Abstract:
The invention provides a method for treating drill cuttings, preferably marine cuttings, preferably in situ, so that the cuttings can be discharged into the environment, preferably back into marine waters without causing oxygen depletion of marine sediment. In a preferred embodiment, the treatment emulsifies and then encapsulates free hydrocarbons in the marine cuttings.

Description:
[0001]    The present application is a continuation-in-part of pending U.S. application Ser. No. 09/691,589, filed Oct. 18, 2000, which is a continuation-in-part of pending U.S. application Ser. No. 09/426,172, filed Oct. 22, 1999. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to an emulsifier composition for treating marine cuttings preferably drilled with invert emulsion drilling fluids to minimize the environmental impact of their discharge into the sea. The treated cuttings and associated hydrocarbons will disperse in the marine environment, eliminating the possibility of organic enrichment.  
         BACKGROUND OF THE INVENTION  
         [0003]    During the drilling of oil and/or gas wells, a drill bit at the end of a rotating drill string, or at the end of a drill motor, is used to penetrate through geologic formations. During this operation, drilling mud is circulated through the drill string, out of the bit, and returned to the surface via the annular space between the drill pipe and the formation. Among other functions, the drilling mud provides a washing action to remove the formation cuttings from the wellbore. The mud returns to the surface along with entrained drill cuttings and typically flows through “shale shakers,” desanders, desilters, hydrocyclones, centrifuges, and/or other known devices to separate the cuttings from the mud. The shale shaker(s), which typically sit above the mud storage area, essentially are screens that are used to separate the drill cuttings from the drilling mud. The drilling mud falls through the screens by gravity and the cuttings pass over the end of the screens.  
           [0004]    Where drilling is offshore, the disposal of the drill cuttings after separation from the drilling mud can present a problem. The most economical way to dispose of the cuttings would be to simply discharge the cuttings back into the surrounding water. However, the cuttings may contain environmentally damaging “free hydrocarbons,” defined herein as hydrocarbons derived either from the drilling mud, from the formation, or both. The potential for environmental contamination could be alleviated by transporting the cuttings to a disposal facility onshore; however, this would increase the cost of the drilling operation considerably, and would not necessarily improve the environmental performance of the drilling operation.  
           [0005]    A typical approach to resolve the problem has been to minimize the toxicity of the base fluids used to make drilling muds, and more recently, to use base fluids which are more biodegradable. Unfortunately, this approach fails to prevent one type of damage that free hydrocarbons can inflict on a marine environment.  
           [0006]    Free hydrocarbons are known to organically enrich marine sediment, which eventually causes oxygen depletion and destruction of the environment surrounding the depleted sediment. As with any other organic matter, hydrocarbons tend to break down or decompose in the presence of oxygen, forming carbon dioxide and water. Oxygen is a limiting resource for this reaction. Marine sediment typically has an oxygen content of only from about 2 to about 8 mg per liter of marine sediment. When drill cuttings containing a high concentration of hydrocarbons are discharged into marine waters and reach the sea floor, the oxygen available in the marine sediment rapidly is used to decompose the hydrocarbons. The resulting oxygen depletion very rapidly causes the marine sediment to become anoxic, creating an environment in which most benthic organisms cannot exist.  
           [0007]    The potential for environmental damage could be reduced by treating the cuttings in situ before discharging the cuttings into marine waters. Methods are need for treating marine cuttings, preferably in situ, to reduce the quantity of hydrocarbons that will be accessible upon discharge to organically enrich marine sediment.  
         SUMMARY OF THE INVENTION  
         [0008]    A method for treating cuttings from an offshore rig comprising:  
           [0009]    providing cuttings produced during drilling of a marine wellbore, said cuttings comprising free hydrocarbons; and,  
           [0010]    treating said cuttings in situ to produce a converted cutting mixture in which said free hydrocarbons are unavailable to induce oxygen depletion of said marine sediment, wherein said treating also changes wettability of said cuttings from oil wettable to water wettable; and,  
           [0011]    discharging said converted cutting mixture into marine waters. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    According to the present invention, marine cuttings are treated, preferably in situ, with an emulsifier composition to minimize their environmental impact upon discharge. The treatment forms a cutting mixture which will not result in oxygen depletion of marine sediment. In a preferred method, free hydrocarbons in the cuttings are converted into “isolated hydrocarbons,” defined herein as hydrocarbons which are unavailable to organically enrich surrounding marine sediment in an amount sufficient to induce oxygen depletion of the marine sediment. For purposes of the present application, the term “oxygen depletion” is defined to mean depletion of oxygen in marine sediment to a level below that required to sustain a typical community of benthic aerobic organisms. Without limiting the invention, typical healthy marine sediments are believed to have an oxygen content of from about 2 mg O 2 /liter to about 8 mg O 2 /liter of sediment.  
