Abstract:
A system and method for removing particles from a well bore penetrating a possible hydrocarbon producing formation, according to which drilling fluid is mixed with a weighted material, and the mixture is introduced into a well bore so that the mixture scours any particles accumulated in the well bore. A well-completion fluid is introduced into the well bore that dissolves the weighted material.

Description:
BACKGROUND  
         [0001]    This invention relates to a system and method for using a weighted sweep material for removing particles from a well bore penetrating a possible hydrocarbon producing formation.  
           [0002]    A problem often encountered in well-drilling operations is the accumulation of drilling cuttings, especially on the low side of deviated, or horizontal, well bores. As these cuttings accumulate within the well bore, it is necessary to occasionally pump a volume of drilling fluid that has been weighted to a density higher than the active drilling fluid system in order to clean, or sweep, the well of the drilling cuttings. This “weighted sweep” material, which is often in the form of barite, or the like, is introduced into the well bore with the drilling fluid and scours the low side of the well bore to clean the well bore of cuttings. The weighted sweep material also provides extra buoyancy to mobilize and remove the cuttings from the well bore, and the cuttings are then separated from the weighted sweep material by shaker screens, or the like.  
           [0003]    Upon completion of the weighted sweep, the weighted sweep material must be removed from the drilling fluid and taken out of the well bore. However, it is virtually impossible to remove all of the weighted sweep material from the well bore. Thus, some of the material will remain in the well bore and become part of the well formation, which may cause several problems. For example, the presence of the non-removable, weighted sweep material could reduce the permeability of the rock being drilled and cause a distortion of the particle size distribution of the bridging materials in the drilling fluid. Also, the presence of the weighted sweep material makes it difficult, if not impossible, to eliminate some plugging of the permeable zones in the formation. Thus, the rock in the drilled pay zone could be damaged, and thus possibly impede the withdrawal of oil and gas.  
           [0004]    Therefore, what is needed is a system and method for removing the cuttings from the well bore without encountering the above problems. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0005]    The drawing is a diagrammatic view depicting the system of an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0006]    According to an embodiment of the invention, and with reference to the drawing, a deviated well bore  10  is formed below ground G and adjacent to an oil or gas reservoir  12 . A drill pipe  14  is disposed in the well bore  10  with the outer diameter of the drill pipe being less than the inner diameter of the well bore, or casing, to form an annulus  16 .  
         [0007]    It will be assumed that, as a result of the well-drilling operation, there is an accumulation of drilling cuttings  18  in the annulus  16 , especially on the lower side thereof as shown, which need to be removed for the reasons described above. To this end, a source  20  of drilling fluid, or mud, is pumped into the well bore via a conduit  22  and by a pump  24 . A source  26  of a weighted material, to be described in detail, is introduced, via a conduit  28 , into the conduit  22  for mixing with the drilling fluid upstream of the well bore  10 .  
         [0008]    The mixture of fluid and weighted material forms a “weighted sweep” material which is introduced into the annulus  16  at the inlet end of the well bore  10 . As the weighted sweep material passes through the annulus  16 , it scours the lower side of the well bore  10  to remove the drilling cuttings  18 . The mixture of the weighted sweep material and the cuttings is circulated back to the surface for separation and further conventional treatment. However, some of the weighted material remains in the annulus and must be removed for the reasons described above.  
         [0009]    One or more well-completion fluids are then introduced into, and flow through, the annulus for various well-completion procedures. These well-completion fluids are conventional and may include mineral acids, organic acids, chelating agents, and ammonium salt solutions.  
         [0010]    According to an embodiment of the invention, the weighted material is of a type that will be dissolved by each of the above well-completion fluids, which, after use, are circulated out of the well bore  10 . The weighted material preferably is a metal salt. More preferably, the weighted material is an oxide, hydroxide, carbonate, sulfate, phosphate, tungstate, fluoride, pyrophosphate or orthosilicate salt of an alkaline earth metal, a transition metal, especially a transition metal from Period 4 of the Periodic Table of Elements, a metal selected from Groups 13, 14 and 15 of the Periodic Table of Elements or a Lanthanide series rare earth metal. Specific examples of metal salts that are absorbable by the well completion fluids and thus are suitable for use as the weighted material include magnesium oxide, barium pyrophosphate, aluminum hydroxide, calcium fluoride, calcium tungstate, magnesium orthosilicate, iron oxide, iron tungstate, manganese oxide, manganese carbonate, manganese tungstate, manganese hydroxide, iron hydroxide, zinc oxide, zinc carbonate, zinc phosphate, zinc sulfate, lanthanum hydroxide, cerium hydroxide, lanthanum oxide, bismuth oxide, hydroxylapatite (hydrated calcium phosphate), anhydrite (calcium sulfate), dolomite (calcium magnesium carbonate), copper oxide, tin oxide, strontium carbonate, and strontium phosphate  
       EXAMPLE  
       [0011]    A weighted material, having a specific gravity of greater than 2.6, is passed from the source  26  into the conduit  22  where it mixes with drilling fluid from the source  20 , which drilling fluid can be in a mud-like form. This forms a weighted sweep material with a fluid weight of approximately 2 to 4 pounds per gallon greater than the drilling fluid in the annulus. The weighted sweep material is pumped into the annulus  16  by the pump  24 , and scours the lower side of the well bore  10  to remove the drilling cuttings  18 . The mixture of the weighted sweep material and the cuttings is then circulated back to the ground surface for separation, and/or further treatment and some of the weighted sweep material accumulates in the annulus  16 . Well-completion fluids are then circulated through the annulus  16  in connection with various conventional well-completion operations well known to those skilled in the art.  
         [0012]    Since the weighted sweep material is soluble in the well completion fluids mentioned above, when the latter fluids are circulated through the annulus  16  as described above, the weighted sweep material is dissolved and removed by the fluids. Thus, the annulus is void of the weighted material and the permeability of the rock drilled through during the drilling operation is not reduced, while plugging of the permeable zones in the formation is eliminated, thus eliminating the problems discussed above.  
       Variations  
       [0013]    It is understood that the above embodiment is not limited to the particular well bore described and shown herein, but is equally applicable to horizontal bores and vertical bores. Also, the embodiment is not limited to the use of one weighted material described above, but rather two or more weighted materials of the above type can be mixed before being introduced into the conduit  22 . Further, the expression “drilling fluid” is meant to cover fluids and muds. Moreover, the fluid that dissolves the weighted material does not have to be a “well-completion” fluid but can be any fluid that dissolves the weighted material. Still further, although the expression “well bore” has been used herein, it is understood that it is meant to cover casings, pipe, strings, conduits, or any other type of device capable of transferring fluids.  
         [0014]    Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many other modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures.