Abstract:
A high-alumina cement composition is disclosed which has thermal stability over a wide temperature range and which develops strength at an early age. The cement composition comprises a high-alumina cement, a halide salt and calcium sulfate. This composition when added to water provides a hydraulic cement composition which can be employed in environments subject to wide temperature fluctuations below and above freezing (e.g. 32° F).

Description:
This is a division of application Ser. No. 431,270 filed Jan. 7, 1974, now U.S. Pat. No. 3,922,172. 
    
    
     BACKGROUND OF THE INVENTION High-alumina cements are known for their high temperature stability. However, they are also known to lose their strength when subjected to thermal variations. For example, it has been shown that a set high-alumina cement having a 10,000 psi compressive strength lost as must as 90 percent or more of its strength when subjected to variations in temperature. It is known that the loss of compressive strengths of aluminous cements subjected to temperature variations can be prevented by incorporating calcium sulfate into the cement. However, calcium sulfate lowers the heat of hydration of the system and thus limits the use of the cement at lower temperatures. Under colder conditions the system may freeze before setting or fail to develop strength rapidly enough to be commercially acceptable as a cold weather cement. The present invention concerns the unexpected discovery that a halide salt, which is known to be a freezing point depressant and also is known to retard the setting of high-alumina cements, at all concentrations, when admixed with a high-alumina cement containing calcium sulfate actually functions to both prevent the hydraulic cement slurry from freezing and also, and surprisingly, greatly increases the early strengths, i.e. functions as an accelerator instead of a retarder in the specific cement system claimed herein. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a cement and a method of cementing in environments containing temperatures varying from those below freezing (i.e. 32° F) to above 100° F and the like. 
     The composition of the present invention contains a high-alumina cement, an effective amount of calcium sulfate to prevent strength degradation of the set cement at elevated temperatures, an effective amount of a halide salt to accelerate the rate of setting and prevent the freezing of said composition at temperatures below about 32° F, and sufficient water to form a hydraulic cement slurry which sets to a solid having adequate strength within a period of time which is practical. 
     In practicing the method of the present invention a cement slurry of the present invention is employed to cement, for example, bore holes and the like in environments wherein temperature fluctuations vary from below freezing (i.e. 32° F) to above about 100° F. The method employs standard cementing equipment and procedures well known in the art. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     High-alumina cement, known also as aluminous cement is not Portland cement. It is made by fusing a mixture of limestone and bauxite with small amounts of silica and titania. In Europe, the process is usually carried out in an open-hearth furnace having a long vertical stack into which the mixture of raw materials is charged. The hot gases produced by a blast of pulverized coal and air pass through the charge and carry off water and carbon dioxide. Fusion occurs when the charge drops from the vertical stack onto the hearth at a temperature of about 1,425° to 1,500° C. A molten liquid is formed and is continuously collected and solidified in steel pans which are carried on an endless belt. Electric arc furnaces also have been used when electric power is cheap. In the United States, the mixture is burned in a rotary kiln similar to that used for Portland cement but provided with a tap hole from which the molten liquid is drawn intermittently. A black solidified sinter is formed and is stored, e.g. in storage piles, from which it is transferred to crushing and grinding mills where it is reduced, without additions, to a fine powder. 
     Aluminous cement is composed of, as percent by weight, from about 36 to 42 percent Al 2  O 3 , about the same amount of CaO, about 7 to 18 percent oxides of iron, about 5 to 10 percent SiO 2 , and small amounts of TiO 2 , MgO and alkalies. A number of other compounds include minor amounts of, for example, CaO . Al 2  O 3  ; 6CaO . 4Al 2  O 3  ; FeO . SiO 2  ; 2CaO . Al 2  O 3  . SiO and ferrites. The setting and hardening of the cement when mixed with water is probably brought about by the formation of calcium aluminate gels, such as, CaO . Al 2  O 3  . 10H 2  O; 2CaO . Al 2  O 3  . 8H 2  O and 3CaO . Al 2  O 3  . 6H 2  O. 
     One of the notable properties of high-alumina cement is its development of very high strengths at early ages. it attains nearly its maximum strength in less than a day, which is much higher than the strength developed by Portland cement at that age. At higher temperatures, however, the strength drops off rapidly. Heat is also evolved rapidly on hydration and results in high setting temperatures. The resistance of the cement to corrosion in sea or sulfate waters, as well as its resistance to weak solutions of mineral acids, is outstanding. An analysis of a typical high-alumina cement is: 
     
         ______________________________________Constituents  Per Cent by Weight______________________________________SiO           8 to 9Al.sub.2 O.sub.3         40 to 41CaO           36 to 37Fe.sub.2 O.sub.3         5 to 6FeO           5 to 7TiO.sub.2     about 2.0MgO           about 1.0S             about 0.2SO.sub.2      about 0.2Metallic Iron trace______________________________________ 
    