         [0013]    Isolated hydrocarbons may be formed in a number of ways, including but not necessarily limited to encapsulation of the free hydrocarbons with a suitable encapsulating material. In a preferred embodiment, isolated hydrocarbons are produced by encapsulating free hydrocarbons on cuttings with an encapsulating material which renders the hydrocarbons wholly or partially inaccessible to biological degradation for a prolonged period of time. In a preferred embodiment, hydrocarbons in the drilling mud are non-toxic and biodegradable, and the encapsulating material allows some release of the hydrocarbons into the surrounding seawater at a rate which is sufficiently low as to allow the microorganisms in the surrounding environment to degrade the hydrocarbons without oxygen depletion of the marine sediment.  
         [0014]    Hydrocarbons released into the seawater are called “leachate.” The quantity of leachate released over a given period of time is defined as a percentage of the total quantity of “oil on cuttings,” or free hydrocarbons. In the laboratory, the isolated hydrocarbons are tested for leachate by placing them in actual or synthetic seawater and measuring the amount of “leachate” over a period of about 150 days. Preferably, isolated hydrocarbons, according to the present invention, permit leachate of 0.5% or less of free hydrocarbons, more preferably about 0.25% or less of free hydrocarbons, and most preferably about 0.05% or less of free hydrocarbons.  
         [0015]    The drilled cuttings may be treated using any suitable system of equipment. After separation from the drilling mud, the contaminated cuttings typically pass through a holding bin into an inlet hopper. The cuttings preferably are treated directly in a batch mixer equipped with an appropriate inlet for the relevant solutions and an apparatus for low shear mixing, such as a paddle mixer.  
         [0016]    In a preferred embodiment, the cuttings are sprayed with an emulsifying solution effective to transform the free hydrocarbons in the cuttings into an emulsion. The emulsion thereafter is treated with an encapsulating material to encapsulate the emulsified hydrocarbons, and the mixture of drill cuttings and encapsulated free hydrocarbons is released into marine waters where it disperses.  
         [0017]    The composition of the emulsifying solution may vary depending upon the type of free hydrocarbons found in the drilling mud. The following emulsifiers have superior (a) environmental compatibility, and (b) produce a very stable emulsion. The emulsifying solution may be a blend of organic acids, inorganic acids, and emulsifiers. The emulsifier(s) may have any ionic nature, including non-ionic, anionic, and cationic. Preferred emulsifying solutions are as non-toxic as possible, and preferably are either non-ionic or a non-ionic/anionic blend (where the drilling mud comprises paraffins) or, a combination of at least a non-ionic surfactant and most preferably a non-ionic and an anionic emulsifier (where the drilling system does not comprise paraffins). Although the compounds called “emulsifiers” herein typically are referred to as surfactants, their function in the present invention is to act as emulsifiers. The emulsifying solution lowers the interfacial tension between the oil and water to produce a sufficiently small droplet size, from about 3 microns to about 20 microns, preferably about 10 microns or less in diameter.  
         [0018]    Preferred emulsifying solutions comprise a sufficient amount of a relatively strong acid to lower the pH to of the solution to at least about 4, preferably to at least about 2 to about 3, and most preferably to about 1. Relatively strong acids include, but are not necessarily limited to phosphoric acid, hydrochloric acid, sulfuric acid, nitric acid, and the like. A preferred acid is phosphoric acid. The emulsifying solution preferably comprises from about 15 wt % to about 45 wt %, preferably about 20 wt % phosphoric acid; about 5 wt % to about 90 wt %, preferably about 65 wt % emulsifiers; and water.  
         [0019]    In order to achieve the desired small droplet size, it is necessary to use emulsifiers with the correct hydrophilic/lipophilic balance (HLB). The required HLB will differ depending upon the oil being emulsified. Preferred non-ionic emulsifiers include, but are not necessarily limited to linear or branched polyoxyethylene alcohols, more preferably linear polyoxyethylene alcohols, comprising (a) from about 8 to about 30, preferably about 8 to about 20 carbon atoms, and (b) comprising about 3 to about 50 moles, most preferably about 3 to about 20 moles ethylene oxide. Most preferred non-ionic emulsifiers are linear polyoxyethylene alcohols having from about 13 to about 15 carbon atoms and comprising about 10 moles ethylene oxide. The following are preferred HLB&#39;s for non-ionic emulsifiers when the drilling mud contains the following oils: polyalphaolefins and paraffins-HLB 12.5; esters-HLB-15.4; synthetic iso-paraffins-HLB 10.9.  
         [0020]    Blends of both non-ionic and anionic emulsifiers have been found to decrease droplet size in most instances. Where such a blend is used, a preferred ratio of non-ionic to anionic emulsifier is about 5/95 to about 95/5, preferably about 50/50 to about 85/15. Any suitable, non-toxic anionic emulsifier may be used in such blends. Preferred anionic emulsifiers include, but are not necessarily limited to those selected from the group consisting of: alkane sulfates, alkane sulfonates, and phosphate esters comprising about 8 to about 18 carbon atoms, preferably about 8 to about 12 carbon atoms.  
         [0021]    The following are preferred emulsifying blends for use with the specified type of drilling muds. The drilling muds indicated by brand name are available from Baker Hughes INTEQ, and the brand name represents a proprietary trademark of Baker Hughes INTEQ:  
         [0022]    A Most Preferred Emulsifying Blend for Use with a Drilling Mud Comprising Isomerized Olefins (SYN-TEQ) (Blend of Emulsifiers with HLB 12.5)  
                                       Secondary alkanesulfonate of sodium or Sodium octyl sulfate   26 wt %       C13/C15 linear alcohol ethoxylate with 10 moles of ethylene   39 wt %       oxide       Water + Phosphoric acid (at 75%)   35 wt %       Ratio of (linear alcohol ethoxylate with 10 moles of EO) to       (secondary alkanesulfonate of sodium or Sodium Octyl       Sulfate) = 60:40       Ratio of active emulsifier to phosphoric acid = 3:23                  
 
