WO1995022150A1

WO1995022150A1 - Radioactive waste disposal

Info

Publication number: WO1995022150A1
Application number: PCT/GB1995/000230
Authority: WO
Inventors: Timothy Hamilton Watts; David Owen Nicholls
Original assignee: Cma International Limited
Priority date: 1994-02-08
Filing date: 1995-02-06
Publication date: 1995-08-17
Also published as: GB2286284B; GB2286284A; AU1582995A; GB9402401D0

Abstract

A technique for the long term storage of radioactive or other hazardous waste is provided. Old oil and gas wells typically 2.5 to 4 km deep are adapted to contain specially packaged units of hazardous waste. Well known drilling technology techniques are used in the emplacement of the units through the existing well head. The well may then be capped and monitoring equipment installed. The techniques may be used for both land based and undersea wells, and a method for transporting the specially packaged units within radioactive shielding to the well head, and for facilitating emplacement through the radioactive shielding into the well, is also described.

Description

RADIOACTIVE WASTE DISPOSAL

The present invention relates to techniques for radioactive waste disposal, and in particular to the long term storage of encapsulated radioactive waste underground.

Disposal of substantial quantities of radioactive waste is, at present, an ever increasing problem. Various techniques exist for the long term storage of such waste, commonly underground. Typically, this involves the construction or utilization of large underground cavern facilities which can store the waste in bulk under carefully controlled conditions. The location of these facilities requires careful choice to avoid potential hazards from adverse geological conditions.

The radioactive waste for disposal is "packaged" in a number of different ways, dependent upon the type and level of the waste. For example, low and intermediate level waste is often matrix bonded in cement, being then encased in some appropriate lining material. High level wastes may be processed using, for example, Synroc technology, or vitrified using borosilicate glass.

It will be understood that although the present description refers throughout to radioactive waste, the techniques of the present invention could readily be applied to non-radioactive, but otherwise highly toxic waste. It is intended that the present specification encompasses such wastes.

It is an object of the present invention to provide a convenient, safe, and cost effective method of underground radioactive or other waste disposal.

In accordance with one aspect of the invention, there is provided a waste disposal technique which uses available boreholes which may have been provided during other drilling activities. Such boreholes can, of course also be purpose-drilled for the application of the present invention. The present invention also provides techniques for the preparation of waste prior to disposal and techniques for the placement of such prepared waste into such boreholes.

In accordance with one embodiment of the present invention, there is provided a method of emplacing encapsulated hazardous waste units into pre- drilled boreholes exceeding 2000m in depth.

Preferably, the borehole comprises an oil or gas well borehole.

In accordance with a preferred embodiment, there is provided a method of: encapsulating the hazardous waste into elongate cylindrical waste sleeve units, and providing each waste sleeve with a wireline latch mechanism at an upper end thereof; emplacing each waste sleeve down the borehole using a wireline from an overhead rig; releasing the wireline from said waste sleeve using a remotely controlled release mechanism; and sealing the top of the borehole with a plug.

In accordance with another preferred embodiment, there is provided a method of encapsulating the hazardous waste into elongate cylindrical waste sleeve units, and providing each waste sleeve with an upper and lower coupling mechanism, said upper coupling mechanisms adapted to cooperate with a corresponding lower coupling mechanism of another waste sleeve; locating successive waste sleeves substantially coaxially within a running barrel; coupling an upper end of said running barrel with the uppermost waste sleeve using a detachable connection; driving the running barrel into the borehole; upon reaching the appropriate depth of borehole, decoupling said running barrel and said uppermost waste sleeve using said detachable connection; and withdrawing said running barrel. In accordance with a further embodiment of the present invention, there is provided a transport package adapted to enclose a waste sleeve unit, including engagement means for engaging with the top of a borehole, and including valve control means allowing the waste sleeve to be remotely and controllably released from the transport package into the borehole.

In accordance with a further embodiment of the present invention there is provided a method of disposal of hazardous waste, encapsulated into waste sleeves, into pre-drilled boreholes including the steps of: conveying the waste sleeves to a well head within a shielded transport package, each transport package being provided with a well head engagement means for coupling one end of the transport package thereto; locating the transport package onto the well head; opening an upper hatch in the transport package to allow coupling of an emplacement device to one end of the waste sleeve; opening a lower hatch in the transport package to allow passage of the waste sleeve and emplacement device through the transport package; once the waste sleeve is emplaced in the borehole, withdrawing the emplacement device, and retrieving the transport package.

