GB2162880A - Processing drilling fluid - Google Patents

Abstract

A self-contained drilling fluid treatment assembly, e.g. in a sea-going vessel 1, includes a treatment unit for connection to at least one storage tank (9-11) of a vessel. The unit comprises a plurality of process tanks, a mixing unit, and a pair of centrifuges. Each tank includes agitation means to maintain solids in suspension in the fluid. <IMAGE>

Description

SPECIFICATION Processing drilling fluid The invention relates to apparatus for preparing and recycling drilling fluids.

Drilling fluids or drilling muds are widely used in the drilling industry to assist the drilling process. Typically, they comprise relatively high density muds of relatively high viscosity some functions of which are to enable drill cuttings to be entrained and removed, to cool and lubricate the drill bit and general well pressure control. Drilling fluids are forced down the drill string in use and pass up and out of the hole being formed. In the past, some used drilling fluids particularly low toxicity oil based fluids, have been passed into storage tanks and shipped to an onshore processing plant where the fluids are reprocessed for further use. This is a time consuming system and is very costly.

We have devised a completely new apparatus in accordance with the present invention by providing a self-contained drilling fluid treatment assembly including a treatment unit for connection to at least one storage tank of a vessel, the unit comprising a plurality of process tanks, a mixing unit, and solids content control means, wherein each tank includes agitation means to maintain solids in suspension in the fluid.

We have discovered that a small sea-going vessel having a capacity as low as 1 500 barrels can provide a processing facility for an offshore rig. Conveniently, the capacity is substantially 3000 barrels. Furthermore, the provision of agitation means in each-tank enables a compact and self-contained drilling fluid preparation and recycling vessel to be con structed.-This vessel enables not only used drilling fluid to be recycled but also the preparation of fresh drilling fluids which is particularly important where changes in the properties (eg weight) of the fluid are required.

The number of storage tanks provided depends on the size of the vessel involved and the quantity of fluid it is desired to store. In general, it is desirable to use a number of small storage tanks rather than one or more large tanks in order to assist movement of fluids between tanks.

Preferably, the agitation means is provided by at least one rotatably driven (conveniently hydraulically driven) paddle. The position of the paddle, its size and rotational speed are determined empirically depending on the size of the tank concerned.

One of the problems which could occur with the containment or storage of fluids containing solids in tanks is settling out of the solids and-this is mitigated to a large extent by the inclusion of agitation means. However, we have found that additionally it is preferable if the storage tanks are partly defined by a skin mounted within the huil of the vessel.

This provides the tanks with a smooth surface which assists the action of the agitation means.

Preferably, the lower corners of each tank are dressed out to minimise solids settling.

However, we have discovered that a significant improvement in the apparatus is achieved by providing in addition to the agitation means, cleaning jets at at-least some of the lowermost corners of the tanks. These jets when provided in conjunction with dressed out corners minimise the settling of solids and are particularly advantageous. Preferably, cleaning jets are provided at each of the lower corners of the. tanks and conveniently a pair of cleaning jets generally angled towards each other are provided at the corners. Typically, the cleaning jets may comprise compressed air jets connected to a source of compressed air.

Conveniently, the agitation means and other processing equipment are hydraulically driven.

In the case of the agitation means, this allows the motors to be positioned within the tanks.

Fluid may be passed between the tanks by any conventional means such as hydraulically driven fluid pumps. Thus all equipment can be driven from a single hydraulic source.

In the past rotary solids separators have been used to remove solids. In the invention, the solids content control means may comprise a decanting centrifuge but preferably comprises two decanting centrifuges arranged in series, the first centrifuge being arranged to separate drilling fluid into a fraction containing solids having a size greater than 5 microns, the remaining fraction being fed in use to the other centrifuge where it is separated into a subsidiary fraction containing solids having a size greater than 1.5 microns which is discarded. This solids content control means enables a significant amount of free liquid to be retained while removing a substantial proportion of the fine solids.

The use of two centrifuges also enables conventional processing to be carried out in parallel.

