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 nineties and is now a mature technology.
This process of cuttings re-injection or CRI has some major benefits compared to the alternative containment option of transporting cuttings to shore for treatment and disposal. Firstly the CRI 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 scale or even produced water.
The limitations of CRI are partly cost related - injection units have a fairly high capex and operating cost which can only be justified for a drilling installation with a reasonably active drilling schedule. Furthermore, the installation of a CRI unit will place certain weight and space requirements on the platform, as well as extra crew, which may pose problems for synthetic mudall installations. The reliability of CRI equipment is also an important factor. Experience has shown that properly designed and managed re-injection operations may be carried out with little down time, but wherever problems do occur drilling shutdowns will result. Lastly, in order to carry out a CRI operation it is necessary to have access to a suitable well head to receive the waste slurry and this in effect restricts the application to permanent platforms. The technology for re-injection into a subsea well head has been demonstrated but is not available for operational use.
1. CRI technical backgound
The process typically involves the collection of cuttings from discharge lines at the shale shakers which are then carried to a cuttings processing unit by an auger or conveyor system. The cuttings are mixed with sea water and fed through a grinding mill to produce a fine paste. Grinding is controlled such that an ideal particle size of about 200 microns is achieved. The slurry is usually then passed to a second tank where further water and / or conditioning chemicals may be added to produce a fluid suitable for injection. The slurry may then be passed to a holding tank or injected immediately. Dedicated injection pumps would normally be used although the standard cement pump is also an option. Unless a dedicated disposal well is being used the slurry is pumped via a modified well head into a selected annulus, typically the 20" by 13 3/8". The fluid is then injected into the formation in a similar manner to standard fracturing principles. On completion of injection activities the annulus would then be cemented.
The main technical distinction separating different commercial CRI units is the method used in grinding the drill cuttings. There are two established methods of grinding cuttings. The first is generally referred to as the Arco method after the patent owners. In this treatment the cuttings are passed through what is essentially a modified centrifugal pump in which the shearing and abrasive actions act to reduce the cuttings size. The second approach is to use a positive action grinding mill such as the Statoil wet autogeneous mill (WAG). The latter has the advantage that the positive action can result in faster more effective grinding but has the disadvantage that wear and maintenance requirements of the mill are typically higher.
A third approach being pursued is the use of ultrasonics technology to produce a cuttings slurry. In this application the cuttings are pumped through an ultrasonic processing chamber in which a fast and efficient particle size reduction may be achieved. The basic processing unit has been built and tested but its incorporation in a fully functional unit is still in the development phase. With each of these systems the processed cuttings are passed through a classification stage with oversize material being re-circulated through the grinding stage.
A lists of commercial suppliers of CRI equipment including the ultrasonic unit is presented at the end.
2. Planning and feasibility
The main requirements for application of a CRI unit may effectively be divided between the surface and the subsurface. The surface considerations that must be addressed in the planning process are principally those of space and weight. A typical re-injection unit would consist of a slurrification skid, with the grinding mill and usually two tanks (- a conditioning tank and a holding tank), and an injection pump skid. The slurrification unit might be in the order of 20' 10' 10' and 15T and a skid with a triplex pump and power pack something similar again. Where space or loading constraints are likely to be problematical a custom unit may be designed with individual components of the plant separated and located wherever possible and tanks sized as permitted. Rig electrical power, air and diesel are also required. A final requirement for additional crew of 2 - 4 men, dependant upon the equipment and drilling program, must also be given consideration.
The subsurface issues that must be treated at an early stage in the planning process relate to the availability of suitable wells for injection. This problem will clearly be field and installation specific but having identified a suitable formation, consideration should be given to the following factors: wellhead design and specification (including erosion / corrosion tolerance); casing design and integrity; cement quality and TOC. It is important to appreciate that whilst a lot of work has been done in predicting fracture behaviour, practice has often deviated a long way from expectation. In this context the value of the experience gained within Company from the CRI operations in Brent should not be under estimated and any project group looking seriously at re-injection would be most strongly recommended to try and benefit from this first hand experience.
In operations where CRI has been identified as the most cost effective option the above issues must clearly be addressed to establish the feasibility of the option. If these requirements can be met the process of identifying a suitable supplier and planning system build and installation must then be followed.