         [0023]    For Use with a Drilling Mud Comprising Isomerized Olefins (SYN-TEQ) (Blend of Emulsifiers with HLB 12.5)  
                                           Secondary alkanesulfonate of sodium or Sodium octyl   9.75   wt %       sulfate       Isodecyl alcohol ethoxylate with 6 moles of ethylene oxide   55.25   wt %       Water + Phosphoric acid (at 75%)   35   wt %       Ratio of (Isodecyl alcohol ethoxylate with 6 moles of EO)       to (secondary alkanesulfonate of sodium or Sodium Octyl       Sulfate) = 85:15       Ratio of active emulsifier to phosphoric acid = 3:23                  
 
         [0024]    For Use with an Ester-Containing Drilling Mud (Blend of Emulsifiers with HLB 15.4)  
                                           Sodium Octyl Sulfate   6.50   wt %       Oleyl alcohol ethoxylate with 20 moles of ethylene oxide   58.50   wt %       Water + Phosphoric acid (at 75%)   35   wt %       Ratio of (Oleyl alcohol ethoxylate with 20 moles of EO) to       Sodium octyl sulfate = 90:10                  
 
         [0025]    For Use with a Paraffin-Containing Mud (PARA-TEQ) ((Emulsifier with HLB 12.5)  
                                           Isodecyl alcohol ethoxylate with 6 moles of ethylene oxide   55.25   wt %       Secondary alkanesulfonate of sodium or sodium octyl   9.75   wt %       sulfate       Water + Phosphoric acid (at 75%)   35   wt %                  
 
