In drilling operations for oil or gas, particularly offshore, one has in recent time become increasingly concerned with questions of environmental influence and pollution. Drilling mud coming up to a drill rig during drilling among other things contains drill cuttings in large amounts, and it is a problem to get rid of this waste which is strongly contaminated and it has been attempted to clean it in various ways.

When treating and cleaning drilling mud containing drill cuttings it is also of much interest to retrieve valuable constituents which may be re-used, i.e. may be recirculated. This in the first place applies to the original drilling mud, including specific constituents being added thereto for attaining desired properties, and these constituents may also be in the form of particles.

Within the oil industry there have been made great efforts to find solutions to these problems and tasks, but without any satisfactory methods having been found thus far. This, inter alia, appears from a report - "OIL BASED DRILLING FLUIDS"- published at the occasion of a conference on the theme in Trondheim, Norway in February 1986. Of particular interest in the present context is an article at page 71 of the report: S. Davies - "An Assessment of the Development of Cuttings Cleaning System within the Context of North Sea Offshore Drilling".

Among previously proposed solutions may be mentioned washing with seawater, briguetting, installation of a large tube on the sea-bed and filling of drill cuttings and waste at the top of the tube, and possibly to pump up oil which with time is collected at the upper part of the tube. Also a ther o-dynamic process has been tried, based on the

grinding of drill cuttings in a mill with development of frictional heat which should bring contaminating oil to vapourise from the drill cuttings.

The present invention is directed to an improved and more complete form of treatment of re-circulated drilling mud during drilling for oil or gas, primarily for use with oil-based mud, alternatively with water-based mud, and comprising in addition to a de-gasser step and a vibration screen unit, also one or more steps for separating coarse, respectively finer drill cuttings from the mud. The invention relates to a combination of process steps with the employment of treatment units or machines or apparatuses which individually per se are to a large extent previously known from various technical areas, but in particular from land-based mining industry.

The novel and specific combination of features in the process according to the invention, in the first place consists in a) Separation in a first step by employing a screw classifier having a closed tube around the screw and a supply of oil, alternatively water in counterflow against the conveying direction of the screw classifier, as well as an overflow for drilling mud from the tank of the screw classifier. b) Transfer of mud from the overflow to cyclone means having associated screen means for separating drill cutting particles from the mud, and c) de-watering in at least one vibration de-watering unit through which there is supplied superheated vapour and chemicals, preferrably in the form of a leaching solution and metasilicate, and which receives drill cuttings separated out in the screw classifier.

Other specific features of the process and a corresponding arrangement according to the invention, for the treatment of recirculated drilling mud, are recited in the claims.

In the following description the invention shall be explained more closely with reference to the drawings, of which:

Figur 1 shows a simplified flow-diagram for a complete arrangement for processing of recirculated drilling mud, in particular on board an offshore drilling rig, and

Figur 2 shows the main components of a screw classifier incorporated in the arrangement of fig. 1.

In Figur 1 there is purely schematically shown a borehole 1 and a return conduit 2 for drilling mud from the borehole. The mud contains the original drilling fluid and besides drill cuttings from the drilling process and possibly other more or less contaminating substances or particles. In the usual manner the returned drilling mud is first passed through a de-gasser as shown at 3 in the drawing, and a usual vibration screen 6. From the screen the resulting coarse fraction as shown at 4 is conveyed to a first separating step in the form of a so-called screw - classifier 10 which is also a type of machine being known per se, for example as designed and built by the firm of Sala International AB, Sala, Sweden.

Reference is now made in particular to fig. 2. The screw classifier 10 as its main components has a rotatable screw 12 which is enclosed by an inclined tube 13 the lower end of which is emersed into a tank 11 which likewise can have sloping walls and bottom. There is shown a filling of drilling mud to a level 16 in the tank 11, whereby the level 16 is determined by an overflow 17 from the tank. The screw 12 with the surrounding tube 13 during operation will have the function approximately like a screw conveyor which takes up and moves drill cuttings from the lower part of the tank 11 upwards to the left in the drawing, the coarse portion of the goods being thereby brought out of the tank 11. Simultaneously the liguid fraction of the tank contents will flow back through the tube 13 so as to be discharged from the tank 11 over the

overflow 17 mentioned above.

At 15 and towards the outlet at the top of the tube 13 there is indicated a spray of oil or another liguid in counterflow against the goods in the form of drill cuttings which is being transported upwards towards this outlet through the tube 13. Thereby there is provided for a thorough washing of the goods at the same time as the processing and pressing which the goods are subjected to in the tube, results in a feed-back of any valuable substances and particles together with the liguid, to the tank 11. Here, among other things, there may be the question of barite which can adhere to the coarse portion of the drill cuttings. Barite and other fine particles together with the drilling fluid will leave the tank 11 at said overflow 17. It is to be explained more closely below how in this manner one can recover chemicals, oil, salts, polymers etc. being present in this liquid phase together with valuable weight particles.