3. CRI economics
The economic viability of CRI is heavily dependant upon the drilling sequence and the level of activity of a given installation and in order consider this properly an evaluation exercise must be carried out to determine which installations will favour CRI and of the effective costs.
For this exercise to be successful, however, it will clearly be important to have accurate input data for the cost of installation and operation of re-injection units on the platforms considered. Whilst approximate costs are available from the various companies offering CRI units, accurate figures will depend both on the installation specific considerations listed above and on the degree of commercial competition at the tender stage. To realise the best quality input possible for the economics evaluation a preliminary study is being carried out for a number of candidate platforms.
As an indication of CRI costs, package prices are likely to be in the £1,000K’s range depending on whether or not a dedicated injection pump skid is required. The option of using the cement pump for injection is therefore quite important to the overall cost as well as space requirements. In addition to this overhead, installation costs are likely to be in the order of £1,000K depending on platform requirements. Finally, during drilling operations a CRI crew of 2 - 4 men will be required at a day rate of approximately £700 each. During top hole drilling 4 men would probably be required whereas later in the well when cuttings generation rates are lower 2 might be sufficient. It is possible that the development of increasingly automated and reliable units will reduce the crew requirements.
Another option for reducing CRI costs would be to exploit the increasing mobility of CRI units to time share a unit between installations. An alternative to mobile CRI units would be to transport cuttings from installations without CRI capability for injection at some central location. This would clearly have the advantage of reducing CRI overheads but has associated problems. Firstly there is legislation restricting the movement of waste material for disposal remote from the site of origin. This legislation has recently been relaxed to allow transfer of waste material within a single field for disposal by re-injection and the current climate seems to be of an increasingly open minded approach to such operations. The second problem with transporting cuttings for re-injection is that all of the logistical problems associated with ship to shore are immediately encountered. The economic implications ned to be evaluated, but the expectation is that if cuttings are to be moved from the rig on which they originate it will be more efficient to return them to shore than to another platform. An exception might be in the case of a mobile rig drilling beside a platform where cuttings transfer might be possible without requiring a vessel.
4. Subsea re-injection
Whilst the process of subsea injection is not an established operation, work has been done to prove the feasibility of the concept. As part of DEA project 026 a CRI permanent guide base (PGB) and wellhead for subsea injection have been built and successfully tested. The DEA project involved an injection test at the end of an MDU drilling operation. After completion of the well a drill pipe stinger was run and engaged with a stab connector on the PGB allowing a quantity of stored cuttings to be injected into the 20" 13 3/8" annulus.
Whilst this trial was a successful proof of concept, a number of areas would need considerable development before the process could be considered for routine application offshore. Firstly the development PGB would need considerable improvement to meet Company standards. More importantly, a satisfactory method of pumping the slurry from the CRI unit at surface to the PGB during drilling operations needs to be found. Furthermore, on single well sites the disposal of cuttings generated before the 13 3/8" casing has been set represents a serious problem.
Despite these problems, however, given the requirement to use OBM in many of the wells drilled from MDU and the difficulties of shipping cuttings off the rig, the value of a subsea CRI facility could be very high.
Appendix – CRI Service suppliers
Apollo Services, Aberdeen Tel. 694098
Burgess and Garrick, Shetland, Tel. 01806 242107
Drexel, Aberdeen, Tel. 899000
Dril-Quip, Aberdeen Tel. 727000
Drilltech Services Ltd, Aberdeen, Tel. 249988
Enaco PLC, Aberdeen Tel. 895511
The Company Group, Aberdeen Tel. 214600
MGA Environmental Services Ltd, Aberdeen, Tel. 210366
Oiltools Ltd, Aberdeen,Tel. 719200
Pac-Land Managament, Inc, Lafayette, LA Tel. 001 318 236 8805
Sea Force Engineering, Aberdeen Tel. 622303
Swaco Geolograph Ltd, Aberdeen Tel. 703771
Sweco Oilfield Services, Aberdeen Tel. 715250
Solids Control Services, Aberdeen Tel. 249220
Thule Rigtech, Aberdeen Tel. 595763
Transocean, Aberdeen Tel. 891118