Abstract:
A process for metal plating the perforation tunnels in the cement sheath between casing and wellbore in a subterranean well to protect the cement from CO 2  attack.

Description:
RELEVANT APPLICATION 
     Application Serial No. 065,171, filed June 16, 1987, now U.S. Pat. No. 4,718,492, by the same inventor, which is a continuation of application Serial No. 827,922, filed Feb. 10, 1986, now abandoned, by the same inventor, is relevant to this application. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a process for passing carbon dioxide through a cased, cemented and perforated well. Carbon dioxide in the presence of moisture leads to carbonic acid attack of oil well cement. This attack, if allowed to proceed unchecked eventually weakens the cement to an unacceptable condition. The major constituent of set cement, hydrated calcium silicate, breaks down by the action of CO 2  as follows: 
     
         3CaO.2SiO.sub.2.3H.sub.2 O+3CO.sub.2 →3CaCO.sub.3 +2SiO.sub.2 +3H.sub.2 O. 
    
     The reaction products calcium carbonate and silica possess less binding power than hydrated calcium silicate. Therefore, a considerable deterioration in cement strength takes place by CO 2  attack. 
     Cement is exposed to CO 2  in wells for injection of CO 2  and in those used for the production of reservoir fluid which, at a certain stage of a CO 2  -flooding project, contains CO 2  as well. Also in source wells, used for the supply of CO 2 , there is exposed cement. 
     The only place of exposure of CO 2  in injection, production and source wells is at the perforations, where the perforation tunnels traverse the cement sheath between casing and borehole. To maintain the integrity of injection, production, and source wells, the exposed cement should be shielded from CO 2  attack. This can be achieved by providing the exposed cement with an impermeable layer of inert material which protects the cement. It has now been found that nickel is very suitable for this purpose. 
     Electroless metal plating has been used for consoliation of loose or incompetent subsurface formations. U.S. Pat. No. 3,393,737 describes an electroless metal plating technique for consolidating loose formations, and discloses that this technique is superior to resin consolidation techniques. U.S. Pat. No. 3,685,582 describes an electroless metal plating technique for consolidating loose formations which can be advantageously used in high temperature formations (250° to 400° F.). 
     SUMMARY OF THE INVENTION 
     The invention concerns a process for electroless metal plating of perforation tunnels in the cement sheath between casing and borehole in a subterranean well. The metal plating process advantageously utilizes a preflush to prepare the surface, an activator to initiate the plating reaction, a spacer, and the plating solution, followed by a final spacer. The metal plating serves to protect the cement from CO 2  attack, when CO 2  is injected into the formation, or produced from it. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The protective layer of nickel is suitably installed as described hereinafter. After cementing the casing string in a CO 2  production or injection well, the casing is perforated, suitably in brine, and the perforations are advantageously washed clean. The nickel plating technique is then carried out, preferably using the following steps: 
     1. If the perforated interval is less than 3 m long, a retrievable packer with a tail pipe is preferably run on tubing and suspended, preferably 0.5-1.5 m above the highest perforation. In the perforated interval is longer than 3 m, a retrievable straddle packer is preferably placed across the lowest 3 m of the interval. As an alternative, a through-tubing expandable/collapsible straddle packer can be run on coiled tubing as a selective placement tool. The bottom end of the tail pipe should reach to below the lowest perforations with the packer in the set position. 
     2. The packer is set and the formation injectivity tested with a temperature recording device opposite the perforations. Based on this information, the appropriate spacer solution is selected. 
     3. The packer is released and the preflushes, activator solution, and enough spacers are injected to fill the tubing and the annulus covering the perforations. 
     4. The packer is reset and the remainder of the spacers injected, followed by the plating solution and the final spacer. 
     5. The tubing and two pore volumes are displaced with brine. 
     6. The packer is released and pulled. 
     7. Where the interval is longer than 3 m, the straddle packer is repositioned to a maximum of 3 m above the previous treatment position and steps 2. through 6. are repeated. 
     8. Step 7. is repeated as many times as required to treat the whole interval. 
     The preflushes serve to remove oil from the formation, and they preferably contain a slug of diesel oil followed by the same volume of isopropyl alcohol. 
     The activator solution preferably contains colloidal palladium. The preferred chemical composition is shown in Table 1. 
     
