As part of the continuous drive to improve environmental performance, drill cuttings discharges must be addressed and recommendations made for reducing the environmental impact of this waste stream.

The options for avoiding OBM discharges are to increase the use of water based drilling mud or implement total containment systems to prevent the discharge of synthetic or oil - based muds (synthetic mud/OBM) to seat.

The methods of total containment (cuttings re-injection - Cuttings Re-Injection), and transport of cuttings to shore for treatment and disposal) are operations involving mature technology.

Other technologies to consider are: high performance water based mud; equipment for sub-sea Cuttings Re-Injection; bulk transport of cuttings to shore; improved on-site cleaning technology and slurrification and discharge.

1. Introduction

In order to reduce the impact of cuttings discharged to sea, a considerable effort has been made over a number of years to develop drilling muds with an environmentally acceptable base fluid. Initially diesel base muds were replaced with low toxicity oil based muds (LTOBM). Although this represented an improvement, the slow rate of degradation of mineral oil based muds results in cuttings piles on the sea bed with a very long lifetime. As a result increasingly stringent legislation on the discharge of all OBM cuttings has been introduced, leading to the situations whereby no cuttings may be discharged if the level of OBM relative to the weight of dry cuttings exceeds 1%.

The principal methods for eliminating OBM discharges are by the containment of drilling waste for subsequent disposal, the use of water based muds (WBM) and the development of synthetic or pseudo oil based muds (synthetic mud / POBM) with similar performance characteristics but faster degrading than OBM.

The high cost of the synthetic base fluid makes pseudo muds considerably more expensive than mineral oil based muds. This has been justified by the fact that for many drilling operations the cost of a total containment system for drill cuttings disposal has been even higher and because the drilling performance of WBM is too low. However,the environmental performance of the synthetic mud  has not matched expectations. In fact, laboratory tests indicate that the environmental behaviour of the majority of synthetic mud is very similar to that of OBM. With this evidence the continued use of costly drilling fluids is increasingly becoming insupportable both on economic and environmental grounds.

It is necessary to assess the environmental impact of all drilling muds in use and to design a practical solids controlhedule for phasing out the discharge of any found unacceptable. This must be done without compromising the need for drilling fluids able to drill demanding wells. This involves identifying the current forecast for the discharge activity in question, and analysis of the reduction drivers impacting that activity: environmental effects, public perception, assessynthetic mudent of legal trends, bench marking (nationally and internationally), best available technology, cost, etc.

The alternatives to disolids controlharging contaminated cuttings to the sea are: the use of genuinely low impact drilling fluids; employing an effective cuttings cleaning technique prior to dumping or containing the drill cuttings and either returning them to shore for treatment and disposal or re-injecting them in a disposal well offshore. These options each have technical limitations as well as economic implications which will differ between specific operations. In order to recognise this a method of comparing and ranking the possible solutions on a case specific basis should be found.

It should be recognised that a number of important factors in considering the handling of drilling waste are changing quite quickly. Such factors include the legislation in place, the level of understanding of the genuine impact of drilling fluids currently in use, the technologies available for cleaning or disposing of cuttings and the development of new drilling fluids.

2. Environmental effects

The environmental effects of disolids controlharging cuttings to sea are well understood. The practical significance of such effects is determined by the nature and composition of contaminants in the discharge, which determine the rate at which organisynthetic muds are able to re-colonise impacted areas. For water based muds, re colonisation typically proceeds rapidly - generally beginning when discharge has ceased.

OBM discharges have longer term effects, since elevated concentrations of hydrocarbons associated with cuttings and surrounding sediments have severe impacts on marine life - either by direct toxicity or by creating anoxic conditions. The rate at with the oil degrades determines the duration of effects. Degradation is typically slow under North Sea conditions, and effects can last for decades.

Environmental concerns with synthetic muds (synthetic muds) also focus on their persistence in the marine environment, and thus on the duration of environmental effects. The muds were developed in response to increasing restrictions on the discharge of oil based mud (OBM) to sea, with their use and discharge supported by claims from mud companies that their products degraded significantly faster in the marine environment than OBM. These claims are based on limited - and sometimes questionable - data sets.

