This invention relates to a method for the remediation of waste drilling muds generated during oil-winning operations. The process involves conversion of the oil in the mud to a water-in-oil microemulsion, and its subsequent extraction from the particulate fraction by floatation. Muds so treated are rendered entirely free of oil. The residue (a mixture of rock particles and colloidal clay) is harmless to the environment and can be safely disposed of by dumping on land or at sea.

The recovered oil can be re-used, burned or biodegraded.

Drilling muds are used as lubricants and stabilisers in the drilling of wells as part of the oil-winning operation. There are a large number of different mud formulations but they can be subdivided into just two groups, those based on oil and those based on water.

Oil-based muds generally perform better than water- based muds and are in common use where drilling operations is particularly difficult. For example, oil-based muds are used almost exclusively in North Sea oil-winning operations, as a consequence of the practice of horizontal (as opposed to vertical)

drilling.

Oil-based muds consist of an oil, which may be a mineral oil or a synthetic oil plus a detergent plus variable amounts of colloidal clay (e. g. bentonite), added as required during the drilling operation. The oil acts basically as a lubricant and the colloidal clay stabilises the walls of the well. During drilling, drilled rock particles (shale, sandstone, limestone) and sea-water accumulate in the drilling mud. Therefore a used drilling mud consists of the original oil-lubricant, variable amounts of colloidal clay, sea-water and drilled rock particles.

Existing technologies for the treatment of used drilling muds are designed to recover the bulk of the oil for reuse. The principle alternatives are (i) the hydrocyclone (a type of industrial-scale high-speed centrifuge) and (ii) the shale-shaker (a vibrating screen which retains particulates of diameter > 100 micrometers, approx.). The latter appears to be in more common usage. These techniques result in the recovery of up to 90% of the oil base, leaving behind a cake of oil-coated cuttings and colloidal clay containing approximately 100g oil/kg. In the North Sea, it has been past practice to dump this waste drilling mud (WDM) overboard, where it has accumulated in piles at the base of the drilling platform. This has a severe destabilising effect on the benthic ecosystem due to the slow leaching of toxic organics and heavy metals and as a consequence legislation has been introduced to make this practice illegal in UK waters. The new legislation will permit overboard dumping of oil-free materials only.

Some work has been carried out in the application of

micro-organisms to degrade the oil component of the WDM. Whilst this area undoubtedly has potential, no commercial technology is available as yet.

Use of industrial scale incinerators, such as the Torbed reactor have been found to be very expensive both in terms of plant and operating costs; in any case such systems could not be placed on an off-shore rig.

A commercially viable WDM remediation/bioremediation technology must reduce the oil content of the WDM to 1% w/w oil or less. The infrastructure to support it must also conform to weight-and power-requirements for installation on the rig should on-rig processing be desired (transport to shore prior to processing will incur a high cost disincentive). Hence relatively stringent performance factors must be met for any potentially viable technology. This necessarily means that relatively straightforward technologies, requiring plant with a minimum of sophistication, will be preferred.

It is an object of the present invention to provide a method to remove the oil component from waste drilling mud and other oil-contaminated particulate material.

According to the present invention there is provided a method for removing oil from oil-contaminated particulate material comprising the steps of mixing the oil-contaminated material with a microemulsion-forming surfactant, and an excess of water, or an aqueous salt solution, and allowing the resulting mixture to separate into an upper microemulsion phase, an intermediate conjugate polar phase (i. e. the aqueous phase in this case) and a lower layer of oil free solids.

In one aspect of the invention the oil-contaminated particulate material is waste drilling mud.

The water or aqueous salt solution may be added before, after or together with the surfactant.

A microemulsion for use in the present invention is preferably one wherein the contact interfacial tension generated between a microemulsion phase and a conjugate polar phase is extremely low. Most preferably the interfacial tension is less than 10-4 mNm~l.

Suitably the microemulsion is chosen from the group comprising sodium bis-2-ethylhexyl sulphosuccinate, sodium dodecyl sulphate, didodecyldimethyl ammonium bromide, trioctyl ammonium chloride, hexadexyltrimethylammonium bromide, polyoxyethylene ethers of aliphatic alcohols, polyoxyethylene ethers of 4-t-octylphenol, and polyoxyetheylene esters of sorbitol and any other cationic, anionic or nonionic detergent either in commercial manufacture, or custom synthesized or biologically manufactured.

The method may further comprise the steps of temperature and/or ionic strength adjustment to enable ultralow surface tensions to be established.

Preferably the invention permits a Winsor Type II system to be established comprising an upper microemulsion phase containing all the oil and surfactant, a middle aqueous phase and a lower solids phase, wherein the solids phase is devoid of oil.

Preferably the method further comprises a step wherein the oil is recovered.

Recovered oil may be reused.

