METHOD OF TREATING OIL-CONTAMINATED DRILL MUDS OR CUTTINGS PRIOR TO REUSE OR DISPOSAL

Method of treating oil-contaminated drill muds or cuttings prior to reuse or disposal.

This invention relates to the treatment of oil-contaminated drill solids, e.g. drilling muds or drill cuttings prior to reuse or disposal.

In subterranean and marine drilling operations using water-based drilling mud, the disposal of the driil cuttings presents little problem from the environmental or ecological point of view. In land-based operations they can be dumped or used as land in-fill at any convenient site, and in marine operations they can be dumped overboard from the drill platform for distribution over the sea bed by tidal currents. However, in many drilling operations, for example, in drilling through shale high in montmoriilonite, which undergoes expansion when contacted by an aqueous drilling mud, it is preferred to use an oil-based drilling mud, containing for example, a substantial amount of diesel oil. Oil-based muds are also preferred since, in general, they permit higher drilling rates, and also they facilitate drilling at an angle to the vertical, for example, when drilling inclined well bores which may be at anything up to 50° or 60° from the vertical or more. However, the drill cuttings from such operations present a disposal problem because of the oil contamination.

Various proposals have been made for the treatment of such oilcontaminated drill cuttings prior to disposal ranging from incineration, which is costly, and detergent washing, which is often not sufficiently effective to meet statutory requirements governing the disposal of oil-contaminated waste, or at least not at economic levels of operation.

In one such method, disclosed in European Patent Application Publication No. 0005273, the hydrocarbon values in such oil-contaminated drill cuttings are proposed to be removed by a combination of vacuum distillation and grinding by heating the cuttings under reduced pressure in a pressure vessel equipped with means for milling or shearing the drill cuttings during the heating process. In another method disclosed in U.S. Patent No. 3 693 951 oil contaminated drill cuttings are passed on a conveyor belt through a preheater for partial drying and then to a high-intensity infrared heating chamber in which the volatile hydrocarbons are expelled from the cuttings at a surface temperature higher than the combustion point of petroleum.

The detergent washing of drilling cuttings is disclosed in U.S. Patents 3 693 733 and 3 860 019.

Yet another approach is adopted in U.5. Patent No. 4 040 866. In this proposal oil-contaminated drill cuttings are processed to remove the contaminating oil by contacting the cuttings with a mixed solvent system consisting of ethylenegiycoi monoisobutyl ether, diethylenegiycol monobutyi ether acetate, ethyl butyl ketone, ethylenegiycoi monobutylether, ethylenegiycol monom ethyl ether acetate, diethylene giycol diethyl ether or diethylene giycol mono-n-hexyl ether as the first component and paraffin oil as the second component in relative proportions of from 10:90 to 80:20. The resultant oil-solvent mixture which forms on the cuttings is readily removable by centrifuging or washing, e.g. with sea water, or a combination of the two.

In accordance with the present invention it has been found that the oil content of oil-contaminated drill cuttings can be economically and efficiently reduced to permitted levels by washing the oil-contaminated drill cuttings, preferably after screening to remove fines and supernatant drilling mud which can be re-used, with a single solvent system consisting of a haiogenated hydrocarbon such as carbon tetrachloride or 1,1,1-trichioroethane, and centrifuging the washed cuttings to remove the oil-solvent solution, which can be recycled to the washing process and/or distilled to recover the washing solvent and an oil residue which can be recycled to the drilling process.

Moreover, it has been found that washing with a single haiogenated hydrocarbon solvent, particularly, 1,1,1-trichloroethane, reduces the drill cuttings to a dry non-sticky, easily friable form which greatly facilitates subsquent transportation and dumping of the decontaminating cuttings. Thus, for example, it has been found that such decontaminated cuttings can be readily transported pneumatically by entrainment in a carrier gas stream, e.g. air. Such cuttings can therefore be readily and easily pipelined away from the drilling platform for dumping at a location remote from the drilling operation. Whilst the invention is particularly described with reference to the decontamination of oil-contaminated drill cuttings prior to disposal, the method of the invention is also applicable to the recovery of the drilling mud itself, particularly drilling muds which are weighted with barytes. Because of the cost element, the recovery of the barytes for reuse can be an important factor in the economics of subterranean and submarine drilling operations, and the present invention provides a simple efficient and economic way in which this can be done. Thus, after screening to remove the cuttings for further decontamination treatment as described, the separated drilling mud can be treated in essentially the same way, for example, by initial screening or centrifuging to separate out the mud solids, and removing from the separated mud solids decontaminating oil by washing with the halohydrocarbon and centrifuging in the manner described.

The invention will be further described with reference to the accompanying drawing in which the single figure represents a diagrammatic flow sheet of a preferred method of treating oil-contaminated drill cuttings in accordance with this invention.

Referring to the drawings, oil-contaminated drill cuttings which have been screened to remove surplus oil-based fluids and fines for re-use in the drilling operation are passed via line 1 to a mixing tank 2. If desired a portion of the separated fines may also be passed to the tank. In the tank 2, the cuttings are contacted with an organic haiohydrocarbon solvent washing liquid, preferably 1,1,1-trichloroethane, fed from a storage tank 12. From the tank 2, the cuttings/solvent slurry is fed to a macerator and pump 3 which reduce the particle size of the cuttings to improve washing contact with the solvent. A recycle stream of macerated slurry is fed via line 4 from the macerator 3 to the tank 2 to maintain a desirable solids concentration in the tank. The main stream of macerated slurry is fed from macerator 3 via line 5 to a continuously running decanter centrifuge 6. Substantially oil-free cuttings are withdrawn from the centrifuge 6 via line 7 for disposal whilst separated oil/solvent washing liquid is withdrawn via line 8. This may be recycled via line 9 to the washing tank 2 whilst a slip stream is taken off via line 10 to a distillation unit II for recovery of the solvent which is returned to the solvent storage tank 12 for re-use and separation of the oil residue via line 13. The oil residue separated via line 13 will contain a proportion of fines and also substantial amounts of the chemical additives added to the original drilling mud and can either be collected for disposal, treated for further recovery of one or more components and/or returned to the drilling operation for use as recycle oilbased drilling mud.

The decontaminated drill cuttings withdrawn via line 7 are in a dry, non-sticky friable powder form suitable for pneumatic transportation in a carrier gas, e.g. air stream, to a disposal point. Alternatively, in submarine operations the decontaminated cuttings can simply be dumped overboard since oil-contamination levels of cuttings treated in accordance with the present invention is well below the minimum standards imposed by government regulations.

It will be seen therefore that the preferred embodiment of the invention, at least, provides a substantially closed cycle for the recovery and reprocessing of the oil-based drilling mud, whilst at the same time substantially solving the disposal problems of the oil-contaminated drill cuttings.

As has already been stated, the above described method and apparatus can readily be adapted for the recovery of the mud solids themselves, e.g. the barytes used in weighied drilling muds.