Results from sea bed surveys and other environmental studies have provided increasing evidence that synthetic muds do not degrade rapidly under field conditions. This has been confirmed by results of laboratory biodegradation tests, carried out by SOAEFD, which indicate that synthetic mud base oils (other than the ester, Petrofree) cannot be differentiated from mineral oils on grounds of persistence.

Following on from this, discharges of OBM and synthetic muds also contribute to overall levels of contamination of the sediments. The significance of this last points needs to be considered against a broader background of reducing inputs of contaminants to the Sea, particularly from land based sources.

3. Public perception

Issues associated with drilling discharges have received a growing degree of public attention as reflected by articles in the New solids controlientist. These articles have not focused specifically on synthetic mud pollution as distinct from mineral oil based muds but as the environmental data described above becomes more established it is clear that the distinction between the two is irrelevant. The issue has also been adopted in the party political arena attracting further recent coverage in local and national media and public interest in the subject is growing. Criticisism is largely directed at the DTI for endorsing less stringent legislation than other European countries.

4. Future liability

The question of future liability and the possible requirement to remove the tens of thousands of tonnes of contaminated cuttings already on the sea bed is a major issue. Although this problem is considered as a decommissioning issue, it is important to recognise that current practices may have longer term implications. Although there is little or no evidence to suggest that oil free cuttings pose any serious threat to the sea bed, if clean cuttings are dumped on top of dirty cuttings it would aggravate any future problems in removing the oily waste. Also, if a commitment is made to remove cuttings in place it become less critical to avoid adding to existing piles in the short term. Likewise, whilst cuttings pile removal is a possibility it would be sensible to make extra efforts to avoid dumping any remaining synthetic mud cuttings on virgin sites. In this context, of the options disolids controlussed below, cuttings re-injection and ship to shore disposal have an advantage over cuttings cleaning techniques or the use of water based mud in that they have no impact on the sea bed.

5. Available techniques

There are a number of possible options for reducing the effect of waste drill cuttings on the environment, falling in three main categories.

1. The ideal solution would be to use a drilling fluid which is genuinely non - harmful to the environment such that cuttings might be discharged with impunity. The synthetic muds were produced with this aim but are now seen to have fallen short of their goal. Irrespective of environmental performance, the application of ester muds is restricted by their temperature limit of 320 ºF. Mud companies are continuing to work on new high performance drilling fluids and as long as their environmental acceptability can be positively demonstrated and is consistent with new legislation this work should be encouraged. Currently WBM is the only environmentally acceptable option allowing the discharge of untreated cuttings. The disadvantage of using WBM is that it has inferior drilling performance to OBM in a number of respects, principally maintaining formation stability and providing good lubrication for high deviation or extended reach wells.

2. The second option is to use OBM and to implement a total containment system allowing disposal of drill cuttings in an environmentally acceptable manner. Currently available methods are either transportation to shore for treatment and disposal or the grinding of cuttings to a slurry which is then pumped back into the ground either in a dedicated disposal well or into the annulus of a standard well. The main advantage of this approach is that it allows continued use of the highest performance and relatively low cost OBMs. The disadvantage is that the costs associated with containment and disposal are substantial.

3. The last option is to continue to use OBM but rather than contain the waste, to clean drill cuttings prior to discharge overboard. A number of approaches to cleaning drill cuttings have been pursued and several have demonstrated technology capable of the required quality of cleaning. None of the existing systems have yet been developed to a commercial stage for offshore applications, however, although the availability of shore based facilities is important in option two above. There is a considerable ongoing effort to develop such technology for offshore use. The advantage of on site cleaning is in eliminating the expensive process of transporting waste to shore and the associated vulnerability to weather related downtime.

The approach of incinerating rather than cleaning the waste offshore would be unlikely to receive government approval and would itself be a source of polluting emissions inconsistent with improved environmental performance. On a practical level, proper incineration of oily cuttings requires a high temperature furnace and heavy fuel consumption and such plant would not be desirable for a rig site location.

Although the 1% target is currently unattainable at the rig site, where limited discharge of contaminated cuttings does continue there will be an increased emphasis on using the best solids control equipment and effective operational controls to minimise the quantity of mud discharged with cuttings.

