GB2289705A - Treating drill cuttings - Google Patents

Abstract

A process is provided for the separation of a low toxicity oil-based drilling mud from drill cuttings, as well as a means for measuring the final oil retention on the drill cuttings. The process utilizes an environmentally safe liquid ester as a wash fluid for the drill cuttings, in conjunction with a plurality of hydrocyclones to separate the washed drill cuttings from the drilling mud. The process further provides for the use of calcium carbonate as a densifying agent for the drilling mud to enhance the separation of the drilling mud from the drill cuttings.

Description

IMPROVED METHOD FOR REMOVING AND MEASURING OIL AND/OR OIL-BASED DRILLING MUD ADDITIVES FROM DRILL CUTTINGS FIELD OF THE INVENTION The present invention relates to a process for removing oil and/or other drilling mud additives from the drill cuttings generated from downhole oil and gas drilling operations.

BACKGROUND OF THE INVENTION In the drilling of oil and gas wells, drilling fluids or "muds" are used to provide wellbore lubrication, to cool the drill bit, protect against corrosion and to provide a pressure head to maintain formation integrity. There are two main types of drilling muds typically used in drilling operations; water-based and oil-based. Oil-based drilling muds are employed where it is desirable to drill at elevated temperatures, improve borehole stability, control shale sloughing, and control water wetting of the formation, such as in clay and some shale formations. Use of oil-based drilling muds is also desirable in "sour gas" wells where use of a water-based mud can cause hydrogen embrittlement of steels used in the drilling operation as a result of the water reacting with the formation sulfur compounds.Use of oil-based muds additionally inhibit corrosion and provide superior lubrication of the drill pipe along the wellbore when employed in directional drilling operations such as those frequently conducted from offshore platforms.

Drilling muds are typically circulated down the inside of a tubular drilling string, outwardly through the drill bit and up the annulus between the drill string and the wellbore.

The drilling muds serve to carry the drill cuttings away from the bit and out of the borehole, cool and lubricate the bit, and provide hydrostatic pressure to overcome the pore pressure of formations encountered. A typical oil-based drilling mud includes a diesel or mineral oil base, oil soluble emulsifiers, water (often salt water), oil wetting agents such as calcium sulfonates, and additives such as gilsonite and organophilic clays to control leak-off into the formation; with the density of the drilling mud adjusted through the use of weighting agents such as barite or hematite. These oil-based drilling muds are very stable oil external-water internal emulsions capable of holding solids, such as drill cuttings, in the oil phase with the use of wetting agents.The drill cuttings, therefore, tend to become oil wet, trapping large quantities of oil-based mud in their intergranular spaces. Because of the increasing environmental concerns regarding disposal of these oil coated drill cuttings, it is essential to have an economic means to remove the oil from the cuttings to allow disposal in an environmentally acceptable manner. This is particularly significant in offshore operations where it is desirable to discharge the solids overboard from the drilling rigs and avoid having to transport them to shore for disposal.

Several different methods for processing drill cuttings contaminated with oil-based mud have been suggested. For example, U.S. Patent Nos. 3,688,781; 3,693,733; 3,716,480; 4,175,039; 4,546,783; and 4,645,608 teach the use of aqueous solutions of detergents in an attempt to wash absorbed oil-based mud from the surface of the drill cuttings. These attempts have been largely unsuccessful since the oil wetting agents incorporated into the oil-based muds will resist the detergent action of the aqueous wash solutions.

Additionally, detergent-laden water, which may be even more toxic to marine organisms than the oil on the drill solids, is continuously discharged into the marine environment.

Solvent washing or extraction techniques to remove oil contamination from cuttings have been suggested in U.S.

