by Jonathan Wills, M.A., Ph.D., M.Inst.Pet.,
for Ekologicheskaya Vahkta Sakhalina
(Sakhalin Environment Watch); 25th May 2000
Minimising Waste Discharges and Their Effects
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Re-injection Offshore (continued)
While the successful and rightly acclaimed Shell Expro CRI project in the North Sea was designed primarily to deal with drill cuttings contaminated with OBMs, it is clear that the technology can also be used to deal with WBM and/or SBM wastes contaminated with crude oil from production reservoirs.
There are other important conclusions from the project:
- The equipment for CRI can be fitted onto normal-sized North Sea production platforms. While these are typically larger than those in Cook Inlet, Alaska, they are similar to offshore installations in Canada, and not much bigger than the Molikpaq platform on the Sakhalin Shelf. Finding enough space should not be an insoluble problem.
- Methods to screen existing wells for use as waste disposal wells are now tried and tested.
- Likewise, the techniques for splitting open geological strata, to receive slurrified waste and ensure it does not escape to the surface or adversely affect nearby producing strata, are now ell known.
- The geology of the northern North Sea appears almost ideal for CRI as a method of waste disposal. Strata are fairly level, faulting not too severe, cap rocks mostly good and seismic activity minor.
Shell Expro is the operator in the UK sector of the North Sea for Esso, an Exxon subsidiary involved in numerous joint ventures with Shell on the UKCS, where the Shell-Esso name appears on dozens of oil and gas leasing blocks. From its involvement with Shell in the North Sea, Exxon has therefore been fully aware of the practicability of offshore CRI since at least November 1997.
The corporation has other sources of information, notably its subsidiary, Exxon Production Research in Irving, Texas, which has been at the forefront of CRI research and development. In February this year, at the International Association of Drilling Contractors / Society of Petroleum Engineers' annual drilling conference in New Orleans, Ronald Steiger of Exxon Production Research and Zissis Moschovidis of PCM Technical, Tulsa, presented the final results and conclusions of an ingenious experiment (Moschovidis, Z. A. and Steiger, R. P. 2000. The Mounds Drill-Cuttings Injection Field Experiment: Final Results and Conclusions. IADC/SPE 59115. Washington, D.C. ) first planned in 1997, about the time the Shell-Esso project in the North Sea was getting under way. The Mounds Drill Cuttings Injection Field Experiment was carried out at an onshore well and designed to find out what happened to the underlying geology when cuttings were injected intermittently, to see if injection pressure was a reliable way of checking that the waste was confined to the target area, to "compile operational practices related to cuttings disposal", to show how seismic and tilt monitoring equipment could assess the size and shape of underground fracturing, and to test and improve theoretical models of waste injection.
The experiment "fulfilled most of its objectives" and, by the use of observation wells intercepting the strata where the injection well was fracturing the rocks and depositing wastes, gave a very clear picture of what actually happens while CRI is going on. The authors and their research team proved they could monitor 12 crucial variables in the process. They made a number of detailed recommendations on creating and controlling the extent of hydraulically-induced fractures in the rocks, using tracer chemicals, choosing suitable geological structures for waste disposal, preparing wells and mixing the slurries.
Although the Mounds experiment was on land, they recognised its relevance to offshore structures where space is at a premium:
Several cuttings slurrifying/injection operations have been carried out over the last few years. Many of the operators use skid-mounted slurrification units for grinding the drill cuttings into a very fine particle slurry. The skid-mounted units are typically about 20ft (6m) long by 12ft (3.6m) high by 10ft (3m) wide and weigh about 10 to 15 tons. They usually have two 50-bbl (8m3) slurrification tanks, centrifugal pump grinders and a shale-shaker for screening the slurry. The centrifugal pump grinders are pumps that have been modified with tungsten carbide coated impellers that function as grinders as well as transfer pumps. Cuttings are usually transported from the drilling fluid shale shakers to the grinding unit by screw conveyors.
In addition, a grinding mill may be required if hard or abrasive formations are expected to be encountered. A slurry storage tank that may range from 150 bbl to 500 bbl (24m3 to 80m3) is typically used in offshore operations for intermittent batch injections. A dedicated triplex pump is used to inject the slurry downhole with the cementing pump used as a backup. Some systems are skid mounted slurry units built to fit platform space requirements. [emphasis added].
Steiger and his colleagues concluded:
Drill cuttings disposal by downhole injection is an economic and environmentally friendly solution for oil and gas operations under zero-discharge requirements. Disposal injections have been applied in several areas around the world and at significant depths where they will not interfere with surface and subsurface potable water sources. The critical issue associated with this technology is the assurance that the cuttings are permanently and safely isolated in a cost-effective manner.