An example of a vessel incorporating a fluid preparation and recycling assembly in accordance with the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a side elevation of the vessel with some parts shown in phantom; Figure 2 is a cross-section through part of the vessel shown in Figure 1 illustrating the fluid preparation and recycling assembly in more detail; Figure 3 is a section taken on the line 3-3 in Figure 2; Figure 4 is a section taken on the line 4-4 in Figure 2; Figure 5 is a perspective view of the fluid preparation and recycling assembly; Figure 6 is a plain of part of the main deck of the vessel shown in Figure 1; Figure 7 is a side elevation of an agitator for use in a storage tank;; Figures 8A and 8B are a section and perspective view respectively of a typical lower corner of a storage tank; and, Figures 9-1 2 illustrate diagrammatically four different treatment processes which can be carried out.

The vessel or ship illustrated in Figure 1 is based on a conventional mud carrying vessel and so will not be described in detail. Essentially, the vessel includes a hull 1 which at its fore end 2 surrounds crew accommodation and the like. Also at the fore end 2 a superstructure 3 including a bridge and the like is provided. The main body of the vessel 1 includes two drive systems 4 (only one shown) connected via respective drive shafts 5 to propellor systems 6. A rudder 7 connected to conventional steering gear 8 (Figure 6) is positioned at the aft end of the vessel.

Six storage tanks are positioned on either side of a mid-portion of the vessel. Three of the storage tanks are indicated at 9, 10, 11.

The total capacity of these six storage tanks is 686 m3. The storage tanks are used for different purposes such as to contain pure fluid prior to mixing prepared drilling fluid after mixing with solids, fluid which has passed through the drill string and been passed back to the vessel from the drilling rig, and any other fluid such as brine. Any one storage tank will be used for one particular substance.

Additional storage capacity for dry solids is provided by four tanks 1 2-1 5. These dry solids include barite, cement, bentonite and the like.

The main deck 1 6 of the vessel includes a sack store 1 7 positioned above the storage tank 9 on the starboard side of the vessel and a set of hydraulic power packs 18 opposite the sack store 1 7 on the port side of the vessel. Between the sack store and the hydraulic power pack system is positioned a mud process room 1 9 with which this invention is primarily concerned.

The mud process room 1 9 together with a set of six tanks (to be described) form a selfcontained mud processing and recycling assembly which is inserted into the conventional mud storage vessel. Below the mud process room 1 9 are positioned two holding tanks 20, 21, two process tanks 22, 23, and two utilities tanks 24, 25 (Figure 4). Each pair of tanks may be connected by respective gate valves 21', 22', 25'. As can be seen in Figure 2, these tanks are positioned just below the main deck 1 6. In addition, the tanks 20-25 are positioned between a pair of laterally spaced storage tanks one of which comprises the storage tank 9. A pair of electrically driven internal transfer pumps 26, 27 are provided adjacent the utilities tank 25.

A perspective view of the full self-contained assembly is illustrated in Figure 5 and it should be understood that the tanks 20-25 are contained within a corrugated metallic wall 28.

Above the tanks 20-25 and within the mud processing room 1 9 are positioned a pair of centrifuges 29, 30. These may be provided by Pioneer Mark I and Mark II centrifuges respectively. The centrifuges 29, 30 are supported above the main deck 1 6 by respective support structures 31, 32. A pair of solids discharge chutes 33, 34 are positioned below the centrifuge 29 and extend into the holding tank 20 and process tank 22 respectively. A flap (not shown) at the apex of the chutes controls the chute along which the solids travel. A second pair of discharge chutes 35, 36 are positioned below an outlet of the centrifuge 30 and extend into the holding tank 21 and process tank 23 respectively. A flap (not shown) at the apex of the chutes controls the chute along which the solids travel.

A surge tank 37 is positioned adjacent the mud process room 1 9 and above a mud mixing unit 38.

Each storage tank includes three paddle agitators of the form shown in-Figure 7. Each agitator comprises an hydraulic motor 39 which drives a shaft 40 via a flexible coupling 41. A set of four blades 42 are mounted to the shaft 40, each blade 42 being spaced from the adjacent blades through a circumferential angle of 90". Each blade 42 is canted through an angle of 45 to the vertical. The sizes of the blades 42 are selected in accordance with the contents of the storage tank concerned. Typically, the distance indicated by the arrow 43 is about 1 metre.

The set of blades 42 are connected to a further shaft 44 which is rotatably received in a mounting 45 secured to the base of the storage tank. The entire assembly shown in Figure 7 is located within the storage tank.

The speed of rotation of the blades 42 is also selected according to the content of the storage tank and may range up to about 250 RPM. As has previously been mentioned, in view of the size of the storage tanks, three agitators are positioned in each-storage tank.