The final option for reducing the impact of any discharges is to reduce cuttings to a fine particle slurry prior to discharge. This has the benefit that the suspension time of solid matter before reaching the sea bed is very much greater resulting in a much wider dispersion area. It is also likely that the increased surface area of the solids, combined with the greater suspension time, will lead to a much more rapid breakdown of any associated drilling fluids. Therefore, although the area over which the cuttings spread is much greater, the thinner layer will have a much reduced impact on the sea bed.

The application of the above techniques involves consideration of a number of factors which will vary on a case by case basis as explained below. It is therefore impossible to attribute simple figures for the cost of widespread application of discharge reduction projects.

Table 1. Available techniques

6. Disolids controlharge reduction strategy

The effect of the withdrawal of OBM on drilling operations will require significant rig upgrades, logistics planning and reappraisal of drilling fluid selection. The costs associated with these changes will also be significant and the phase out strategy should be designed in the context of a business case to minize the impact. Given the uncertainty in a number of key considerations, it is important to retain a high degree of flexibility in any planning.

Planning a co-ordinated approach to phasing out the use of OBM involves comparison of the costs associated with the various options applicable to each drilling project on a well section by well section basis. Furthermore, the cost efficiency of the cuttings re-injection option is heavily dependant upon the level of drilling activity of each installation over a number of years i.e., the long term drilling sequence.

There are currently three options for managing the drilling waste from any well section.

1. The simplest is the use of WBM as this allows the unrestricted discharge of drill cuttings. Where the performance of WBM is compatible with the demands of the hole section to be drilled this should be the first choice for environmental and economic perspectives.

2. The second option is to use an OBM drilling mud in conjunction with a total containment system (cuttings re-injection or shipment of waste to shore for treatment and disposal). Where a containment system is planned cheaper mineral OBM is prefered to the more expensive synthetic mud. A containment system will have associated cost penalties but allows the use of high performance drilling muds where required.

3. The third option, rarely available today, is the use of synthetic mud and discharge of cuttings to the sea. Where this option is selected the use of optimum solids control equipment to minimise the amount of synthetic mud discharged will be important. In the short term improved solids control is likely to be an important consideration as it is possible to reduce the oil on cuttings (oil on cuttings (OOC)) level, and hence the volume of oil discharged, by approximately 50% at relatively low expense.

Where WBM is not cost effective for a hole section, as a result of performance penalties compared to OBM, the best alternative must be identified. If the Cuttings Re-Injection option is economically favourable, this will be the preferred option. It should be recognised that Cuttings Re-Injection is only mature in the context of platform drilling. Subsea re-injection is not available as routine technology. At this stage the only containment option considered for mobile rigs is ship to shore.

Once containment costs have been calculated the adjusted cost of using OBM should be compared again with the cost of WBM and associated performance penalties.

The option of cleaning drilling waste at the rig site and then disolids controlharging to sea is not currently available, although there is a considerable effort being made within the industry to develop the necessary technology to achieve this. This and a number of other technologies are reviewed below.

7. Strategy implementation

The process of identifying the preferred containment option for each drilling operation is to collate drilling sequence, mud performance and containment cost data. Platforms where Cuttings Re-Injection is the best option are identified and for remaining operations the skip and ship option has to be compared with the use of WBM. Where hole sections of 16" or larger require OBM the limitations of skip and ship will be problematical and the use of ester muds may be considered against the option of drilling at reduced speeds.

It should also be recognised that having identified the choice of drilling fluid or containment system for a particular project there will in many cases be a substantial amount of work required to implement this solution. This is particularly true in the case of cuttings re-injection where the installation of a unit requires detailed appraisal of platform facilities, a tender bid to select one of the several suppliers and in most cases a significant amount of engineering work to install and commission the unit. Similarly in the case of shipment of cuttings to shore, if this policy is to be adopted on a significant solids controlale, as will be required, detailed attention must be given to considerations such as the capacity of the company's platform supply vessel resources and the ability of the waste handling industry to cope with the consequent increase in volume of material to be treated on shore. The impact on rig contracting strategy should also be considered.