The present invention will now be described by way of example, and with reference to the accompanying drawings in which:

Figure 1 shows a schematic cross-sectional view through a waste sleeve constructed in accordance with one embodiment of the present invention; Figure 2 shows a schematic cross-sectional view of a first embodiment of a transport package in accordance with the present invention and suitable for use with a riserless wireline emplacement technique;

Figure 3 shows a schematic cross-sectional view of a second embodiment of a transport package in accordance with the present invention and suitable for use with a drill-pipe emplacement technique; Figure 4 shows a schematic diagram illustrating the transfer of transport packages using riserless emplacement from a semi-submersible vessel or dynamically positioned vessel to a sub-sea well head;

Figure 5 shows a schematic cross-sectional diagram through a waste sleeve, running barrel and oil well casing;

Figure 6 shows a schematic cross-sectional diagram of the upper and lower extremities of a waste sleeve assembly and cooperating portions of a running barrel.

Although underground radioactive waste disposal is well known, it has previously been limited to purpose built storage sites at a relatively shallow minable depth, for example to a maximum of 2000m. By contrast, the present invention proposes the use of existing or specially drilled ultra deep boreholes, particularly those drilled for oil and gas, at depths greater than 2.5 kilometres below sea level, which may be adapted to accommodate highly toxic waste, for example radioactive substances.

In the preferred situation, existing oil or gas wells are used, and these include boreholes which are lined with steel casing and thus direct contact of the waste material with geological formations and circulating groundwater can be avoided.

Individual exploratory boreholes, depleted sub-sea well clusters or pre¬ existing installations such as concrete or steel legged oil production platforms sited over oil or gas fields that have or will be come depleted, can be adapted to handle specially formed waste packages which will be emplaced using either drillpipe/coiled tube or cable/wireline methods. The waste packages will then be cemented in place, the borehole plugged and a containment barrier of cement/concrete will then be pumped into place above the plug. Dependent on the borehole depth and the amount of waste emplaced, the barrier could be over 4.0 kilometres thick.

Multiple waste sleeves can be run into the hole and stacked up to a depth permitted by the minimum containment or critical mass limitations required. Well heads may be fitted with monitoring equipment and after a suitable period of observation any surface installations can then be removed.

A: Choice of Locations In determining the ideal locations of target sites for the application of the waste disposal technique described herein, a large number of factors must be considered, exemplified by those below. The testing techniques used are all those which are well known to, and can be applied by, the person skilled in the art of oil exploration and oil extraction engineering.

The inventor has recognized that oil and gas reservoirs are particularly suited for adaptation to waste storage sites because by their very nature they have proven to hold fluid trapped over millions of years and as such are sites where groundwater flows are proven to be minimal or non existent. Other sites such as failed exploration boreholes which have encountered low permeability non-reservoir rocks, may also be considered.

Boreholes can, with present technology, be surveyed to an accuracy of centimetres, so the location of the waste once emplaced in the disposal sites will be known precisely. Intervention, should it be necessary at a later date, will be possible by drilling either from another borehole or by re-entry of the existing borehole. HYDROCHRONOLOGY a. Stagnant pore water sites are favoured as target sites. These sites are where the isotopic dating of pore waters is similar to the timing of hydrocarbon fill. b. Where age dating of reservoir waters shows it to be part of a regional aquifer system with a dynamic flow regime, then such locations are preferably avoided.

GROUNDWATER FLOW a. Relatively impermeable zones known as aquitards and aquicludes are favoured as target sites. These zones are where groundwater flow can be proved to be slow enough to ensure that during, for example, the decay of the more persistent radionuclides, fluid cannot move far enough to cause an environmental or health hazard. b. Regional aquifers however, are preferably avoided particularly where recharge of overlying zones is apparent or where horizontal movement of water between boreholes is apparent. c. Reservoirs where dynamic trapping has occurred, i.e. those that have a through flow of water with respect to hydrocarbons, are also preferably avoided.

GEOCHEMICAL ENVIRONMENT

Pore fluids whose properties would compromise steel casing (or other casing material encapsulating the waste) such as zones which contain embrittling gas such as hydrogen sulphide or carbon dioxide, are also preferably avoided. Highly corrosive saline fluid environments are preferably avoided and full consideration of the Eh-Ph conditions of groundwaters will also be necessary. FORMATION STABILITY

Highly faulted and fractured reservoirs are preferably avoided, whilst homogeneous and overpressured reservoirs which have been carefully managed to ensure the original stress state has been maintained as much as possible, are favoured as target sites.

HEAT FLOW

Modelling of the potential heat flow from the waste material can be performed to evaluate the impact any changed heat flow patterns will have on the environs of the well bore. The heat generation capacity of the waste in situ will be matched with the existing geothermal temperature and sites where large differentials are likely to occur, are preferably avoided.