The agitators will be driven from a central hydraulic control (not shown). The speed is monitored by means not shown so that if a significant change occurs an alarm will sound and an operator can take correcting action.

Additionally, each of the tanks 20-25 includes a single agitator 46 (Figures 2-4) each of these agitators will have a form similar to that shown in Figure 7 except that the distance indicated by the arrow 43 will range between about 0.6 metres for the tanks 20, 21, 24, 25 and 0.8 metres for the tanks 22, 23. A further difference between these agitators and those in the storage tanks is that the hydraulic motor 29 is positioned outside the tanks as indicated in Figure 2.

Typically, the agitators in the storage tanks may rotate at a speed of about 70 RPM while the agitators in the tanks 20-25 will rotate at a higher speed, for example 87.5 RPM.

Although the agitators provide a reasonable degree of agitation it is particularly important in the present context to prevent significant settling of solids within any of the tanks. To this end, each of the lowermost corners of the storage tanks and the tanks 20-25 are modified in a manner similar to that shown in Figure 8A and 8B. In each corner, a triangular dressing plate 47 is mounted to prevent solids settling into corners of the tanks. In addition, a pair of compressed air jets 48 are mounted adjacent the dressing plate 47 and are angled to direct compressed air across the plate in a plane generally parallel with the plate. The jets 48 are about 0.75 inches long and are positioned about 1 inch from the tank bottom, 1 inch from the tank side and 1 inch from the adjacent corners of the dressing plate 47.

They are angled towards each other at an included angle of about 120 . Each jet 48 is provided with a rubber non-return valve (not shown) on its nozzle to prevent fluid in the tank passing back into the jet. In use; compressed air is fed to the jets from a source (not. shown) via a valve 49. This removes settled solids from the plate 47 in the area of jets allowing solids further up the plate to slide down.

The storage tank and the process tanks 2025 are provided with suitable inlet ports and outlet ports and connecting pipes to enable the material stored in the tanks to be moved between tanks and to the mud process room 1 9. Where gravitational movement is not possible, the movement of the materials is assisted by the internal transfer pumps 26, 27.

A variety of treatments can be performed by the assembly by suitably connecting the process tanks and storage tanks with the centrifuges 29, 30. Figure 9 illustrates a liquid phase recovery process. In this example, the two centrifuges 29, 30 work in parallel to separate a high-cost liquid phase together with the "fines", typicallv less than 2 microns, from the solids. Used mud from one or more of the storage tanks is fed to both process tanks 22, 23 with the valve 22 open and from there is pumped in parallel to each centrifuge 29, 30. The solid phase separated by the centrifuges 29, 30 passes into the holding tanks 20, 21 with the valve 21' open while the clean liquid phase from each centrifuge passes into the utilities tanks 24, 25 with the valve 25 open. This clean liquid phase is then pumped to appropriate storage tanks for reuse.The essentially dry solids in the holding tanks 20, 21 are then either dumped (where this is ecologically permissible) or mixed to a thick slurry and returned to the fluid storage tanks for subsequent disposal ashore. This process would normally be carried out at the end of a well or hole section, when a change of mud type is required.

It should be noted that this process avoids the current practice of shipping all the used mud to shore to be processed. About 240 m3 of mud can be processed and the liquid phase returned to the rig within 24 hours.

Figure 10 illustrates a barite salvage process. This is similar to the process shown in Figure 9 with the two centrifuges 29, 30 working in parallel and the tanks connected in pairs. The damp, barite solid phase in the holding tanks 20, 21 is pumped to the mixer 38 where it is mixed with fresh liquid phase from the storage tanks and then either returned to the rig or stored in appropriate storage tanks. The waste liquid phase is fed to the utilities tanks 24,25 and then either discharged overboard (if permissible) or returned to waste fluid storage tanks.

Figure 11 illustrates a two stage centifuging system. In this process, the two centrifuges 29, 30 work in series and the valves 21' 22', and 25' are closed. The object of this process is to control "viscosity increasing fines". The fines to be discarded are mostly in the range 2.0 micron. As the small particles cannot be separated from the liquid phase it is necessary to limit the amount of material falling into this range.

Solids particles in drilling fluids degrade with mechanical handling; by removing particles of a size just greater than 2 micron then the quantity of particles degrading to a size of less than 2 micron is controlled.