This is further evidence that lack of space is unlikely to be a problem when installing CRI equipment aboard offshore production facilities on the Sakhalin Shelf.
If obstacles do exist, they could be geological. As the E&P; Forum has noted, "It is important to ensure that the disposal is complete, with no chance of break-out of the waste to surface sediments and potable water sources, and that the integrity of the injection well is not compromised. (E&P; Forum. 1993. Guidelines for the Planning of Downhole Injection Programmes for Oil Based Mud Wastes and Associated Cuttings from Offshore Wells. Report No. 2.56/187. E&P; Forum, London.)
In particular, badly faulted rocks are often unsuitable for CRI. There may also be difficulties if rocks overlying the intended waste disposal strata do not provide a reliable seal, or where severe seismic activity threatens the integrity of a waste deposit. The way to determine a rock formation's suitability for CRI is to conduct and publish an independent, expert geological survey, and to apply the methodology developed and tested by Exxon Production Research and specialist contractors such as Terralog Technologies Inc. (TTI) (Terralog Technologies Inc. 2000. Slurry Fracture Injection Waste Management - Oilfield Applications. Website http://www.terralog.com/sfi.htm Calgary, Alberta.) of Calgary, Alberta.
TTI was one of the expert contractors sponsored by the oil company ARCO (now part of BP Amoco) in a 1997 Drilling Engineering Association study on the current status of slurry fracture injection into unconsolidated sand deposits. In western Canada alone, TTI currently disposes of a million barrels of slurry a year by this method and says the technique is "expanding rapidly worldwide".
Although offshore re-injection is certainly practicable, TTI admit that there can be difficulties. In a letter to the author on 28 April 2000, a company representative explained:
The concerns about the SFI equipment being used offshore are certainly valid. Depending on the amount of equipment you need, problems can arise with respect to space on an offshore platform. With respect to geological issues, large scale injection of waste material into shales is problematic and probably not technically viable in the long term. We generally recommend large scale injection into thick, unconsolidated formations, above or below a production reservoir. The geomechanical and flow response of the formation is such that any adverse effects on nearby wells and production activities can be mitigated. With proper formation management, large volume disposal of oil field wastes into deep wells is very effective.
The equipment that we employ can handle maximum grain sizes of 3/16" [4.76mm] and optimum grain sizes of 1/16" [1.58mm]. So in most cases we do not have to grind the material before slurrification and injection. Generally we do not use grinding equipment as part of our disposal operations. If the waste stream we are dealing with is fairly wet - large amounts of mix water for slurrification may not be required, which will further reduce the amount of equipment that is required for injection/disposal purposes. Our equipment can typically deal with a slurry stream of 20-35% by volume solids concentration. However, if required, a complete slurry disposal unit (our design) that can handle 'dry' material will take up an area of about 25m x 19m.
For offshore applications, the easiest method is to bring the waste material back to shore where it is possible to dispose of the material through deep well injection/SFI with relative ease. Terralog is currently involved with such an offshore operation in Indonesia. If this approach is cost prohibitive, an injection-disposal system can be put together for an offshore installation. The amount of equipment/space required will depend on the grain size of the material and the amount of slurrification required prior to disposal. Any concerns with respect to geology can be managed effectively if suitable geology is present in the area.
Where the geology is suitable, there appear to be very considerable financial advantages in using CRI, particularly when very large volumes of water-based drilling mud are involved. Another recent study (Nugrohu, A. 2000. Mud-Waste Handling: One Operator's Experience with On-Site Treatment. PT Caltex Pacific Indonesia. IADC/SPE paper No. 39385) published by the Society of Petroleum Engineers, shows how the treatment and disposal costs of WBM wastes at an onshore Indonesian oilfield may be halved by treating the waste on site and pumping it down the well in a "closed loop / zero discharge" system, rather than shipping it to a centralised mud treatment facility.
The official view of the UK Department of Trade and Industry is: "There are many examples, both in the UK and Norwegian sectors, of the successful slurrification and re-injection of drill cuttings."
It appears that the UK Government does not keep a central register of cuttings re-injection activity but these installations in the North Sea are known to have carried out re-injection, mainly of OBM and SBM cuttings: (DTI spokesman, 2 May 2000, pers. comm.)
Table 13: Re-injection in the North Sea
Courtesy of DTI, Aberdeen.
|BP Amoco ||Andrew; Harding; Magnus; Bruce; Marnock |
|Conoco ||Murdoch |
|Shell/Esso ||North Cormorant; Tern Alpha; Dunlin Alpha; Brent|
|Total ||North Alwyn; Dunbar|
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