, however, keep at disposal and in stirring circulation an amount of cement mixture corresponding to the sum of the amount that can be accommodated by the three chambers and the amount accommodated by the pipeline loop. The central upper mixing chamber with agitator may have a lower wall than the annular output chamber surrounding it, so that in a completed cement mixture from the annular output chamber, in such a situation in which the supplies of cement, water and chemicals to the plant are temporarily stopped, has the possibility of re-entering the upper, central mixing chamber. Thereby is enabled a continuous circulation of cement mixture in such a stand-by situation, in which completed cement mixture flows through the pipeline loop, the upper annular output chamber, out of this chamber again, partly to the pipeline loop, partly to the upper, central mixing chamber equipped with the agitator, and therefrom to the underlying, wider chamber, and from this chamber up into the abovelying, annular output chamber and so forth. The method of mixing and the mixer for preparing cement mixtures of the kind in question are described in detail in the following, with reference to the accompanying drawings, in which

Fig. 1 is a schematic vertical section/side view of a mixer for preparing cement mixtures in accordance with the method according to the invention;

Fig. 2 shows a top view of the same mixer, and shows the circumferential shape of three chambers built together, in which the mixing takes place;

Fig. 3 shows a top view of a mixing plant, in which the mixer according to Figs. 1 and 2 is included, and in which is further included a cement silo with feeding screw, pipelines with nozzles for supplying the upper, central mixing chamber of the mixer cement, water and chemicals, respectively, arranged thereto, and in which mixing plant there is further included a pipeline loop connected to the output chamber for circulating complete cement mixture to a pump-out unit with shut-off valve, alternatively back to the output chamber; and

Fig. 4 is a side view of the mixing plant of Fig. 3.

A mixer 10 which is included in a mixing plant (Figs. 3 and 4) appears best from Figs. 1 and 2. The mixer 10 consists of an upper, central, relatively narrow chamber 12 which has an longitudinally oval circumference.

The central chamber 12 is provided with a symmetrically placed, driven agitator consisting of two vertical, rotatable shafts 14, 14', each provided with stirring organs 16, 18 in the form of knife-blade-like pairs of blades or pairs of propeller blades, maybe of varying shapes. The lowermost pair of blades 16' is arranged in a transition opening 20 between the central mixing chamber 12 and the underlying chamber 22 of circular circumference and larger width than the central, upper mixing chamber 12 with the agitator 14,14', 16,16', 18, so that it surrounds the projection thereof, see Fig. 2.

A third, annular upper chamber 24 of circular circumference and larger diameter than the underlying, second chamber 22, surrounds the central, upper chamber 12 concentrically, as shown in both Figs . 1 and 2.

The circumferential wall of the upper, inner, central, first mixing chamber 12 is terminated at a distance below a lid or a ceiling 26 which is shared by the two upper chambers, the first and the third chamber 12 and 24, respectively, so that in a given situation, for circulation purposes, mixture shall have the possibility of flowing over the upper edge of the wall of the first chamber 12 and into that chamber. This will be explained in further detail in connection with Figs. 3 and 4.

In Fig. 1 is indicated the downstream end of a worm screw 28 serving as a feeding screw for cement from a cement silo 36 to an area immediately above a central, upper opening 12' of the first chamber 12. Immediately below the cement inlet point above the opening 12' to the mixer 12,22,24 are nozzles 32', 34' on pipelines 32, 34 carrying water and chemicals. On each vertical stirrer shaft 14, 14', in the area of water and chemicals supply, is arranged a pair of blades 18 ' to start the mixing process . The worm screw 28 has its upstream end portion positioned below a downward outlet 38 of a cement silo 36 and works in a pipe or similar 40.

According to Fig. 4 the longitudinal axis of the worm screw 28 extends in a slightly upward slope from the outlet 38 of the silo 36 to the downstream end portion above the upper inlet opening 12 ' of the first chamber 12.

When cement fed from the feeding screw 28, water fed from the nozzle 32 ' and chemicals fed from the nozzle 3 ' come to- gether in the upper portion of the first, central mixing chamber 12 equipped with the driven mixing plant 14, 14', 16,16', 18,18', the mixing plant provides an intensive rough mixing of the components of the resulting cement mixture, while all the time the mass is being worked downwards, to where the lowest pair of blades subjects it to the most shear forces. Through the transition opening 20 the mass is forced down into the second, underlying chamber 22 and further up into the abovelying camber 24 through openings 42.

A third, upper, annular chamber 24 constitutes the output chamber of the built -together and fluid communicating cambers 12, 22, 24.

For this purpose the third, upper, annular output chamber 24 is provided with a pipe-socket-like outlet 44 which is connected to a pipeline 46, which, in accordance with the em- bodiment of the mixing plant shown in Figs. 3 and 4, is taken in a closed circuit back to an inlet 48 leading to the output chamber 24, approximately on the diametrically opposite side. At a distance from both the outlet 44 and the inlet 48 of the output chamber 24, the pipeline 46 extending in a closed circuit is connected to a first pump 50 serving to circulate complete cement mixture in the pipeline and possibly also in the output chamber 24. A second pump 52 serves to pump completely mixed cement mixture out to the place of use, for example a well bore in which casing is to be fixed by cementation.