B: The Disposal Technology LOW & INTERMEDIATE LEVEL WASTE COLUMNS

In a presently preferred embodiment, and as shown in figure 1, radioactive or other waste is formed into columns 10 of approximately 10cm diameter and 10m in length. The waste material will form a matrix 11 bonded by cement, of the type already proposed for disposal in drums in the Sellafield repository. These columns 10 will be sleeved with a thin protective steel lining 12 sufficient to provide mechanical strength to the column, and contain the volume of waste. Other materials with the appropriate mechanical strength may be considered. The protective steel lining 12 will have either tapering threaded ends 13 as shown in the figure, or wireline latch mechanisms 14 (as shown in the embodiment of figure 2) machined at the top. Corresponding tapering thread end 13a may be provided at the lower end of the column. This whole assembly is referred to as the "waste sleeve" 15. HIGH LEVEL WASTE DISCS

High level wastes, for example, the residues from reprocessed spent fuel, will preferably be processed using Synroc technology or vitrified using, for example, borosilicate glass, into 10-20cm diameter discs. These discs will then be inserted coaxially into a thin metal similar to the steel lining 12, of about 10m in length. Similarly, this unit may be provided with a wireline latch at the top, and will then constitute the waste sleeve 15.

TRANSPACKS With reference to figure 2, the waste sleeves 15 will be transported to and handled at, for example, an offshore installation in protective housings 30 or transport packages, hereinafter termed "transpacks" . These are constructed from an appropriate shielding material — eg. a suitable radiation absorption material. The transpacks preferably incorporate buoyancy packs if waste is being emplaced directly from a ship or barge into a borehole at the seabed. Transpacks could also be towed intact under water to the site by, for example, suspension beneath a surface vessel, or could be conveyed on or in a submarine vessel.

An internal gas or vacuum jacket 31 surrounds the waste sleeve 15 and isolates it from the shielding material 32. This is bled off or equalised before the transpack is opened, using gas bleed-off lines 33. These lines are typically remote vehicle operated when the transpack has been positioned at a well-head, particularly where this takes place on the seabed. A double jacket may be necessary in order to connect the attachment of the wireline latch, before pressure equalisation of the main jacket. A retaining plate 37 keeps the waste sleeve in position until it is ready for release into a borehole, and may also be operated by remote vehicle. Upper and lower hatch plates 38a, 38b may be operated to facilitate attachment of the wireline and release of the waste sleeve into the borehole, respectively. A further hatch plate may be provided if it is necessary to facilitate attachment of the wireline prior to equalization of pressure in the internal jacket.

In figure 3, there is shown a slightly modified version of a transpack, suitable for use with a drillpipe emplacement technique to be described hereinafter. Gas bleed off valves 33 and upper and lower hatch plates 38a, 38b operate in similar manner to that already described.

These transpacks 30 are used as temporary containment during transportation at sea and will be latched on to the well head 34 (or upper annular blowout preventer) or at the rig floor using a latching mechanism 35 well known in the art of oil well engineering.

The waste sleeve can then be released and lowered into the bore hole through the transpack and then the transpack then retrieved. The transpack can be washed in seawater to minimise contamination before backloading.

THE SURFACE HANDLING SYSTEM

A schematic diagram of a handling technique to transfer the transpacks from ship or rig to the wellhead is shown in figure 4. Wherever possible automatic handling systems will be used to transfer the transpack and its contents whilst on board the rig.

For waste that is too radioactive to bring onto the emplacement vessel or installation, it will be transferred directly from under the transportation barge 40 or ship via a moon pool door 41, down to the wellhead 42 by cable 43. The transpack 30 will be locked into position at the wellhead 42 using latch assembly 35, the gas jacket will be equalised by operation of gas bleed off lines 33 and the upper hatch plate 38a opened. For wireline emplacement techniques, the wireline 45 is attached to the sleeve through latching port 36, and the retaining plate and lower hatch opened. The waste sleeve 15 is then lowered into the borehole.

For drillpipe type emplacement, the running assembly of drillpipe or coiled tube will be fed over the waste sleeve through the upper hatch plate 38a, using a remotely operated vehicle. The lower hatch plate 38b can then be opened and the waste sleeve lowered into the borehole.

As an alternative, for low or intermediate level waste that can be handled quicker via the rig floor, the transpack can be placed on the rig 50 floor and either the wireline 45 or drillpipe/coiled tube (not shown) connected to the waste sleeve at the surface.

All operations can be done without a marine riser to avoid the unnecessary rig contamination unless it is necessary to recover liquids such as wet cement, drilling fluid (mud) or extraneous debris that has to be pumped out of the borehole. Remotely operated equipment will be used wherever possible.

