Compositions and Methods for Well Completions

Abstract

Well-cementing compositions for use in high-pressure, high-temperature (HPHT) wells are often densified, and contain weighting agents such as hematite, ilmenite, barite and hausmannite. The weighting agents are usually finely divided to help keep them suspended in the cement slurry. At high temperatures, finely divided weighting agents based on metal oxides react with the calcium-silicate-hydrate binder in set Portland cement, leading to cement deterioration. Finely divided weighting agents based on metal sulfates are inert with respect to calcium silicate hydrate; consequently, set-cement stability is preserved.

1 . A well-cementing composition, comprising water and solids comprising Portland cement, silica and an additive comprising one or more metal sulfates in the list comprising barite, celestine and anglesite, wherein the median particle size of the additive is smaller than about 10 μm. 
     
     
         2 . The composition of  claim 1 , wherein the density of the composition is higher than about 2035 kg/m 3 . 
     
     
         3 . The composition of  claim 1 , wherein the additive concentration is between about 1% and about 150% by weight of cement. 
     
     
         4 . The composition of  claim 1 , further comprising one or more additives in the list comprising: accelerators, retarders, extenders, fluid-loss additives, dispersants, gas-generating agents, antifoam agents, chemical-expansion agents, flexible additives, pozzolans and fibers. 
     
     
         5 . The composition of  claim 1 , wherein the solids are present in at least two particle-size ranges. 
     
     
         6 . The composition of  claim 1 , wherein the viscosity of said composition is lower than 1000 mPa-s at a shear rate of 100 s −1 . 
     
     
         7 . The composition of  claim 1 , wherein the median particle size of the additive is smaller than about 5 μm. 
     
     
         8 . A method for maintaining the compressive strength of a well-cementing composition, comprising:
 (i) providing a cement slurry comprising water, Portland cement and silica; and   (ii) incorporating into the cement slurry an additive comprising one or more metal sulfates in the list comprising barite, celestine and anglesite, wherein the average particle size of the additive is smaller than about 10 μm; and   (iii) curing the cement slurry at a temperature higher than or equal to about 200° C.   
     
     
         9 . The method of  claim 8 , wherein the density of the composition is higher than about 2035 kg/m 3 . 
     
     
         10 . The method of  claim 8 , wherein the additive concentration is between about 1% and about 150% by weight of cement. 
     
     
         11 . The method of  claims 8 , wherein the composition further comprises one or more additives in the list comprising: accelerators, retarders, extenders, fluid-loss additives, dispersants, gas-generating agents, antifoam agents, chemical-expansion agents, flexible additives, pozzolans and fibers. 
     
     
         12 . The method of  claim 8 , wherein the solids in the composition are present in at least two particle-size ranges. 
     
     
         13 . The method of  claim 8 , wherein the cement slurry has a viscosity lower than 1000 mPa-s at a shear rate of 100 s −1 . 
     
     
         14 . The method of  claim 8 , wherein the median particle size of the additive is smaller than about 5 μm. 
     
     
         15 . A method for cementing a subterranean well, comprising:
 (i) providing a cement slurry comprising water, Portland cement and silica; and   (ii) incorporating into the cement slurry an additive comprising one or more metal sulfates in the list comprising barite, celestine and anglesite, wherein the average particle size of the additive is smaller than about 10 μm; and   (iii) placing the slurry into the well,   wherein, the bottomhole temperature in the well is higher than or equal to about 200° C.   
     
     
         16 . The method of  claim 15 , wherein the density of the composition is higher than about 2035 kg/m 3 . 
     
     
         17 . The method of  claim 15 , wherein the additive concentration is between about 1% and about 150% by weight of cement. 
     
     
         18 . The method of  claim 15 , wherein the composition further comprises one or more additives in the list comprising: accelerators, retarders, extenders, fluid-loss additives, dispersants, gas-generating agents, antifoam agents, chemical-expansion agents, flexible additives, pozzolans and fibers. 
     
     
         19 . The method of  claim 15 , wherein the solids in the composition are present in at least two particle-size ranges. 
     
     
         20 . The method of  claim 15 , wherein the cement slurry has a viscosity lower than 1000 mPa-s at a shear rate of 100 s −1 .