Alternatively, the method may comprise a step wherein the oil in microemulsion form is biodegraded by inoculation with hydrocarbon degrading micro-organisms.

Suitably bacteria belonging to the genera Rhodococcus or Gordona or Tsukamurella or a mixture of these may be used to degrade the oil.

The invention further provides an apparatus for carrying out the method as disclosed herein.

In one embodiment the apparatus comprises a tank reactor for batch-mode separation.

In an alternative embodiment the apparatus comprises a centrifugal reactor for continuous separation.

This invention relates to a process for the remediation of waste drilling muds. The novel element of the invention is the conversion of the oil in the WDM into an oil-continuous (water-in-oil, w/o) microemulsion.

This is achieved by the addition of surfactant, water and (optionally) a salt. The surfactant is one of a particular group of surfactants which stabilise microemulsions. In this particular invention an excess amount of water is added such that the resulting system consists of two phases: an upper oil-continuous microemulsion phase (containing all of the oil, all of the surfactant and some water) and a lower water phase containing most of the salt (if present). This is known as a Winsor Type II system.

Microemulsions, and multiphase systems such as the Winsor II, in which one of the phases is a

microemulsion, are a recognised and singular group of colloidal systems which are characterised by the fact that they are thermodynamically stable. This single factor distinguishes microemulsions from all other multicomponent systems containing surfactant, including ordinary emulsions which, by definition, are unstable with respect to a system of separate phases.

Microemulsions form spontaneously when their components are mixed (this is the corollary of thermodynamic stability) and, once formed, remain so unless measures are taken to break the microemulsion (usually by adding salt or changing the temperature).

Many microemulsion-forming surfactants are described in the literature, and it is possible to determine, for any given surfactant, whether it will or will not form a microemulsion. Microemulsion-forming surfactants can be selected from the range of existing commercially available surfactants, or they can be custom-designed, or they can be purified from a living organism, biosurfactants.

The specific property of microemulsions relevant to this invention is the fact that the contact interfacial tension generated between a microemulsion phase and a conjugate polar phase (e. g. water, air, or a solid material such as clay) is extremely low. Under certain conditions it can be immeasurably low (<10-4 mNm~l). By way of contrast, the interfacial tension of an oil such as n-heptane and water is of the order of 100mNm-1, i. e. higher by a factor of at least 106. Ultralow surface tensions, hence water or microemulsion formation, is established, for a given surfactant, by appropriate adjustment of temperature and salt concentration (ionic strength).

This invention is based on the concept that, on microemulsification, the interfacial surface tension between the oil and the particulate phases of the WDM will decrease to essentially zero and that this will facilitate the separation of the two phases, on addition of water or aqueous salts solution.

Phenomenonologically, the process can be envisaged in two stages: (i) conversion of the oil to a w/o microemulsion and (ii) separation of the microemulsified oil from the cuttings by floatation, once again using water or aqueous salts solution. In practice these two steps can be combined by establishing conditions which permit the formation of a Winsor II system (namely by stirring the WDM with surfactant, water and (optionally) salt, at a suitable temperature for a suitable length of time. Following microemulsification (which is normally complete in a matter of seconds or minutes), the system is permitted to phase-separate (this process may be facilitated by low-speed centrifugation). The resulting Winsor II consists of an upper microemulsion phase, containing all of the oil and surfactant, a middle aqueous and a lower oil free solids phase, absolutely devoid of oil.

The oil and surfactant may be separated (i. e. the microemulsion decomposed or broken) using well- documented procedures.

In one example, after removal of solids the recovered Winsor II is warmed or cooled such that the surfactant partitions entirely out of the oil-phase, resulting in a two-phase system, one of pure oil and one of an aqueous surfactant solution. The direction of the temperature change is dictated by the type of surfactant used.

In another example the recovered microemulsion phase is stirred with water containing no salts. On standing the mixture separates out into an oil phase containing no surfactant and aqueous solution of the surfactant.

Which ever method is applied, the resulting aqueous surfactant phase can be recycled to the phase- separation reactor. The recovered, pure oil can be reused, burned, or biodegraded.

The scope of the invention includes the option to biodegrade the w/o microemulsion, rather than recover the surfactant. In this case the Winsor II is inoculated with micro-organisms which degrade the oil (preferably oil-tolerant, hydrocarbon-oxidising organisms belonging to the genus Rhodococcus). On stirring, the Winsor system becomes an emulsion consisting of droplets of water-in-oil microemulsion in a continuous aqueous phase. The microemulsified nature of the oil confers the specific advantage that the degree of dispersion of the microemulsion phase obtained for a given stirring rate (shear force), and hence the total interfacial surface area generated, is much greater than for the equivalent water+oil (no surfactant) system. This again is a consequence of the ultralow surface-tension condition. Good dispersion facilitates bioremediation on the basis the availability of interfacial surface area is rate limiting in hydrocarbon biodegradation.