Patent Nos. 4,040,866; 4,434,028; and 4,836,302. In particular, U.S. Patent No. 4,040,866 teaches the use of a mutual solvent, one soluble in both oil and water, to clean oily drill cuttings. In this process, oily liquid is removed from the solids with a mutual solvent, such as ethylene glycol monobutyl ether, with the remaining oil and solvent mixture which remains on the cuttings being washed away with water, wherein the water-washed cuttings are then centrifuged to recover the cuttings. This method has proven impractical since two undesirable process streams are created, with large quantities of solvent, having a volume approximately equal to that of the original oily liquid on the solids, being washed from the solids with water and discharged with the water into the environment.

Additionally, large volumes of mutual solvent become contaminated with dissolved oil and must either be discarded, or purified and recycled. The cost of mutual solvents prohibits simple disposal, and furthermore, the high boiling point and high latent heat of vaporization of the mutual solvents makes their separation from oil by distillation difficult, expensive and hazardous.

SUMMARY OF THE INVENTION Briefly, the present invention relates to a new method for the separation of an oleophylic substance and mud from mineral particles contaminated with the oleophylic substance and mud. More particularly, the present invention relates to a new method for the separation of low toxicity oil-based drilling mud from drill cuttings, and recovery of the drilling mud for reuse, resulting in the cuttings having a significantly reduced oil retention value. The invention employs a liquid ester or any synthetic or natural oil approved for environmental discharge as an agitation liquid to wash the cuttings discharge and displace the mud prior to being fed through a centrifugal process to further separate the drilling fluid and any associated oil from the cuttings.

In one presently preferred mode, the method of this invention includes the steps of transporting oil-based mud-laden cuttings to a feed tank wherein the oil-based cuttings are washed and subjected to a turbulent mixing with a liquid ester or synthetic oil to separate the drill-cutting solids from the drilling mud and oil contaminants. To enhance the mud-cutting separation, a calcium carbonate densified mud is used in lieu of the conventional barite mud. The washed cutting solids are then fed to a centrifuging stage to separate the cutting solids from the drilling mud and oil allowing reclamation of both.

The oil retention on the cutting solids is then measured to identify quantitatively the amount of discharge approved wash fluid, such as an ester, as well as quantitatively measuring the amounts of other oils remaining on the cutting solids. If additional cleaning is warranted, the separation steps can be repeated in the manner described above.

Drill-cutting solids, as used herein, includes all solids that may be separated from the drilling mud returned from a wellbore during drilling operations. Although the majority of these solids comprise the actual borehole material cut from the formation, other solids materials will also usually be present including additives which are conventionally used in formulating drilling muds. These additives may be present in finely ground form, as is usual with weighting agents, or in larger regular or irregular forms, as may be the case with fluid loss additives. The actual borehole material contained in the solids will generally comprise a wide spectrum of sizes ranging from extremely fine particles to relatively coarse particles, with the relative proportions varying extensively with the types of formations being drilled.

Through the use of the present invention, the low toxicity oil of the oil-based mud and other desirable fine particles are removed from the cuttings so that the cuttings have an oil retention value within safe limits to allow discharge to the sea environment, and the oil-based mud and desirable solids can be reused in the mud system for the oil well.

DESCRIPTION OF THE PREFERRED EMBODIMENT The apparatus employed in practicing the method of the preferred embodiment utilizes a low toxicity drilling mud, and is illustrated in the Figure. Typically, the drill cuttings circulated through the mud system on an offshore oil well drilling platform, and are covered by the low toxicity oil-based drilling mud on all external surfaces, and further have the low tox base oil trapped within the intergranular spaces or voids of the drill-cutting solids.

The method and apparatus illustrated in the Figure are provided for removing the oil-based drilling mud both from the surfaces and from the intergranular spaces of the drill-cutting solids.

With reference to the Figure, a slurry of low toxicity oil-based drilling mud and cutting solids enters the apparatus of this invention through line 10. The drill cuttings in this slurry typically may contain about 50% by volume of oil-based drilling mud absorbed within the intergranular spaces or voids of the drill-cutting solids.