If the place of use is capable of accepting as large an amount of cement mixture per time unit as the mixing plant is capable of producing per said time unit, the process may be planned for continuous operation as long as the cementation work lasts.

If the cementation work is intermittent, the process may be adjusted for batch production and delivery/pumping out of cement mixture, as necessary.

Contrary to this, if the place of use is not able to accept finished cement mixture at the rate of production, there may be kept ready in the mixing plant, awaiting delivery shortly, the amount of cement mixture which can be accommodated in the pipe line loop and the three mixing chambers 12, 22, 24. Of course, the supply of cement, water and chemicals is stopped immediately, once the pipe loop and chambers are filled, after the shut-off valve arranged to the second pump 52 has stopped the supply to the place of use.

In such a case cement mixture in the third, annular, chamber 24 may flow over into the first, central, mixing chamber 12, while, at the same time, the third annular, output chamber 24 has the possibility of feeding mixture to the pipeline loop 46, in which the first pump 50 maintains a circulation of the cement mixture through the mixing plant .

Claims

C l a i m s

1. A method of preparing a cement mixture for use in subsea wells, wherein cement, water and possible chemical additives are added to a first mixing vessel (12) in which the compo- nents of the cement mixture are stirred by a mechanical mixing plant (14,16,18) which gives rise to currents in the wet mixture enhancing the mixing of the cement, and in which a mixing takes place at two levels, in an upper and in a lower mixing zone (12 and 22), c h a r a c t e r i z e d i n that the wet matter under mixing, is brought to flow up from said underlying mixing zone, which is formed by a lower, second mixing chamber (22), into a third, abovelying chamber (24) surrounding said first mixing chamber (12) and forming the output chamber (24) of the mixing plant.

2. A method as claimed in claim 1, c h a r a c t e r i z e d i n that the finished cement mixture at the outlet (44) of the third chamber, the output chamber (24), is lead into a pipeline loop (46) connected to the outlet (44) to be able further to circulate forcedly before the cement mixture is pumped into said subsea well.

3. A method as claimed in claim 2, c h a r a c t e r i z e d i n that the finished cement mixture is circulated - when a valve connected to said pump (52) is closed - back to the output chamber (24) through an inlet (48) and is then brought to flow in a closed circuit created by the pipeline loop (46) and the output chamber (24) .

4. A mixing plant for the preparation of a cement mixture to be used in subsea wells, in which cement, water and possible chemical additives are added separately through separate supply organs to a first mixing vessel (12) equipped with a mechanical agitator (14,16,18) to create currents in the wet matter, enhancing the mixing, and in which the first mixing vessel (12) is formed by an annular partition wall extending upward from the bottom wall of a wider vessel and is terminated at a distance from the ceiling wall of this wider vessel, so that the first mixing vessel (12) and an annular, surrounding vessel (24) communicate with each other at the top, c h a r a c t e r i z e d i n that said first chamber (12) and the annular chamber (24) surrounding it, constitute the first and third chambers of a mixing chamber aggregate comprising three chambers (12, 22, 24) in which said first chamber (12) forms a central, first chamber which is in fluid communication with an underlying, second mixing chamber (22), which in turn is in fluid communication with the abovelying, annular, third chamber (24) which has a larger horizontal extent than the underlying, second mixing chamber (22) , and that a bottom opening of the first mixing chamber (12) commu- nicates with the underlying, second mixing chamber (22), while a bottom opening (42) of the third, annular chamber (24) communicates with the underlying, second mixing chamber (22), as the direction of flow through the latter opening (42) is from the bottom upwards, while that through the bot- tom opening (20) of the first mixing chamber (12) is from the top downwards .

5. A mixing plant as claimed in claim 4, c h a r a c t e r i z e d i n that the output chamber (24) surrounds the first chamber (12) concentrically, and that the wall of the first chamber (12) in an upper portion surrounded by the output chamber (24) , has a perforation or similar so that the first chamber is in limited fluid communication with the output chamber (24) .

6. A mixing plant as claimed in claim 5, c h a r a c t e r i z e d i n that the output chamber (24) with a pipe-socket-like connecting organ (44) is connected to a pipe line loop (46) which has a circulation pump (50) arranged thereto, and that the pipe line loop (46) has another, closable pump (52) arranged thereto for pumping complete cement mixture forward to the place of use.

7. A mixing plant as claimed in claim 4, 5 or 6, c h a r a c t e r i z e d i n that said first chamber (12) equipped with agitator (14,14', 16,16', 18,18'), above its inlet opening has the downstream end of a worm screw (28) arranged thereto, for feeding cement from a cement silo (36) , and that above said inlet opening are arranged nozzles (32 ' , 34 ' ) on the end of pipelines for separate adding of water and chemical additives.

8. A mixing plant as claimed in claim 7, c h a r a c t e r i z e d i n that the agitator in the form of pairs of blades or similar (16') are arranged in the bottom opening (20) of the first chamber (12) which forms the transition opening between the first and second chambers (12, 22), thus forming a rotating choking organ in this opening (20) .