8. WBM and drilling fluid performance

WBM systems have been the traditional drilling fluids since the early days of the oil industry. Deeper, hotter wells, more deviated and with increasing well path tortuosity have brought the WBM systems to their limits of economic applicability. Continued efforts in the development of WBM systems have allowed the industry to use systems ranging from the classic Water-Bentonite suspension via the highly dispersive Lignosulphonate drilling fluids to the KCl-Polyglycol drilling fluid presently known as the most inhibitive WBM to-date.

9. Cuttings re-injection

The process of grinding drill cuttings to form a slurry which is then pumped downhole, either into a dedicated disposal well or into the annulus of an existing well, has been used since the early 90's and is a mature technology. This process of Cuttings Re-Injection has some major benefits compared to the alternative containment option of transporting cuttings to shore for treatment and disposal. Firstly the Cuttings Re-Injection process is able to handle volumes of drill cuttings compatible with the fastest realistic rates of drilling even in the larger 16" or 17 ½" hole sections (this can be 30 - 40 tonnes per hour), unlike the ship to shore option. Furthermore, because there is no requirement to move the cuttings off the rig there are no demands on crane facilities, deck space or supply vessels and perhaps most importantly no vulnerability to weather related downtime. The last factor is the largest uncertain variable in the planning of containment operations, and may correspond to drilling shut downs where the ship to shore option is used in the winter months. Finally, the re-injection option has the benefit that where the re-injection facility is available it may be further employed for the disposal of other problematical waste streams such as low radioactivity LSA solids controlale or even produced water.

10. Transport of cuttings to shore for disposal

The option of transporting drill cuttings from the rig has a number of attractions. Unlike the use of WBM or Cuttings Re-Injection, the skip and ship option can be applied to any drilling operation from platform or MDU. The rig upgrades required to allow this containment option are minimal and can be implemented quickly and cheaply. Since the rig site equipment is simple and reliable, equipment failure is less likely than for Cuttings Re-Injection. A number of process technologies have been developed for treating the cuttings which recover and recycle base oil from the cuttings leaving an inert material that can be used as building material.

11. Solids control

Good solids control equipment is a requirement for any drilling operation. The level of oil on cuttings for both OBM and synthetic mud discharges has been the focus of a great deal of attention in the cuttings discharge debate and the solids control industry has responded by developing equipment that will greatly reduce the oil on cuttings (OOC) compared to simple shale shaker processing. Although there is no mechanical equipment able to reach the 1% oil on cuttings (OOC). It is recommended that adoption of the highest performance solids control quickly becomes a requirement for operations planning to use and discharge synthetic mud.

A standard arrangement of linear shale shakers will typically leave an oil on cuttings (OOC) level of 10 - 15%. The level is usually lower in top hole sections where cuttings are large and higher in lower sections where cuttings hold down effects and greater residence time in the borehole often result in finely ground cuttings. Improved solids control can reduce this oil on cuttings (OOC) level to 5% or better at relatively low cost, effectively halving the amount of synthetic mud discharged on cuttings. Furthermore the additional mud recovered may represent a substential saving per well section, making it attractive in financial as well as environmental terms.

It is recommended that the use of this kind of solids control technology and other performance enhancing equipment, such as the active flow divider to replace the header box, be pursued on a rig specific basis with the solids control companies.

As well as reducing the amount of synthetic mud discharged on cuttings it should be recognised that significant surface losses of synthetic mud result from other sources such as shaker overflow, centrifuge waste and disposal of cement contaminated mud. All of these waste streams should be targeted for reduction.

12. MUD 10 system

The MUD 10 system is effectively a vibrating centrifuge designed to process cuttings delivered straight from the shakers by screw conveyor. The system has a high throughput and can handle cuttings at the maximum rates generated in drilling and has the advantage of also coping with shaker overflow. The fluid recovered by the unit requires secondary processing with a decanting centrifuge to produce re-usable drilling mud. The solids waste leaving the unit will typically have an oil on cuttings (OOC) level reduced to approximately 2 - 4%. This can equate to the recovery of 100 - 150 bbls of drilling mud per 1000 ft drilled which represents a significant saving when expensive drilling muds are being used.

