Field of the Invention

This invention relates to the cleaning of oil contaminated cuttings arising from drilling operations.

Background to the Invention

During the drilling of oil and gas wells large quantities of rock particles are produced from the bore hole. The rock particles, typically referred to as cuttings, are carried to the surface of the hole by a continuously circulating fluid, referred to as the drilling fluid or drilling mud. At the surface the cuttings are separated from the bulk of the fluid by various units of mechanical equipment, typically vibrating screen shakers, hydrocyclones, and centrifuges. Separated cuttings are, however, still wetted by the drilling fluid.

In cases where the drilling fluid is an oil based mud, which typically consists of an emulsion of aqueous salt solution in a light mineral oil base, the separated cuttings may have a considerable quantity of oil associated with them.

Oil is retained on cuttings in three ways: as oil mud in a viscous layer on the surface of the cuttings; as oil mud in "free" form trapped in agglomerates of cuttings; and as free oil absorbed into rock pores. Excluding accidental

oil spills, the largest source of oil discharge from oil field operations is in the form of oil mud associated with drill cuttings. To combat this, legislation has been introduced which limits the concentration of oil on cuttings which can lawfully be discharged. For example, countries bordering the North Sea are working to standardise and improve current legislation, and it is expected that oil discharge levels will be made more stringent in the future.

To meet current legislative requirements, cleaning systems have been developed for offshore application which, under most circumstances, will reduce the oil content on discharged cuttings to below 10% by weight. All of these systems employ an initial washing technique followed by centrifugal separation and discharge of cleaned solids, and limited recycling of the wash fluid. One known system employs mud based oil as the washing fluid but this suffers from the disadvantage of leaving a coating of oil on the solids and therefore limits the degree to which the cuttings can be cleaned. Another known method utilises a water/surfactant wash solution which, whilst theoretically producing cleaner solids, results in the secondary problem of producing large volumes of oil contaminated wash water for disposal.

Several other thermal methods have been attempted which employ either thermal destruction of the hydrocarbons associated with the cuttings, or thermal evaporation of the oil followed by condensation and recovery of oil values. Such systems have suffered the disadvantages of high mechanical wear, corrosion, and difficulty in process control due to the variability in properties of the cuttings. In particular, such systems have also had a

high energy demand due, in large part, to the necessity to evaporate all water associated with the cuttings.

The present invention aims to provide a novel method of and apparatus for cleaning cuttings which meets the more stringent cleaning requirements shortly to be imposed and which is capable of overcoming the disadvantages of known cleaning systems.

Summary of the Invention

According to the invention a method of removing oil from oily cuttings obtained from a drilling operation comprises

- exposing the oily cuttings to a solvent for oil and thereby dissolving the oil in the solvent, and separating the cuttings from the solvent with the oil dissolved therein.

The solvent (eg toluene) and the oily cuttings are preferably exposed to the solvent in an extractor chamber through which the cuttings and solvent pass in opposite directions in a continuous process, the cuttings being fed into an inlet end of the extractor chamber and exiting from an outlet end thereof, with the solvent discharge being collected at the inlet end of the extractor chamber, cleaned and then recirculated back to the outlet end of the extractor chamber. This ensures that the cleanest solvent contacts the cuttings at the outlet end of the extractor chamber.

The cuttings are conveniently fed through the extractor chamber by means of a conveyor belt and the solvent may be sprayed into the extractor chamber at the outlet end thereof.

The extractor chamber is conveniently divided into a plurality of compartments through which the cuttings are transported in succession by the belt, the solvent being extracted at the base of each compartment and being pumped to the adjacent compartment into which the solvent is sprayed. In each compartment, the solvent is preferably allowed to filter through the cuttings and belt and is then collected in a sump at the base of the compartment.

The cuttings obtained from the outlet end of the extractor chamber will retain a certain amount of solvent and absorbed water. To remove these, the cuttings thus obtained are preferably fed to a dryer unit which removes the solvent, leaving clean cuttings in the form of solids having an oil content typically not greater than 1% by weight.

