Abstract:
A method for heating/removing and/or preventing solids in onshore or subsea pipe or wellbore tubular includes pumping a solution or suspension containing carbon nanotubes (CNT) and/or CNT based derivatives into the pipe or wellbore to a position proximate the solids and applying electromagnetic energy or electric current at one or more selected frequencies to the pipe proximate the solution. The pipe or wellbore tubular itself may be pre-coated with carbon nanotube containing material and exposed to radio frequency energy, microwaves, and electric current upon formation of solids therein.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Priority is claimed from U.S. Provisional Application No. 61/679,823 filed on Aug. 6, 2012 and U.S. Provisional Application No. 61/705,357 filed on Sep. 25, 2012 both of which applications are incorporated herein by reference for all purposes. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND 
       [0003]    This disclosure relates generally to the field of fusible solids removal from pipes used to produce hydrocarbons from subsurface reservoirs. More specifically, the invention relates to compositions of materials and methods of activating such materials to facilitate removal of heat sensitive solids containing hydrates, wax, paraffins, asphaltenes etc 
         [0004]    Thermodynamic conditions favoring solids formation, e.g., hydrate formation, are often found in pipelines. This is highly undesirable because the clathrate crystals might agglomerate and plug the pipeline or flowline and cause flow assurance failure and damage valves and instrumentation. The results can range from flow reduction to equipment damage. Hydrates may be formed in deepwater applications specifically in a tieback (a line that connects a producing well having a sea floor disposed outlet to a central collection and/or processing facility). Currently, to remove a hydrate plug, pressure reductions/pumping methanol is attempted to dissolve the plug, however such method has only shown limited success. Other known techniques include heating a solution of salts on the surface and pumping them down the line, but in long tiebacks the solutions cools too quickly and cannot be reheated. There is currently no known convenient way to heat up such lines and heat/melt/remove the hydrates once a hydrate plug is formed. 
         [0005]    Similarly, solids such as asphaltenes, waxes and other paraffins may deposit within tubular components of wellbores during production of hydrocarbons from subsurface reservoirs. Such solids may also be deposited in pipelines if temperature and pressure conditions favor such deposition. 
         [0006]    What is needed is a method and system to enable relatively easy removal of solids if and when formed in such subsea or buried onshore lines or wellbores 
       SUMMARY 
       [0007]    One aspect is a method for heating and/or removing and/or preventing fusible solids in a subsea pipe or wellbore which includes pumping a solution or suspension containing carbon nanotubes into the pipe or wellbore to a position proximate the solids and applying electromagnetic energy or electric current at a selected frequency to the pipe or wellbore proximate the solution. The pipe or wellbore tubular itself may be pre-coated or manufactured with carbon nanotubes (CNT) and CNT based derivatives containing material and exposed to radio frequency electromagnetic energy upon formation of solids therein. 
         [0008]    Other aspects and advantages will be apparent from the description and claims which follow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows an example electromagnetic energy generating instrument in a wellbore. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    In one example, carbon nanotubes (CNT) and/or CNT based derivatives may be introduced into solutions or liquid suspensions, pumped into a pipe having hydrates and then heating the CNT and/or CNT based derivatives (“collectively “CNT”) in the solution or suspension as needed when the CNT solution or suspension comes into contact with hydrates or surfaces where hydrates adhere, for example, in deepwater flow lines or tiebacks. In some examples, a concentration of the carbon nanotubes and/or CNT based derivatives in the solution or suspension may be in the range of 10 milligrams per liter to 90% by weight. 