     The cement composition of the present invention comprises a high-alumina cement and, based on the weight of high-alumina cement, from about 5 to about 30 percent by weight of calcium sulfate, and based on the weight of high-alumina cement and calcium sulfate, from about 35 to about 50 percent by weight of water. A halide salt is present in an amount ranging from about 5 to about 15 percent by weight of water. Preferably the cement slurry contains a high-alumina cement and from about 20 to about 25 percent by weight of calcium sulfate. Water is employed in an amount ranging from about 38 to 44 percent by weight based on the weight of high-alumina cement and calcium sulfate and the halide salt is present in an amount ranging from about 8 to about 12 percent by weight of the water. 
     Halide salts which can be employed include any which lower the freezing point of water and are not detrimental to the cement composition. Preferred salts are sodium chloride, potassium chloride and lithium chloride. The halide salt may be dry blended with the dry high aluminum cement or may be present in the form of a brine which is used to prepare the hydraulic cement slurry. 
     Calcium sulfate in an anhydrous or hydrated form can be employed. The amount of calcium sulfate to be employed is based on anhydrous calcium sulfate. 
     Other ingredients well known in the art can be included in the composition of the invention to perform such functions as prevent fluid loss from the hydraulic cement slurry, to increase the density of the hydraulic cement slurry and the like. For example silica flour, sand, barite, various polymeric fluid loss additives and the like can be incorporated into the composition of the present invention. 
     The hydraulic cement slurry of the invention can be employed to cement casing into petroleum producing boreholes in environments wherein the surface conditions are below freezing and the temperature at the lower end of the well bore may be as much as 100° F or higher. Well known cementing techniques and equipment can be employed to emplace the cement slurry. 
     EXAMPLE 1 
     In this first example, nine high-alumina cement slurries were prepared containing the constituents set forth in the following Table I. Each of these slurries was then cast into 2 inch cubes and kept at a temperature of 40° F for 3 days at the end of which their compressive strengths were determined. The samples were then left for one day at 100° F and their compressive strength again determined. The samples were then maintained at a temperature of 160° F for three days. The compressive strength was determined at the end of one day, and then again at the end of the third day. The compressive strengths were determined by breaking the cubes on a Tinis Olsen Compressive Strength tester according to the procedure set forth in API RP10B, 17th Ed., 1971. The results of these tests are also set forth in the following Table I. 
     
                       TABLE I______________________________________*Constituents inHydraulicCement slurry    Compressive Strength psi at EndCaSO.sub.4           of Time Period at TemperatureTest Anhy-                 40° F                            100° F                                  160° F                                        160° FNo.  drous    NaCl    H.sub.2 O                      3 days                            1 day 1 day 3 days______________________________________1    20               40   3925  6450  4790  64402    --       --      38   7440  9450  3050  28903    10               40   6780  8600  2120  23854    25               40   3025  4665  4515  62255    22.5             40   2800  4950  5400  58156    20               40   3335  4040  4050  52907    20        3      40   6210  6925  6900  57008    20       10      40   6675  6750  6425  63609    20        3      40   5850  7875  8260  900010   22.5     10      40   5625  6090  7350  701511   --       12      46   2200  4600   680   730______________________________________ *In this and the following tables CaSO.sub.4 is expressed as per cent by weight of the high-alumina cement, water is expressed as per cent by weight of the total of CaSO.sub.4 and high-alumina cement and the halide salt as per cent by weight of water. 
    
     EXAMPLE 2 
     In this example three different cement compositions were prepared containing a high-alumina cement and anhydrous calcium sulfate. Two of the mixtures also contained sodium chloride and the third did not. These three cement compositions were cast into cubes in the manner similar to that described in Example 1 and their compressive strengths were determined at the end of certain periods of time over a period of 15 days. During this time the cubes were subjected to variations in temperature ranging from 20° F to 80° F. The hydraulic slurry composition contained the constituents set forth in the following Table II. 
     
                       TABLE II______________________________________      Calcium  Sodium      Sulfate  Chloride   Water______________________________________Composition A        20%        10%        40%Composition B        22.5       10         40Composition C        22.5        0         40______________________________________ 
    
     The results of these tests are set forth in the following Table III. 
     
                       TABLE III______________________________________Temperature ° F 24 Hour                Composition &amp;Periods Immediately Prior                Compressiveto Compressive Strength                Strength psiDay   Determination      A       B     C______________________________________1     402     403     204     80                 7100    6925  43005     206     207     208     80                 5400    8100  43259     2010    80                 7100    7325  382511    2012    2013    2014    2015    80                 5575    8300  4375______________________________________ 
    
     EXAMPLE 3 
     In this example various amounts of sodium chloride were added to a hydraulic cement slurry containing a high-alumina cement, 22.5% by weight of calcium sulfate, based on the weight of alumina cement, and 40% by weight of water, based on the weight of alumina cement and calcium sulfate. The hydraulic cement slurries were than cast into cubes as in the manner set forth in Example 1 and maintained at a temperature of 40° F for a period of 3 days. The compressive strength of each sample was determined at the end of a certain period of time as set forth in the following Table IV. The amount of salt at various compressive strengths are also set forth in the following Table IV. In Test No. 8 no calcium sulfate was present. 
     
                       TABLE IV______________________________________Test   Wt. %     Compressive Strength psiNo.    NaCl      16 Hr.   24 Hr. 48 Hr.  72 Hr.______________________________________1       0         85       990   1450   18052       3         300     2100   4875   56503       8        1945     3575   4875   5815.sup.(3)4      10        2045     3760   4750   60905      12        1870     3380   4190   49656      14        1515     2950   4690   43407      16        1090     2460   3750   4475 8.sup.(2)  12        not set  not set                            --     22009      12 LiCl   1250     1430   --      --10     12 KCl     265     2040   --      --______________________________________ .sup.(1) This is average of compressive strengths determined on 3 samples of same composition. .sup.(2) No CaSO.sub.4 .sup.(3) In Test Nos. 3 and 5-7 the compressive strengths for 48 hr. and 72 hr. periods were determined on the same compositions but prepared at a later time.