The solvent discharge from the extractor chamber normally contains fine solids, in addition to oil/solvent and water. For solvents such as toluene, the oil/solvent and water are in the form of an emulsion. In one method, the fine solids are separated from the emulsion which is then delivered to an evaporator in which the emulsion is broken down into an oil component and a solvent plus water vapour component. Fine solids may be extracted, as by gravity separation, either upstream or downstream of the evaporator. However, the high latent heat of the evaporated water demands a great deal of energy for this process. To overcome this problem, in another preferred method, the emulsion is broken down by a demulsification process in which the fine solids and the emulsion are exposed to a demulsifying agent which effects separation into a fine solids component, an oil/solvent component and

a water component. In both methods the solvent is preferably recovered, together with solvent from the dryer unit, and recirculated to be fed into the extractor chamber.

In a modification of the method, an alternative extracting means is employed, comprising an agitated vessel to which the oily cuttings and the solvent are added, and the resultant solids/solvent slurry is then fed, for example by a positive displacement pump, to a centrifuge, preferably a solid bowl decanting centrifuge.

According to a further feature of the invention, water is added to the solids/solvent slurry prior to centrifugation. This reduces the amount of oil carried through with the solids, and avoids any requirement to heat the solids phase prior to its discharge.

The invention also includes within its scope apparatus for removing oil from oily cuttings obtained from a drilling operation, the apparatus comprising an extractor in which the oily cuttings are exposed to an organic solvent for oil and in which the oil is dissolved in the solvent, the extractor having separate outlets for the cuttings and for the solvent and oil dissolved therein.

Description of Embodiments

Eight methods according to the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a block diagram illustrating a first method.

Figure 2 is a block diagram illustrating a second method, and

Figures 3 to 8 are block diagrams illustrating six further methods.

Referring first to Figure 1, drill cuttings from a drilling operation are fed as indicated by arrow 10 into an extractor in which oil is extracted (as indicated at 12) by the application of a solvent, eg toluene. Instead of being fed directly into the extractor, the cuttings may optionally be pre-mixed with the solvent to present a slurry feed to the extractor, as shown at 14.

The extractor consists of a closed chamber, separated into a plurality of compartments. A continuous fine mesh belt conveys the cuttings from an inlet end of the extractor, through the compartments in turn and discharges the solids from an outlet end of the extractor into a hopper. Clean solvent is sprayed onto the cuttings in the compartment at the outlet end of the extractor, the solvent filtering through the cuttings and belt and being collected in a sump at the base of the compartment. This solvent is then re-circulated by means of a pump to the adjacent compartment immediately upstream (in relation to the direction of movement of the cuttings through the extractor) and the process repeated. Solvent and cuttings are therefore transported in mutually opposite directions through the extractor, with the cleanest solvent contacting the cleanest cuttings at the outlet end of the extractor.

The bulk solids discharged from the extractor at 16 will still be wet, ie they will contain solvent and any water

absorbed into the particles of the cuttings. These solids are therefore transferred to a dryer 18 where the solvent is evaporated at 20 and the clean dry solids discharged at 22, where the oil content of the solids is typically not greater than about 1% by weight.

The fluid discharge 24 from the extractor consists of fine solids plus a solution of water in an oil emulsion dissolved in the solvent. Fine solids are removed at 26 by, for example, a decanting centrifuge and are fed at 28 into t; e bulk solids which are supplied to the dryer 18. The liquid phase 30 from the centrifuge is transferred to an evaporator 32 where the solvent and water are evaporated from the oil. The oil is recovered at 34 as a useable product and the solvent plus water vapour 36 is combined with the solvent and water vapour 20 from the dryer unit 18 and fed into a condenser/separator 38 where the vapours are conder ed. Condensed water and solvents separate by gravity, the water being discharged at 40 and the solvent being re-cycled at 42 back to the extractor at 12.

In the modified process shown in Figure 2, equivalent process steps bear the same reference numerals as used in Figure 1. The process is similar except that the fluid discharge 24 from the extractor is subjected to demulsification in order to separate water from the oil/solvent. Hence, the fine solids plus the oil/solvent and water are fed to a demulsification unit 44 where they are exposed to a demulsifying agent. The fluid mixture is then transferred to a three phase separator unit 46, for example a disc stacked centrifuge, where wet solids, oil/solvent and water are separated. As before, the fine solids 28 are reunited with the bulk solids which are fed

to the dryer 18, and water is discharged at 48 via a separate treatment system if required. The oil/solvent solution 50 is transferred to the evaporater 32, but this will require less energy than in the evaporator 32 of Figure 1 because a large amount of water has been removed at 48 as a result of the demulsification and subsequent separation.