Drilling fluid from the overflow 17 with its possible contents of fine particles will be pumped together with the fine fraction from the screen 6, through a conduit 9 to a drilling fluid cleaning step comprising a number of cyclones 20 and a screen device 22. The coarse fractions from the cyclones and the screen are conveyed through a conduit 27 to a buffer tank 91 with following specific process steps for drill cuttings and particles, whereas the liquid fraction from the overflow of the cyclones 20 goes to a mud tank 100. The same happens to the liquid fraction from the screen 22. Before these liquid fractions enter the mud tank 100 they may be subjected to sampling in the unit 29 shown. In the usual manner tests can be taken here by means of samples.

As shown with dashed line connections there can be provided a centrifuge 25 between the cyclone-screen device 20/22 and the mud tank 100 with the sampling unit 29, this centrifuge 25 providing the possibility for controlling the

viscosity of drilling mud or fluid being re-circulated to the tank 100. Return flow from the centrifuge 25 may be lead to the conduit 27 as also indicated in the drawing. The generally coarse goods or drill cuttings being discharged by the screw 12 from the outlet at the top of the tube 13 in the particle separator, i.e. the screw classifier 10 (see fig. 2), at 7 is supplied to a first vibration de- watering unit 30 which serves to wash away the residue of drilling fluids, oil and small particles. The liquid phase and the particles are conveyed to the buffer tank 91 which can be equipped with an agitator. Water and chemicals can be introduced into the tank as shown at 96. The overflow material, i.e. the coarse particles and constituents from the vibration de-watering unit 30 then proceed to a washing and mixing unit 40 to be described more closely below. As an alternative these particles and constituents can be conveyed directly to a second vibration de-watering unit 50.

More specifically the vibration de-watering unit 30 (and 50) can be of the type which by means of vibration ^" effect conveys the goods from an input end of the unit towards a threshold at the output end of the unit, thereby causing a damming up and thus compression of the goods against the output end, whereby the liquid is squeezed out and can leave the unit, for example through slits ("overflows") in the side walls and the end walls at the input end. The vibration de-watering unit 30 can for example be of a type being produced by the company IFE, Waidhofen/Ybbs, Austria.

A particular feature of the vibration de-watering unit 30 is that it is surrounded by a closed chamber 30A which serves to delimit a space being filled with superheated vapour supplied at 31 as shown. Also chemicals are supplied at 31.

A quite essential step in the treatment being illustrated in the drawing, consists in a washing and mixing unit 40 which preferrably is in the form of a mill, through

which drill cuttings and particles supplied at 33 from the vibration de-watering unit 30, are treated and ground. This mill unit 40 has a double function?

1. It provides the possibility of mechanical washing and scrubbing of the particle surfaces.

2. It serves to crush and grind the rock contained in the drill cuttings, so that oil or liquid which may be entrapped in large pieces and particles, can be released.

Together with the two effects mentioned above, it is an essential feature in the complete treatment process that emulsifying chemicals are supplied to the mill 40 as shown at 42, while water at the same time is added together with these chemicals. It is obvious that the treatment taking place in the mill 40 will be not only pure washing or scrubbing and grinding, but also a thorough mixing of the whole contents in the mill so that the emulsifying chemicals immediately and effectively can have their effect.

A mill being suitable for the treatment step just mentioned, may be of the type SRR being supplied by the above Swedish company.

In order to obtain a more effective crushing or grinding there can be provided a cyclone in association with the mill 40,.as indicated at 45, with recirculation of separated coarse particles to the mill for additional or repeated crushing. Thus the cyclone 45 is included in a closed circuit together with the mill 40.

The next main step in the treatment is a second vibration de-watering unit 50. As mentioned this may be of the same type as the vibration de-watering unit 30, with vapour and chemicals being introduced as shown at 52, preferrably together with water for additional washing of the particle mass and enhancing the emulsifying effect. The vibration de-watering unit 50 thereby represents an additional separating step for particles and liquid, and if required can be directly series connected to the first vibration de-watering unit 30, and similar thereto can also

be provided with a surrounding chamber 50A.

The overflow 55 from the vibration de-watering unit 50 gives a well-cleaned and pollution-free goods or drill cuttings which can be dumped right into the sea after sampling as shown at 69. The liquid phase from the vibration de-watering unit 50, i.e. the emulsion of water and oil with fine particles, is directed to a filter 92, if required through a centrifuge.