Example A sample of waste drilling mud was obtained from a North Sea drilling platform. The material consists of oil (13'/-1% by weight), the remainder being solid material.

This material was treated as follows: 50 millilitres of the waste drilling mud was mixed with 0.4g of the surfactant sodium bis-2- ethylhexylsulphosuccinate (AOT) and 50 millilitres of aqueous sodium chloride (4.5g/l NaCl) (saline). The mixture was stirred until the surfactant was completely dissolved. The mixture was then allowed to phase separate into an upper layer of oil, a middle layer of aqueous salt and a lower layer of particulates. The solids were recovered and the oil content determined by distillation followed by weighing the distilled oil, to be 3'/1%. When AOT was omitted from the mixture (so that there was no microemulsion) formation and any cleaning was done simply to solvent washing the recovered solids are saved at 10+/-1% w/w oil.

Preferred embodiments: The invention extends to -Mixtures of waste drilling mud, AOT and saline, in any desired proportion.

-Any desired aqueous NaCl concentration in the range 0-ll. Og/1.

-Any salt, supplementary to or as replacement for NaCl, including salts of divalent cations, at any desired concentration.

-Any temperature especially in the range 0-60°C.

-Any surfactant ("detergent"), ionic, or non-ionic, including but not restricted to, sodium bis-2- ethylhexyl sulphosuccinate; sodium dodecyl

sulphate; didodecyldimethyl ammonium bromide; trioctylammonium chloride; hexadexyltrimethylammonium bromide; polyoxyethylene ethers of aliphatic alcohols. e. g.

Brij 56, Brij 96; polyoxyethylene ethers of 4-t- octylphenol, e. g. Triton X-100, Nonidet P40; polyoxyethylene esters of sorbitol, e. g. Tween 85.

Biosurfactants may also be used.

Any mixture of surfactants.

Addition of any other type of surface-active component (commonly known as a co-surfactant), which complements the function of the designated surfactant. Examples include but are not restricted to any member alcohol compound, any carboxylic acid compound and any halogenated hydrocarbon compound.

Addition of any substance which may act as a floculating agent, including any commercially available or custom synthesized floculant.

Any oil-contaminated particulate material, as a replacement for waste-drilling mud. The term "oil"is taken to mean a water-immiscible hydrocarbon compound or silicone compound, or derivatives or mixtures thereof.

Any reactor configuration used to accomplish the phase separation and segregation of the phases, including those based on a centrifugal mechanism.

The scope of the invention covers: 1. The use of any microemulsion-stabilising

surfactant, or a mixture of several, and includes both biodegradable or non-biodegradable surfactants of any origin including biosurfactants. Any salt may be used.

Surfactants and salts may be used in any combination.

2. Oil-based drilling muds of any formulation.

3. Any method of achieving phase-separation. Batch- mode separation can be carried out in a simple tank reactor (separation of the microemulsion, water and particulate phases according to buoyant density, under gravity). In an alternative design a Winsor II system (i. e. water, surfactant and a volume of clean oil to prime the system) is spun in a centrifugal reactor to create film consisting of an inner microemulsion-phase (less dense) and an outer water-phase. WDM introduced at the centre-of-rotation passes under centripetal acceleration through the microemulsion phase, where the oil is retained, and the clean particulate material passes through into the water-phase (thus the microemulsion phase acts as a"liquid membrane"). This can be modified for continuous operation.

The invention is designed principally to deal with WDMs but can be extended to include any oil-contaminated particulate material, including soils contaminated with petrochemicals and sand and swarf contaminated with oil-based cutting fluids, generated in metal fabrication.

Advantages a) Phase Separation The invention has the advantage that it requires no sophisticated plant or expensive material for its operation. The only commodity required, apart from water and salt, is the surfactant. The invention is flexible enough to take cost into account in choosing the surfactant. There are no unwanted by-products. The surfactant can be recovered and recycled with high efficiency. b) Biodegradation of Microemulsified Oil Biodegradation of the oil in Winsor II microemulsion form, in a continuously stirred tank reactor configuration, has the advantage that extremely high surface areas are generated. This leads to good contact between bacteria and oil and hence best possible rates of oil-degradation.

Bacteria belonging to the genus Rhodococcus are specified on the basis of their oil-tolerance and well-documented ability to degrade a wide range of hydrocarbons. If non-biodegradable surfactants are used there remains the possibility of recovery and recycling following oil degradation.

The advantages of separating the oil from the particulate matter prior to biodegradation (or any other further treatment are: 1. The volume of the oil alone is far smaller (about 10%) of the volume of the WDM. Hence effectively the same amount of WDM is bio-treated in a reactor of correspondingly smaller dimension. In addition

the cost of transportation to an on-shore site, if necessary, is reduced.

2. The abrasive action of the solid materials in the WDM has been eliminated, therefor reactor lifetimes are extended.