The drill cuttings may have already been subjected to some type of settling tank to allow some part of the oil-based drilling mud to separate by gravity from the drill-cutting solids in order to reduce the amount by volume of oil on the drill-cutting solids as described. However, it is within the scope of this invention to utilize the method of this invention with oil-based drill-cutting solids which are taken directly out of the mud circulation system of the offshore drilling rig.

In a preferred mode, the slurry of cutting solids in suspension in the drilling mud are gravity fed via line 10 to shale shaker 15. One advantage of utilizing a gravity feed system in the present method is that any oil-based mud inadvertently spilled off the end of the shaker is captured and processed back to the mud system 20. As appreciated by those skilled in the art, shaker 15 is fitted with a series of course screens superimposed over a series of finer mesh screens.

The cuttings are next gravity fed into a trough or chute and directed to a solids feed tank 25. It is to be noted that for large cutting solids captured by the course screen shakers, the relatively small surface area of these cuttings as compared with their overall mass, may allow direct discharge into the sea environment since the oil on cuttings valve, expressed in grams of oil per kilogram of cuttings, may be within discharge limits. The feed tank 25 contains a wash fluid comprised of a water and liquid ester such as Petrofrees by Bariod Limited or discharged approved synthetic oil or natural oil combination, having a water/oil-ester ratio based on the type of cuttings processed.As appreciated by those skilled in the art, cuttings from younger geologic time tend to have more water associated with the rock and therefore do not require as much water in the wash fluid. For these cuttings a ratio of approximately 25/75 water to oil or ester is initially used, where for cuttings from an older geologic age a ratio of approximately 40/60 is warranted. Once in the solids tank the cuttings are mixed with the wash fluid using an agitator 30, which comprises a high speed paddle connected via a drive shaft to a motor positioned above the tank. As the agitator moves the wash fluid within the tank, the mud adhering to the cutting solids is subjected to dilution via osmosis and the stirring action and is replaced with the discharge approved wash fluid.If excessive fluids enter the solids feed tank 25, an overflow compartment 35 is provided within the solids feed tank to capture the fluid.

As appreciated by those skill in the art, additional alarms and pump cut-off switches, which are not shown, are provided to prevent accidental spillage of the cuttings-wash fluid combination within the feed tank. It is an important feature of the present invention that the drilling mud may also utilize calcium carbonate as a weighting agent. Unlike the conventional densifying agents, such as barite, the calcium carbonate mud cleans quite readily. However, a limitation in the calcium carbonate is that the final mud density achievable is +/- 13 pounds per gallon (ppg), which for most offshore applications, particularly in the North Sea, is well within operating limitations; particularly in developmental wells.

Located at the bottom of the solids feed tank 25 are drain lines 40 having isolation valves 45 interposed in-line. The wash fluid is transferred via the drain lines by pump 50 to either the active mud system 20, or directed on to a cyclone separation zone, where the centrifugal forces generated by centrifuges 55 separates the wash fluid and any remaining drilling mud from the cutting solids. While a pair of centrifuges are depicted in the Figure, it should be understood that multiple centrifuges arranged to operate in parallel or series may be used.Particulate solids now having a significantly reduced oil on cuttings value are discharged from the centrifuges via line 60 to spectrometer 65 to measure both the quantity of base oil from the mud system, as well as the quantity of dischargeable oil from the wash fluid which remain on the cuttings, while the separated wash fluid is returned to the holding tanks 70 via line 63 for recycling.

The washing and agitation step in feed tank 20 utilizes a liquid ester or environmentally safe synthetic oil, such as Petrofrees to remove the low toxicity oil-based mud from the cutting solids surface. In order to quantitatively measure the exact quantities of both the low tox base oil, which is subject to discharge regulations because of its impact on the sea environment, and the liquid ester, which is environmentally safe, an infrared absorption method is used.