         [0026]    For Use with a Synthetic Isoparaffin-Containing Mud (Blend of Emulsifiers with HLB 10.9)  
                                           Isotridecyl ethoxylate with 3 moles of ethylene oxide   32.5   wt %       (HLB 8)       Isotridecyl ethoxylate with 10 moles of ethylene oxide   32.5   wt %       (HLB 13.8)       Water + Phosphoric acid (at 75%)   35   wt %       Ratio of Isotridecyl ethoxylate with 3 moles of EO/Isotri-       decyl ethoxylate with 10 moles of EO = 50/50                  
 
         [0027]    An excess of the emulsifier solution is added to the cuttings, preferably in the inlet hopper. The amount of emulsifier added will depend upon the concentration of free hydrocarbons in the cuttings as measured by any suitable means, such as “retort,” or distillation and measurement of the oil content. After addition of the emulsifying solution, the wt/wt ratio of emulsifying blend in the cuttings should be about 0.2 wt % to about 5 wt % for cuttings contaminated with from about 2 wt % to about 18 wt % free hydrocarbons, respectively. The cuttings and emulsifying solution may be agitated so that substantially all of the free hydrocarbons are removed from the cuttings and emulsified or dispersed in the emulsifier solution. Thereafter, the encapsulating material is added.  
         [0028]    The encapsulating material may be substantially any encapsulating material that surrounds the emulsified hydrocarbon droplets and solidifies. Suitable encapsulating materials include, but are not necessarily limited to silicates and reactive microencapsulating materials. A preferred encapsulating material is a silicate solution.  
         [0029]    A preferred silicate solution for forming the encapsulating material has the following composition:  
                                                                         Potassium or Sodium Silicate   33-58   wt %           Waterglass solution   0.01 to 2.0   wt %           Aluminum Trihydrate   0.01 to 2.0   wt %           Titanium   0.01 to 2.0   wt %           Glycol   1.0 to 4.0   wt %                Water   Balance                      
 
         [0030]    The amount of silicate solution that is added to the emulsified solution preferably is about 1 to about 2 times the amount of emulsifying solution added.  
         [0031]    The emulsifier rapidly and substantially completely disperses the free hydrocarbons in the cuttings into small droplets. Where the encapsulating material is silicate, the application of the silicate solution to the emulsified oil converts the emulsified oil into a thick gel, which can be water-washed off of the cuttings, leaving a substantially clean surface. When allowed to dry, the gel is even more amenable to subsequent removal by water-washing. Although the emulsified solution has a relatively low pH, of about 4 or less, preferably from about 2 to about 3, and most preferably about 1, the final product has a pH of from about 6 to about 7, preferably about 7.  
         [0032]    Suitable reactive microencapsulating materials include, but are not necessarily limited to those materials that comprise a polymerizable unsaturated carbon—carbon bond, preferably a vinyl group. An example is methyl methacrylate (MMA). The MMA monomer is added to the cuttings with a suitable emulsifier solution a suitable initiator is added. Suitable emulsifier solutions comprise a salt of an alkyl sulfate, preferably a sodium alkyl sulfate. Preferred emulsifier packages include, but are not necessarily limited to the emulsifier packages given above for use with SYN-TEQ and PARA-TEQ. Suitable initiators include, but are not necessarily limited to lauryl peroxide, dicetylperoxydicarbonate, and 2,2[asobis(2-amidinopropane)hydrochloride.  
         [0033]    While feeding the monomer to the system, adequate stirring is required to prevent a free monomer layer from forming. The temperature preferably is increased to from about 60° C. to about 80° C.  
         [0034]    Because the emulsifier removes hydrocarbons (hydrophobic materials) from the cuttings and because the emulsifying solution is very hydrophilic, the wettability of the cuttings is changed from oil wettable to water wettable. The more hydrophilic cuttings have less tendency to agglomerate, and tend to more widely disperse, both in the seawater as they travel toward the ocean floor, and eventually in the marine sediment.  
         [0035]    The combination of (a) encapsulation of free hydrocarbons from the cuttings (which decreases accessibility to the hydrocarbons over time), and (b) change in the wettability of the cuttings from oil wet to water wet (which results in greater spatial dispersion of the hydrocarbons) greatly minimizes the organic load on the marine sediment and helps to prevent oxygen depletion.  
         [0036]    Persons of skill in the art will appreciate that many modifications may be made to the embodiments described herein without departing from the spirit of the present invention. Accordingly, the embodiments described herein are illustrative only and are not intended to limit the scope of the present invention.