Rental for a unit is in the order of £2 to 3000 per day, with the requirement of a second, decanting centrifuge adding possible additional cost. The unit would probable also require an additional operator, approximately £1000 per day.

If the system is used in conjunction with a total containment system for a ship to shore operation there will be a benefit from reducing the total volume of material to be transported. It should be recognised, however, that the very dry solids produced may be harder to handle. Further, if the cuttings are to be processed on shore for base oil recovery the financial advantage of recovering mud on the rig is reduced.

The M.U.D. 10 unit is not efficient for processing hydrated cuttings. When drilling soft shales and chalks the efficiency of the unit will be reduced unless other, granular cuttings are added into the feed stock.

13. The High G drier

The High G drier is a high performance shale shaker. The unit has a large screen with a three dimensional pyramid profile that increases the effective working area and prevents clogging. The unit also has a high impact vibration motion which produces a considerable performance improvement over standard linear motion shakers. The High G drier should produce cuttings with oil on cuttings (OOC) levels in the order of 5 - 8%. Although the level of oil recovery is not as high as the M.U.D. 10 unit there is no requirement for an additional centrifuge and the processed cuttings are easier to handle. This unit will again be most effective for mud recovery from top hole sections.

14. Mud / cuttings separator

This unit works on the principal of a screw press whereby cuttings and associated mud are transported via an auger type screw through a cylindrical wedge wire screen fitted with an adjustable pressure plate at the discharge port. The action of the screw pushing cuttings against the pressure plate generates a squeezing effect forcing the associated mud through the screen. This mud is collected and returned to the active system, whilst processed cuttings exit the periphery of the compressed pressure plate.

Suppliers claim the unit will reduce oil on cuttings (OOC) to approximately 5 - 6% which may equate to a compaction ratio of 30 - 40%, although this will depend on the type of cuttings material and the quality of the primary shakers. The unit has the advantage of being synthetic mudall (1.7m × 0.5m × 0.5m) and is simple in operation with low maintenance. A dedicated operator is required initially but the responsibility for unit maintenance should go to the Drilling Contractor.

The unit was developed in Norway where a number of systems are in use and is now marketed in the UK by two companies, Sea Force Engineering and Apollo Services UK Ltd.

15. On site cuttings cleaning technology

The ability to clean cuttings at the rig site to a level better than the 1% oil on cuttings (OOC) limit for unrestricted discharge would increase the range of options to allow the continued use of synthetic mud or OBM. Where Cuttings Re-Injection is possible, i.e. for platform operations, on site cleaning is unlikely to be a competitive option but for MDU drilling subsea wells there are a number of advantages over the alternative of ship to shore. The requirement to stock pile large volumes of cuttings on the rig will be avoided as well as vulnerability to weather related down time.

Whilst the technology is available for this type of processing on shore, however, there are a number of problems associated with transferring it to the rig site. The first difficulty is that processing units are typically large and producing a facility that could realistically be installed on a drilling rig is in itself a major challenge. Secondly, the available technologies all utilise either high temperature processing or the use of volatile solvents for cleaning. These operations clearly have safety implications which will need to addressed in detail before any offshore application could be considered. Finally, the plant in use onshore is highly sophisticated and may be hard to operate and maintain reliably offshore. In an application where any downtime is likely to result in an immediate drilling stoppage this is potentially a major concern. Some concepts are described below:

TORBED

The toroidal bed reactor or TORBED is an established technology with a number of applications, particularly in the food processing industry.

The basic principal of the process is the use of high temperature steam directly in contact with the cuttings to produce highly efficient thermal cleaning of the drilling waste. The process is controlled such that hydrocarbons are recovered without cracking for re-use. A prototype unit has been built and used in a proof of concept trial which successfully demonstrated cleaning performance in excess of the 1% oil on cuttings (OOC) target and a second reliability and versatility trial is in its final planning stage.

The TORBED project has attracted firm interest within the service industry and if the current trials are successful it is expected that commercial development of an onshore facility could be quite rapid. The evolution of the process for offshore application is still at a conceptual stage.