In the methods illustrated in Figures 3 to 8, the same reference numerals are again used for the process steps equivalent to those of Figures 1 and 2.

Referring to the drawings collectively, drill cuttings from a drilling operation are fed as indicated by arrow 10 into an extracting means, in which oil is extracted (as indicated at 12A, 12B) by the application of a solvent, eg toluene or other suitable stabilised gasoline fraction.

The solids discharged from the extracting means at 16 will be wet, ie they will contain solvent and any water absorbed into the cuttings particles. These solids, possibly after second stage washing, are transferred to a dryer 18 where the solvent is evaporated (at 20) and the clean cuttings discharged at 22.

The fluid discharge 24 from the extracting means again consists of fine solids plus a solution of water in oil emulsion dissolved in the solvent. Fine solids may be removed at 26 by, for example, a decanting centrifuge. After optional fine solids removal, the fluid discharge 24 is transferred to an evaporator 32 where the solvent and water are evaporated from the oil. The oil is recovered at 34 as a usable product and the solvent plus water vapour 36, possibly combined with the solvent and water

vapour 20 from the dryer unit 18, is fed into a condenser/separator 38 where the vapours are condensed. Condensed water and solvents separate by gravity, the water being discharged at 40 and the solvent being re¬ cycled at 42 back to the extractor.

Two examples of the invention, practised in accordance with the methods of any one of Figures 3 to 8, are as follows:-

EXAMP E 1

In one series of tests oil contaminated drill cuttings of average composition 80.6% w/w solids, 10.0% w/w water, and

9.4% w/w oil were added at the rate of 6.2 tonnes per hour to an agitated vessel 12A into which was also added a

3 solvent at the rate 7.0 m per hour. The solvent used in this case was a stabilised gasoline fraction ex BP

Refinery, Grangemouth.

The average residence time of the cuttings in the vessel was 1 minute, and the temperature was 21°C.

The resultant solids/solvent slurry was fed at the rate of 10 m 3, via a positive displacement pump, into a solid bowl decanting centrifuge 12B. The centrifuge effected solids/liquid separation to yield a centrate (solvent) phase of average composition 88.3% w/w solvent, 10.4% w/w oil, and 1.1% w/w water. Solids phase exiting the centrifuge had an average composition of 84.9% w/w dry solids, 0.9% w/w oil, 9.2% w/w water, and 5.0% w/w solvent.

The oil dissolved in the solvent phase was recovered by

evaporation of the solvent which was subsequently condensed and recovered for re-use. The resultant oil phase, containing 5.1% w/w water and 1.0% w/w residual solvent was considered suitable for re-use in oil based mud preparations.

Solids exiting from the centrifuge were heated to effect evaporation of associated solvent, which was subsequently recovered by condensation. Solids discharged from the dryer unit had an average composition of 89.36% w/w dry solids, 0.95% w/w oil, 9.48% w/w water, and 0.21% w/w solvent.

EXAMPLE 2

The procedure as described in Example 1 above was repeated, using the same ratios and composition of oil contaminated drill cuttings and solvent, but with the addition of water at the rate of 500 litres per hour to the cuttings/solvent slurry immediately prior to centrifugation, as indicated at 12C in Figures 3 to 8.

The resultant centrate (solvent) phase from the centrifuge 12B had a composition of 86.5% w/w solvent, 9.7% w/w oil, and 3.8% w/w water. Water was allowed to separate by gravity in an intermediate tank and discharged to drain. Solvent was removed by evaporation and subsequently recondensed to yield a fluid of composition of 99.6% w/w solvent and 0.4% w/w water, which was recycled back to the process. Residual oil phase remaining in the evaporator had a composition of 97.0% w/w oil, 2.1% w/w water, and 0.9% w/w solvent, and was considered suitable for re-use in oil based mud preparations.

Solids phase exiting from the centrifuge had an average composition of 82.9% w/w solids, 14.8% w/w water, 0.8% w/w oil, and 1.5% w/w solvent indicating that the addition of water to the centrifuge feed slurry had resulted in a significant lowering of solvent carry through with solids. In this case it was considered that no further solvent removal by heating of the solids phase was necessary prior to discharge.