The advantage of such an absorption technique over conventional retort analysis is that the response obtained is proportional to the concentration over the operational range, as per the Beer Lambert Law. Under this law the proportion of light absorbed by a solute in a transparent solid is independent of the intensity of the incident light, being proportional only to the number of absorbing molecules in the light path. This relationship can be expressed as follows:

where lo = intensity of incident light I = intensity of transmitted light X = molar absorptivity or extinction coefficient c = concentration 1 = path length A = absorbance From the above formula, it is apparent that absorbance is proportional to concentration (c).For low tox base oils, measured amounts can be determined by infrared absorption by measuring the intensity of the C-H stretching band situated in the region of 2900-3000 cm1. Likewise, the liquid ester can be measured by the intensity of its carbonyl (C=0) stretching frequency in the region of 1750 cm1. Since the liquid ester also has C-H stretching frequencies, it will reinforce any C-H signal due to base oil. In the preferred embodiment of the present method, a Fourier Transform infrared spectroscopy is used to identify quantitatively the amounts of ester and low toxicity base oil on the cutting solids, utilizing a Nicolet 205 FTIR spectrometer.

The cuttings, after being measured for residual amounts of base oil and dischargeable oil, are either processed back through the wash and centrifuge operations via line 67, if the retention value is too high to allow environmentally safe discharge, or are transferred for disposal if the oil on cuttings valve is deemed safe. The slurry, cleaned of cutting solids from the hydrocyclones, is returned to holding tank 70 via drain line 75. Again, overflows 80 are provided within the holding tanks to aid in fluid transfer and prevent accidental spillage. As the liquid volume within the holding tanks is increased, the wash fluid is transferred via line 85 to the solids feed tank 25. It is to be noted that since any spilled mud initially processed through shale shaker 15 is captured and directed to the mud system 20, and since the separated wash fluid from the processed slurry is transferred via line 85 to the solids feed tank, which in turn directs the unprocessed slurry either to the centrifuges 55 or to the mud system 20 via line 90, the present system can be seen as a closed loop which allows the liquid ester/water ratio of the wash fluid to remain relatively constant.

Thus having been described in detail, it is understood that the foregoing description of the present invention is not intended to be limiting, but only exemplary of the inventive features which are defined in the claims.

Claims (9)

1. A process for cleaning drilling mud containing a oleophylic substance from drill cutting solids in order to prepare the cuttings for environmentally acceptable disposal, said process comprising: collecting drill cutting solids which are contaminated with said drilling mud; contacting the contaminated drill cutting solids with a wash fluid further comprising an environmentally safe processing fluid and water combination to form a slurry; mixing the slurry to remove a substantial portion of the oleophylic substance from a surface of said cutting solids; introducing the resultant slurry to a cyclone separation zone to segregate the drill cutting solids from the slurry to provide drill cutting solids which are substantially free- of oleophylic contamination; recovering the processed slurry, having the cutting solids removed, for further handling;; measuring a respective quantity of oleophylic substance and environmentally safe processing fluid remaining on the surface of said drill cutting solids; and recycling those measured drill cutting solids which require further processing with said wash fluid and said hydrocyclones.

2. A process according to claim 1, wherein the oleophylic substance comprises a low toxicity oil and the processing fluid comprises a liquid ester.

3. A process according to claim 1 or 2, wherein the cyclone separation zone comprises a plurality of hydrocyclones.

4. A process according to claim 1, 2, or 3, wherein the drilling mud utilizes a calcium carbonate densifying agent.

5. A process according to claim 4, wherein the measure of the respective oleophylic substance and processing fluid components is accomplished with an infrared absorption method.

6. A process according to claim 5, wherein the infrared absorption method further comprises the measuring of the intensity of a C-H stretching band for the oleophylic substance and measuring the intensity of a carbonyl for the processing fluid.

7. A process according to claim 6, wherein the intensity of the C-H band is measured in the region of about 3000 cm1, and the carbonyl is measured in the region of about 1750 cm1.

8. A process according to claim 7, wherein the measurements are conducted utilizing a FTIR spectrometer.

9. A process for cleaning drilling mud containing a oleophylic substance from drill cutting solids in order to prepare the cuttings for environmentally acceptable disposal, substantially as hereinbefore described, with reference to the accompanying drawing.