The Star Tech process

Another cuttings cleaning treatment application is the "Star Tech" process. This is also a thermal application, designed to allow recovery of hydrocarbons for re-cycling. The unique feature of this process is that the plant operates under partial vacuum of 21 inches of mercury. This has the effect of increasing the efficiency of the hydrocarbon distillation process at lower temperature. The system also incorporates a condensing and secondary filtration stage to recover and clean hydrocarbons in the vapour phase exhaust.

Ultrasonic cleaning

The field of ultrasonics is an area in which there is potential for cuttings cleaning applications.

Solvent extraction techniques

The cuttings are mixed with a solvent in an agitated reaction vessel in which the oil / water emulsion is broken allowing the oil to dissolve into the solvent. The solvent / cuttings mix is then fed to a centrifuge for separation and the cleaned dried cuttings are discharged overboard. As a result of the hydrophilic nature of the cuttings solids, the water content of the solvent phase is very low. The solvent / oil mix is then passed through an evaporator and condensation column to separate the two, with the oil returned to the mud pit and the solvent to a holding tank for re-use.

16. Bulk shipment of cuttings

Tthe biggest barriers to the widespread adoption of the transportation of cuttings to shore for disposal are the high cost of shipping and the weather down time. Storage of cuttings on the rig in tanks rather than containers might allow a greater storage volume and therefore a longer period of drilling before off-loading of cuttings was required. Furthermore, bulk haulage vessels would also carry a greater tonnage of waste, allowing a reduction in the number of dedicated vessels required to service a fleet of drilling rigs. Of even greater potential advantage, however, would be the ability to pump cuttings from rig to ship via a floating hose if this would allow the waste to be transferred in weather where normal loading operations would be shut down.

Reciprocating pumps developed in the mining industry are available which will pump cuttings in the form they leave the shale shakers over considerable distances. If a low price vessel were available that could hold its position off the rig even in bad weather, such a pump might be used to transfer drill cuttings irrespective of the sea / wind state. It might be noted that the transfer would be easier if the cuttings were slurrified prior to being pumped, but the formation of a slurry requires the addition of significant volumes of water which would add to the volume to be transported and greatly increase the cost of the final processing of the cuttings to remove the oil.

17. Slurrification and discharge

This option has considerable potential for a low cost, highly effective solution.

The main concern associated with the discharge of cuttings contaminated with synthetic muds is that the base fluid can take a long time to degrade, especially when trapped inside a substantial cuttings pile. Sea bed contaminated with synthetic mud recovery times can be many tens of years. It is considered entirely possible that if cuttings are ground to produce a fine particle slurry prior to discharge the greatly increased suspension time of the particles before settling to the sea bed might result in significantly increased rates of degradation. Furthermore, although the area over which the cuttings are dispersed will be much greater, the resulting in very thin cuttings layers on the sea bed will be likely to continue to degrade quickly and have a minimal and short lived effect on the sea bed.

The process of slurrification prior to discharge has already been practised, primarily to avoid the production of cuttings piles which can be problematical to sub sea well heads. The ultrasonic slurrification unit is ideal for this application and can easily be installed between the cuttings ditch and discharge chute.

Very little is known, however, about the impact of slurrification on degradation rates and the process may be criticised for dispersing the cuttings waste over a wider area, possibly into foreign waters, and a parallel with the banned process of sewage discharge has been drawn. It is possible, however, that this process may represent a cheap solution with very low environmental impact and a study to determine the actual behaviour of slurrified cuttings on the sea bed is strongly recommended.

18. Silicate WBM development

The use of silicate mud is likely to offer a cost effective solution in a wide range of well sections.

19. Subsea cuttings re-injection

The basic process of injection via a subsea wellhead has been demonstrated, but for the operation to be carried out in earnest a considerable amount of work needs to be spent developing the injection guide base and designing a suitable injection riser.

20. Mud Containment

One of the recommendations of the UKOOA was that all UK operators should ensure that all installations contracted to or operated by them have oil mud containment systems in place. The guidelines produced contained an idealised list of mechanical containment methods which should be considered when designing new rigs and installations.