THE RUNNING ASSEMBLY a: wireline

A cable (wireline) conveyed system of emplacement is used in preference to the drillpipe/coiled tube system because of the speed of operation and the minimal equipment surface area liable to contamination. This system will be best suited for emplacement of high level waste.

The cable 45 will be either attached at the surface or on the seabed and run into a latching port 36 in the top of the transpack. The wireline 45 will be located through the transpack by remotely operated vehicle in the case of riserless emplacement. Latched onto the wellhead or on the rig floor, the waste sleeve assembly 15 will then be released from the transpack 30 and lowered to the bottom of the hole where the cable 45 will be released by either electrical control or overpull on a weak point. The cable 45 will then be pulled to the surface and the transpack 30 retrieved.

b: drillpipe/coiled tube

For emplacement of multiple waste sleeves 15 or where the angle of the borehole is sufficiently non-perpendicular for the wireline assembly to be run easily to the bottom, a drillpipe/coiled tube conveyed assembly will be used. This will also allow cementing operations to be performed before the drill pipe is removed.

With reference to figure 5, the cemented or solidified waste 11 , moulded into columns 10 and housed in the thin protective metal sleeves 12 have tapered thread connections 13 top and bottom. These waste sleeves 15 can then be screwed together, referred to here as the "waste sleeve assembly".

The waste sleeves are then inserted into 15-25cm diameter adapted or standard oilfield coring or "running" barrels 50. Single or multiple running barrels up to 120m long can be screwed together.

The running barrels 50 are passed down the borehole, which itself is lined with, typically, an eighteen to twenty-four centimetre O/D metal casing

52, which is cemented into the borehole with cement layer 53 between the casing 52 and rock formation 54. This casing cement layer 53 is pumped in during the course of normal drilling completion operations.

The bottom most waste sleeve 15a (see figure 6) would have a protective perforated nose 60, protruding from the throat of the lowermost running barrel 50 to prevent solid debris entering the annulus 61 between the waste sleeve 15a and the inside of the running barrel 50. Perforations 63 would, however, allow the ingress of liquids into the annulus 61 while the running barrel and waste sleeve 15a are being driven downwards. Such a perforated nose would preferably be shaped to assist in guidance down the borehole as shown in figure 6.

The waste sleeve assembly would be released from the running barrel 50 by a primary coarse back-off tapered thread (Acme thread) 62, which would be capable of holding the entire weight of the waste sleeve assembly in air. This release system could be supported by a series of secondary commercially available release mechanisms which can be activated in an emergency.

The running barrels 50 may also have a back-off system similar to the primary device on the top waste sleeve so that the barrels 50 can be left downhole in the event of cementing problems or a total failure to release the waste sleeve assembly.

THE CEMENTING PROCEDURE a: drillpipe/coiled tube emplacement: After the waste sleeve 15 assembly is backed off downhole, the running barrel 50 is pulled back up the borehole slightly and cement is pumped downhole and around the waste sleeves. The cement will bond the waste sleeve to the borehole casing 52 creating an impermeable lateral barrier. The running barrel 50 is pulled out before the cement sets. More cement can then be circulated through the drillpipe/coiled tube and running assembly before pulling them out of the hole.

b: drillpipe/coiled tube & wireline emplacement:

Normal oil well cement plug techniques will be used to achieve cement emplacement, however new sintered refractory slag based drilling fluids which convert to cement without the need to circulate radioactively contaminated mud out of the borehole can also be used particularly to create the containment barrier itself.

THE CONTAINMENT BARRIER

An oil well bridge plug (not shown) will be set on top of the uppermost waste sleeve 15 and then further cement and concrete can be pumped above the bridge plug providing a vertical containment barrier.

Lateral containment will be achieved by preventing fluid movement between the waste sleeve and the formation by means of the casing steel 52. Should any leakage occur then the lack of movement of the formation pore waters will prevent dispersion of radioactively contaminated fluid.

The cement forming the vertical containment will be formed to just beneath the sea bed using sintered slag cement then monitoring equipment will be installed. The monitoring equipment will be accessible from the seabed.

Should the waste need to be examined at a later date then the containment barrier can be drilled and the waste cored for samples.

The ultimate failsafe barrier will be the ocean itself, which will disperse any leakage which might occur in the unlikely event of catastrophic failure or seepage losses.

MONITORING & INSTALLATION REMOVAL

Monitoring equipment will be installed just below the seabed. This will allow telemetry of data on radioactivity, heat and moisture levels for example, to be logged and analysed. After a suitable period of observation, bulky sea bed equipment can be replaced by borehole caps that will not interfere with future marine activity, particularly fishing. These well locations will have sonar beacons so their location will be detectable for future reference.