                       TABLE l______________________________________Preferred Chemical Composition of Buffered Activator Solution              Quantity per m.sup.3Component          of Solution______________________________________Water              968.97 literGum Arabic          0.13 kilogramHydrazine Hydrate (85%)              1.61 literPalladium Chloride Solution              4.00 literFormic Acid (90%)  4.03 literSodium Formate     19.97 kilogram______________________________________Remarks(i)     Chemicals are added to the water in the order listed   with complete mixing and dissolving before adding   the next chemical.(ii)    One liter of PdCl.sub.2 solution contains: 16 g PdCl.sub.2,   100 ml HCl (38%), and 900 ml water.(iii)   This solution contains 64 grams PdCl.sub.2 per m.sup.3 of acti-   vator.______________________________________ 
    
     The spacer contains a dilute palladium solution, followed by a plating nickel solution. 
     The plating solution preferably contains a nickel salt and a reducing agent. One plating solution is used for low temperature application (15°-50° C.), and one for intermediate temperature application (40°-85° C.). The preferred chemical compositions are presented in Tables 2 and 3, respectively. 
     
                       TABLE 2______________________________________Preferred Chemical Composition of Plating SolutionFor Use at Low Temperatures            Quantity per m.sup.3Component        of Solution______________________________________Water            854.6 literNiCl.sub.2.6H.sub.2 O            37.88 kilogramNaH.sub.2 PO.sub.2.H.sub.2 O            45.49 kilogramNH.sub.4 Cl      62.46 kilogram30% ammonia solution             52.13 literNa-Saccharide 2H.sub.2 O             0-13 kilogram______________________________________Remarks(i)    Quantity of Na-Saccharide 2H.sub.2 O depends on in-  jection rate and temperature.______________________________________ 
    
     
                       TABLE 3______________________________________Preferred Chemical Composition of Plating SolutionFor Use At High Temperatures            Quantity per m.sup.3Component        of Solution______________________________________NiSO.sub.4.6H.sub.2 O             84.14 kilogramNaH.sub.2 PO.sub.2.H.sub.2 O            120.36 kilogramSuccinic Acid     13.12 kilogramNaOH (30% solution)            22.31 literSodium Formate    44.98 kilogramFormic Acid (90%)            25 to 175 literWater            866.98-(25 to 175)            liter______________________________________Remarks(i)    Range of Formic Acid depends on injection rate,  temperature, and specific surface of medium treated.(ii)   Quantity of water added is 866.98 liters less the  amount of formic acid added.______________________________________ 
    
     The final spacer advantageously is an ammoniacal buffer solution. 
     The following fluid volumes (this includes the perforation volumes) are preferably applied: 
     
         ______________________________________Preflush (i)      4 to 6 pore volumesPreflush (ii)     4 to 6 pore volumesActivator solution             45 to 55 pore volumesSpacer (i)        4 to 6 pore volumesSpacer (ii)       4 to 6 pore volumesPlating solution  90 to 110 pore volumesFinal spacer      4 to 6 pore volumes______________________________________ 
    
     The nickel plating technique is dynamic, i.e., precipitation of nickel takes place while the plating solution passes through the perforations. The amount of nickel plated out per cm 2  of cement depends on the fluid flow rate through the perforations and the prevailing bottom hole temperature. The protective layer applied on the walls of the perforation tunnels should be strong enough to resist the high shear rates normally occurring in well perforations, yet thin enough to prevent injectivity or productivity impairment. 
     The invention will now be further elucidated by the following example to which it is by no means restricted. 
     EXAMPLE 
     A CO 2  injection well is completed with 9-5/8-inch casing (59.41 kg/m) in 12-1/4-inch hole through sandstone with an average permeability and porosity of 1200 mD and 15%, respectively, at 2250 m. The casing is perforated in brine (density 1100 kg/m 3 ) from 2246 to 2249 m with 1/2-inch holes, 13 per m, total 39 holes. The perforations are internally coated as follows: 
     A retrievable 9-5/8-inch packer with a 4.5 m 2-∛-inch tail pipe (7.0 kg/m) is run on 2-∛-inch tubing (7.0 kg/m) to 2245 m. 
     The packer is set at 2245 m, an injectivity test conducted and the bottom hole temperature measured. An injectivity rate of 0.5 m 3  /min is possible, and the dynamic bottom hole temperature is 70° C. A plating fluid with the composition shown in Table 3 is selected. 
     The packer is released and the following fluid pumped down the hole: 310 l liters of preflush 1, followed by 400 liters of preflush 2, and then by 400 liters of activator solution. 
     The packer is reset at 2245 m and pumping continued at a rate of 0.5 m 3  /min as follows: 400 liters of spacer 1, followed by 400 liters of spacer 2, then by 8000 liters of plating solution, and finally by 400 liters of final spacer. The fluids are chased by 4900 liters of brine. The packer is then released and pulled out of the hole. 
     In this treatment, the perforations are exposed to 8000 liters of plating solution, containing 150 kg nickel. Assuming that 1/4% of the available nickel precipitates on the perforation tunnel walls, then an 840 μm protective layer is formed.