The filter 92 is a very significant unit in the arrangement. Together with the emulsion from the vibration de-watering unit 50 the filter recieves a batch or volume controlled supply of the mixed contents of the buffer tank 91, which among other things makes it possible to incorporate stepwise or batchwise operating treatment stages in the cleaning process being otherwise in principle continuous. If the filter 92 is a press filter it works batch-wise, whereas a band filter will have a continuous function. Both these and various other, commonly known, filter types can be employed. Irrespective of the type of filter it is a substantial advantage according to the invention to arrange the filter 92 in a closed chamber 92A or an autoclave for treatment with super-heated vapour during the filtering. An additionally enhanced effect of the filter step is obtained by also adding chemicals such as a leaching solution and metasilicate during filtering. The supply of vapour and chemicals is illustrated at 94. In the case of a band filter chemicals, vapour, water and so forth can each be introduced at separate and subsequent zones along the band, possibly in an alternating manner along the band. In the usual way the filter band can consist of a textile-based filter screen, perforated rubber or the like. The filter step 92 with vapour and chemical treatment in a closed chamber contributes substantially to the attainment of a high degree of cleaning. From the filter the particles and the solid substances can be dumped into the sea as shown, preferrably after the taking of samples in the

sampling unit 69.

At several places in the arrangement there can be provided centrifuges each of which can advantageously be placed in a ^" closed chamber or an autoclave for treatment with superheated vapour during centrifugation. This centrifugation as well as the filter 92 and other units of the arrangement, which according to this description are provided in a closed chamber for vapour treatment, can comprise sluice devices for feeding in and feeding out so that the vapour pressure in the chambers can be maintained substantially at the desired level. The repeated treatment with vapour and chemicals in several of the cleaning steps is very advantageous to the total effect of the process, so that drill cuttings and other waste will be so well cleaned that it can safely be dumped for example in the sea, whereas valuable constituents of the used drilling fluid can be recirculated for re-use.

The liquid phase from the filter 92 after possible centrifugation in a centrifuge 65 with enclosure 65A and possible vapour supply 64, is conveyed to a treatment tank 70 provided with an agitator 71. An essential feature of the treatment tank 70 is that there are added emulsion reversing chemicals as shown at 72. Accordingly the desired and useful emulsion through the preceeding treatment steps is broken up or dissolved, which results in the two phases oil and water. In this connection there is reason to emphasise that essential or active constituents in the chemicals being added in the different units, for example at 42 in the mill 40 as well as at 31 and 52 in the vibration de-watering units 30 and 50 respectively, can comprise a leaching solution and metasilicates or similar substances, whereas the substance added at 72 in the treatment tank 70 in such case preferrably comprises sulphuric acid. Such a combination of chemicals has particular advantages in this connection.

The mixture from the tank 70 is fed to a last treatment

step in the form of a flotation tank 80 which also can have an agitation (not shown). More essential however is the provision of means for air injection at the bottom of the flotation tank 80, as schematically shown at 84, which is known per se from flotation methods for example in the mining industry. In the tank 80 therefore there will take place a separation of drilling fluid or oil which collects at the top of the liquid filling in the tank and from there can be pumped up and subjected to sampling and then to be transferred to the above mentioned drilling fluid or mud tank 100 for recirculation and re-use. As an alternative collected oil or drilling fluid from the flotation tank 80 and besides from the cyclone-screen device 20/22 and the centrifuge 25 and other units can be fed to other tanks, for example for transportation onshore as indicated at 105 or to another storage tank as indicated at 110.

As a last precaution aiming at the termination of the emulsifying effect emulsion reversing chemicals can also be added to the flotation tank 80 at 82. Clean water can be returned through a conduit 88 for possible sampling at 69 and dumping into the sea together with clean drill cutting particles. On the drill rig fresh water constitutes a valuable resource. Collection and recirculation of clean water from the treatment described, in particular from the conduit 88, can be very advantageous. Thus there can be provided a conduit (not shown) for feeding back recovered water for use in the mill 40.

In figure 1 there is shown a possibility of providing a magnet separator 90 for the case where the separated material from, inter alia, the screen device 22 contains valuable magnetic particles which advantageously can be recirculated, for example to the tank 100 as indicated. Like the plurality of the other treatment units involved in the system of the drawing, the magnet separator 90 is also a type of apparatus or machine being known per se, and which should not need any further detailed description.

The process and the complete arrangement as shown in the drawings are primarily intended for treating oil-based drilling fluids, which apparently represent the most serious problems with respect to pollution, but also water-based drilling fluids can be treated with advantage in the arrangement, since the specific separation by means of the screw classifier 10 with its associated vibration de- watering unit 30 at one hand and the cyclone-screen device 20/22 on the other hand, result in an efficient treatment. In contrast to known methods which involve the dumping of the drill cuttings waste after coarse separation with possible cleaning and /or additional treatment en a cyclone- screen device, a process and an arrangement according to the invention as here described, lead to a fully acceptable cleaning of both solid substances and water and other liquid involved in the process. Thereby the environmental and pollution problems associated with recirculated drilling mud, will be substantially eliminated.