In API (American Petroleum Institute) barite some 9% of the material falls within the size range of 2 to 6 micron. All solids in the range 1.5 to 5 micron will be removed. In oil base muds where the process is mose useful the resultant removal will be all solids in the range 2.5 to 6 micron.

Dirty mud from a rig or storage tanks is pumped to the process tank 22 from where it is pumped to the centrifuge 29. Solids larger than six microns are separated out by the centrifuge 29 and flow down the chute 33 to the holding tank 20. The separated fluid phase with solids smaller than 6 microns is fed to the process tank 23 and then pumped to the centrifuge 30.

The separated solid phase from the centrifuge 30 which will comprise solids larger than two microns passes along the chute 35 to the holding tank 21. The fluid and solids smaller than two microns from the centrifuge 30 pass back to the holding tank 20 for mixing with the solids larger than 6 microns and the process mud from the tank 20 is then fed either directly to the rig or to storage tanks.

Sea water is fed into the holding tank 21 for mixing with the solids larger than 2 microns and the reslurried solids are dumped in the sea.

The process shown in Figure 11 is particularly useful in the following applications: 1-) On newly mixed mud prior to drilling 2) As a semi-continuous process, taking a batch of mud from the rig, processing it, and passing it back to the rig for remixing in the active system, repeating the process as required.

3) As an end of well (or hole section) process, where the newly processed mud would be stored in the fluid storage tanks until required by the rig.

The advantages of this process are all "downhole"; the use of the process in addition to on-rig solids control treatment will enable drilling to be carried out with mud "on line" with the intended mud programme. This will undoubtedly result in higher rate of penetrations. No estimate of the cost of "stuck pipe" can be made until the pipe is 'freed': good tnud means good filter cake and a reduced possibility of differential sticking.

Similarly, good control of mud properties in reservoir sections is of prime importance.

The vessel mounted system is advantageous because few rigs have the space for the necessary tankage and equipment.

Figure 12 illustrates a fourth process involving the removal of the free-liquid adhering to shale- shaker cuttings, desilter and desander underflow. To utilise this facility, the material to be treated requires to be diluted to give a pumpable slurry. This slurry is fed to the process tanks 22, 23 with the valve 22' open and from there is fed to the centrifuges 29, 30 working in parallel. The solids are removed by the centrifuges and fed to the holding tanks 20, 21 with the valve 21 open and to which sea water is also fed. The contents of the holding tanks 20, 21 are then fed to the mixer 38 for mixing with further sea water and the resulting slurry is dumped in the sea.

The liquid phases from the centrifuges 29, 30 are fed to the utilities tanks 24, 25 with the valve 25 open from where it is fed to a rig or storage tanks.

Claims (11)

1. A self-contained drilling fluid treatment assembly including a treatment unit for connection to at least one storage tank of a vessel, the unit comprising a plurality of process tanks, a mixing unit, and solids content control means, wherein each tank includes agitation means to maintain solids in suspension in the fluid.

2. An assembly according to claim 1, wherein the agitation means is provided by at least one rotatably driven paddle.

3. An assembly according to claim 1 or claim 2, wherein the lower corners of each tank are dressed out to minimise solids settling.

4. An assembly according to claim 3, further comprising at least one cleaning jet associated with at least some of the lowermost corners of the tanks to minimise solids settling.

5. An assembly according to claim 4, wherein a pair of cleaning jets are provided at at least some of the lowermost corners of the tanks, the jets being angled generally towards each other.

6. A self-contained drilling fluid treatment assembly substantially as hereinbefore described with reference to the accompanying drawings

7. A vessel comprising a plurality of storage tanks; and an assembly according to any of the preceding claims, the treatment unit of the assembly being connected to at least one of the storage tanks.

8. A vessel according to claim 7, wherein each storage tank includes agitation means to maintain solids in suspension in the fluid.

9. An assembly according to claim 8, wherein the lower corners of each tank are dressed out to minimise solids settling.

1 0. An assembly according to claims 9, further comprising at least one cleaning jet associated with at least some of the lowermost corners of the tanks to minimise solids settling.

11. An assembly according to claim 10, wherein a pair of cleaning jets are provided at least some of the lowermost corners of the tanks, the jets being angled generally towards each other.

1 2. A vessel substantially as hereinbefore described with reference to the accompanying drawings.