Claims

1. A transport package adapted to enclose a waste sleeve unit, including engagement means for engaging with the top of a borehole, and including release control means allowing the waste sleeve to be remotely and controllably released from the transport package into the borehole.

2. A transport package according to claim 1 wherein the engagement means is adapted to engage with a sea bed borehole installation.

3. A transport package according to claim 1 wherein the engagement means is adapted to engage with a surface well head installation.

4. A transport package according to claim 1, 2 or 3 wherein the engagement means includes means for cooperating with a blowout prevention unit.

5. A transport package according to claim 1 or 2 including buoyancy control means.

6. A transport package according to claim 5 wherein the buoyancy control means comprises an internal gas or vacuum jacket.

7. A transport package according to claim 6 wherein the buoyancy control means further includes valve means for controlling the bleeding off of the internal gas or vacuum jacket.

8. A transport package according to claim 7 wherein the valve means are remotely actuable.

9. A transport package according to any preceding claim comprising an elongate casing with a central cavity for housing a waste sleeve unit, and including, at an upper end, means for attaching an emplacement means to the waste sleeve while the waste sleeve is still within the transport package, and release means, at a lower end, for permitting release of the waste sleeve from the package through the release means.

10. A transport package according to claim 9 wherein the release means comprises a remotely actuable retaining plate or hatch.

11. A transport package according to claim 1 including means for suspending beneath or towing behind a sea surface or submarine vessel.

12. A method of disposal of hazardous waste, encapsulated into waste sleeves, into pre-drilled boreholes including the steps of: conveying the waste sleeves to a well head within a shielded transport package, each transport package being provided with a well head engagement means for coupling one end of the transport package thereto; locating the transport package onto the well head; opening an upper hatch in the transport package to allow coupling of an emplacement device to one end of the waste sleeve; opening a lower hatch in the transport package to allow passage of the waste sleeve and emplacement device through the transport package; once the waste sleeve is emplaced in the borehole, withdrawing the emplacement device, and retrieving the transport package.

13. A method of remotely emplacing encapsulated hazardous waste units into pre-drilled boreholes exceeding 2000m in depth.

14. A method according to claim 13 wherein the pre-drilled borehole comprises an oil or gas well.

15. A method according to claim 13 or claim 14 wherein the pre-drilled borehole is on a seabed.

0 16. A method accrding to claim 15 including the steps of controlling remote emplacement equipment at the sea surface.

17. A method according to any one of claims 13 to 16 wherein the borehole is provided with a cemented-in metal casing.

18. A method according to any one of claims 13 to 17, including the steps of: encapsulating the hazardous waste into elongate cylindrical waste sleeve units, and providing each waste sleeve with a wireline latch mechanism at an upper end thereof; emplacing each waste sleeve down the borehole using a wireline from an overhead rig; releasing the wireline from said waste sleeve using a remotely controlled release mechanism; sealing the top of the borehole with a plug.

19. A method according to claim 18 wherein the releasing step includes breaking a weak link through deliberate overpull.

20. A method according to claim 18 wherein the step of sealing the top of the borehole includes pumping in a cement, or other setting compound.

21. A method according to any one of claims 13 to 17, including the steps of: encapsulating the hazardous waste into elongate cylindrical waste sleeve units, and providing each waste sleeve widi an upper and lower coupling mechanism, said upper coupling mechanisms adapted to cooperate with a corresponding lower coupling mechanism of another waste sleeve; locating successive waste sleeves substantially coaxially within a running barrel; coupling an upper end of said running barrel with the uppermost waste sleeve using a detachable connection; driving the running barrel into the borehole; upon reaching the appropriate depth of borehole, decoupling said running barrel and said uppermost waste sleeve using said detachable connection; and withdrawing said running barrel.

22 A method according to claim 21 wherein said withdrawing step includes the step of pumping cement or other setting compound though said running barrel and out into the volume vacated by said running barrel.

23. A method according to claim 21 wherein the detachable connector includes a primary coarse back-off tapered thread.

24. A method according to any one of claims 13 to 23 including the step of installing monitoring equipment within the borehole prior to plugging the top to seal in the waste.

25. A waste sleeve unit comprising an elongate metal structure of less than 20cm diameter filled with toxic waste.

26. An encapsulated hazardous waste unit adapted to fit within a borehole of less than approximately 32cm diameter, and an emplacement device adapted to emplace the unit at a depth of greater than 2000m within said borehole.

27. A method according to claim 14 further including the use of a submarine transportation system to convey the encapsulated waste units to the pre-drilled borehole.