         [0011]    The solution/suspension of CNT may be pumped into a line with a remotely operated vehicle (ROV) or pre engineered subsea system and heated remotely by applying selected frequency or frequencies electromagnetic energy to the part of the line having the hydrates formed therein. Example solutions may include radio frequency (RF) absorption enhancers (e.g., CNT) added to a salt water solution such as sea water, solutions containing salt water, and salt water mixtures prior to applying electromagnetic energy to enhance the effects of the electromagnetic energy on the salt water, e.g., enhanced heating. The absorption enhancers may be particles made from, for example, RF absorbing materials that absorb one or more frequencies of an electromagnetic signal substantially more than other materials, e.g., the CNT. This may permit the electromagnetic signal to heat salt water (or any solution containing salt water or salt water mixture) containing electromagnetic energy absorbing enhancers, e.g., the CNT, substantially more than it would salt water (or salt water solution or salt water mixture) that does not contain additional electromagnetic energy absorption enhancers. 
         [0012]    The electromagnetic energy may be applied, for example, as a 13.56 MHz RF signal, which is expected to be effective to heat RF absorbing carbon molecules and compounds. RF absorption enhancers using these RF absorbing particles are also expected to be effective at slightly higher frequencies, such as those having a frequency on the order of the second or third harmonics of 13.56 MHz. The electromagnetic energy may be applied by having suitable equipment on an ROV, or as will be shown with reference to  FIG. 1 , on a wireline conveyed instrument within a wellbore casing. The selected frequency is not limited to the foregoing example of 13.56 MHz and may extend into the microwave range, e.g., several GHz or more. 
         [0013]    In test experiments, a 250 mg/L suspension of carbon nanotubes in water got as hot as 45° C. within 25 seconds when treated with RF electromagnetic energy. 
         [0014]    In other examples, coatings to be applied to the pipe surfaces or added directly to the surface of the metals which form the pipe, CNT could be included in the coatings or bonded to the materials&#39; surface and heated in the same manner. In these cases electric current application may be the more efficient source of EM energy. 
         [0015]    In yet other examples, solids consisting of asphaltenes, waxes and/or other paraffins may be deposited in pipelines or wellbore tubular (e.g., casing or production tubing) as a result of producing hydrocarbons from subsurface reservoirs depending on pressure and temperature conditions within the wellbore tubular. The above described method may be used to equal effect on fusing and removal of such solids from wellbore tubulars. 
         [0016]      FIG. 1  shows an example electromagnetic energy generating instrument disposed in a wellbore to perform example procedures such as those described hereinabove. The instrument is illustrated generally at  100  during the operation of the instrument  100  in a wellbore  125  drilled through subsurface formations  126 . 
         [0017]    The instrument  100  may be connected to a wire line  101  which is stored on a wire line truck  102  used to reel in and reel out the wire line  101  as is known. The instrument  100  may be initially positioned within a lubricator  103  on the top of a wellhead  104  and the instrument  100  is lowered on the wire line  101  to the position of interest within a well casing  110 . The wire line truck  102  has an associated generator  111  which is connected to a power control unit (PCU)  112  which provides the necessary power to the wire line truck  102  and which, in turn, provides the proper power to the wire line  101  and to the instrument  100 . The well casing  110  may include perforations  131  proximate a producing formation and a plug  130  at the bottom thereof. 
         [0018]    The instrument  100  may be disposed in a pressure resistant housing  132 , and circuits shown generally at  114  may generate electromagnetic energy at the desired frequency to heat the CNTs. Electromagnetic energy may be radiated by transmitter coils  114 A in the instrument  100 . In other examples, the coils  114 A may be substituted by electrodes (also shown at  114 ) so that electric current at a selected frequency may be passed through the CNTs to heat them in a manner similar to imparting electromagnetic energy. 
         [0019]    A pump P may be used to pump mixtures containing CNTs as explained above into the wellbore casing  110  so that areas therein requiring heating may be heated according to the example methods described above. 
         [0020]    The example shown in  FIG. 1  is only one possible method for conveying an electromagnetic energy generating instrument to places in a pipe or wellbore requiring heating according to the techniques described above. Accordingly, the present disclosure is not limited to wireline conveyance within wellbores, but may extend to any form of conveyance and to any pipe or conduit that may require heating as explained herein. 
         [0021]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.