Cement slurries, unaffected by salts of magnesium, for cementing wells traversing salt formations, and related cementing processes
This invention concerns the cementing of drilling wells and in particular of oil wells. 
     In drilling a well through underground formations in order to produce or inject fluids therein, the well must be cemented both to preserve its integrity and to keep control of the fluids injected therein or produced therefrom, among other purposes. 
     It is common practice for the production of fluids, such as for production of crude oil, to lower steel tubing into the drill hole. After this, a cement slurry with suitable properties is prepared at the surface and injected into the tubing. Thereafter, the cement slurry is displaced, meaning that a displacement fluid is injected behind it up to a level of a few meters above the shoe of the tubing. Nearly all the cement slurry is thus pushed back into the annular space or annulus between the tubing and the wall of the well. 
     The slurry having been placed, it is allowed to set and its mechanical strength is allowed to develop to obtain sound support for the tubing, suitable bonding with the surrounding rocks of various compositions and isolation of the respective zones from fluid migrations. 
     A great number of slurry formulations are known which enable the properties of the cement slurry (mainly its density, setting time or thickening time, rheology, fluid loss, free water, compressive strength, permeability and resistance to chemical attack) to be adapted to specific well requirements. 
     A good mix design is especially important when the cement slurry is used to cement a well drilled through salt formations, as the equilibrium conditions between the cement and the saline zones are difficult to establish. Failing to establish these equilibrium conditions soon results in the salts dissolving and the alkalies, alkaline earths and chlorides migrating into the cement, which slows the setting of the cement and consequently adversely affects its mechanical strength, and produces a poor bond between the cement and the surrounding formations. 
     Cement slurry mix designs designed to solve the problems associated with the traversing of salt formations have been proposed in French patent application No. 83 20303, filed on Dec. 19, 1983. These mixes are moderately to heavily salt-laden by sodium chloride and contain admixtures such as sodium silicate adapted to effect a novel fibrous bonding between the cement and the traversed formations. Provided other admixtures, such as citric acid, are further introduced, these mixtures can be used even in the context of magnesium contamination amounting to 3%. 
    
    
     The present invention is directed to providing improved cement slurry mixes for cementing wells, and in particular oil wells, exhibiting the following general properties: 
     (1) the slurry remains liquid (and thus pumpable) whether or not it has been contaminated by soluble magnesium mineral products, such as carnallite, bischofite and/or tachyhydrite, even in very large quantities; 
     (2) the setting time of the slurry is substantially constant, irrespective of magnesium contamination or non-contamination, even in very large quantities; 
     (3) and the cement&#39;s properties are not significantly deteriorated even if the slurry has been highly contaminated by a soluble salt of magnesium. 
     With the cement slurries known hereto, the traversing (or merely the risk of traversing) a layer containing soluble salts of magnesium raises the following well-known, yet serious problems: 
     (a) If a cement slurry which is not adapted to a magnesic contamination is subjected to such a contamination, its viscosity increases as the result of the precipitation of magnesium hydroxide to such an extent that it can no longer be pumped. 
     If the contamination is light, the slurry may remain pumpable, but its setting will nevertheless be accelerated by the magnesium so that the cementing operation being carried out will be adversely affected. 
     (b) If admixtures, such as retarders and the like are added to the cement slurry in anticipation of magnesium contamination and its accelerating effect, and such contamination does not in fact occur, or occurs to a lesser extent than anticipated, said admixtures will obviously have a very detrimental effect, such as excessively retarding setting or preventing setting altogether and thus severely disturbing cementing. 
     (c) In any case, a magnesium contamination adversely affects the cement&#39;s mechanical properties and in particular greatly reduces compressive strength. 
     These problems are avoided by the invention, which provides a slurry that is practically unaffected by the presence or absence of magnesium contamination. 
     The cement slurry compositions according to the invention contain a cement, mixing water and: 
     phosphoric acid (H 3  PO 4 ), in a proportion of more than 0.5% and preferably less than 1.5% by weight of cement (BWOC), 
     hydroxyethylenediphosphonate (HEDP), in a proportion of more than 0.25% BWOC (HEDP also acting as a retarder), 
     NaCl, from 18% to saturation, 
     sodium silicate, from 35 liters per metric ton of slurry up to approximately 150 l/t, 
     various common admixtures (antifoam agents, etc.). 
     Such compositions are compatible, ie. exhibit the three abovementioned properties (1 through 3), with magnesium chloride (MgCl 2 ) present in a proportion of 0 to 40% of bischofite equivalents (MgCl 2 , 6H 2  O) by weight of fresh water 
     Said slurry compositions are thus unaffected by the presence or absence of magnesium contamination. 
     The invention thus resides in the presence of an &#34;agent of compatibility with magnesium&#34; consisting of H 3  PO 4  +HEDP in the proportions as indicated. 
     Nevertheless, if the quantity of sodium silicate is reduced or if the latter is eliminated entirely, very serious disturbances occur. In particular, the delay in setting considerably increases, the cement&#39;s compressive strength drops and the slurry&#39;s flow characteristics deteriorate. The greatest disadvantage by far in this case is the added delay in setting time. 
     The single appended table summarizes a number of non-limiting examples of mixes according to the invention and of comparative examples of mixes without the improvement of the invention. 
     Comparative examples 1*, 2*, 3* and 4* show that, without the combination of H 3  PO 4  and HEDP according to the invention, the slurries are not compatible with magnesium. 
     Comparative examples 5* and 6* show that said combination of H 3  PO 4  +HEDP does not produce the desired effect below a certain threshold level of H 3  PO 4  and HEDP content. Example 5 illustrates the critical nature of the lower thresholds according to the invention. It represents a boundary case. 
     Examples 7 through 14, of mixes according to the invention, show that when the slurry contains the H 3  PO 4  +HEDP combination in suitable proportions: 
     (a) thickening times are practically independent of the presence or absence of magnesium chloride and 
     (b) the cement&#39;s compressive strength is not seriously affected by a bischofite equivalent contamination of up to 30% by weight of water. 
     In fact, the compressive strength in Example 8 (Mg contamination) is even greater than that of the cement in Example 7 (no Mg contamination), which is surprising. 
     These examples also show that the proportion of HEDP used in the compatibility agent is related to temperature: at low bottomhole circulation temperatures (BHCT=150° F.) an appropriate value is 0.5 (Examples 9 and 10); going up to 1 at 185° F. (Examples 11 and 12); then 1.5 at 250° F. (Examples 13 and 14). Obviously, these values are given only as an indication. 
     Finally, and particularly at high temperatures, the mixes according to the invention can be provided with an additional retarder such as borax. 
     
                                           TABLE                                   
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                  Mg                                                      
             H.sub.3                                                      
                  CL.sub.2                                                
   NaCl HEDP PO.sub.4                                                     
                  6H.sub.2 O                                              
                           kg/                                            
                              YV                                          
Ex.                                                                       
   %    %    %    %    TT  cm.sup.2                                       
                              dyne                                        
                                 PV                                       
No BWOW BWOC BWOC BWOW (min)                                              
                           (1)                                            
                              cm.sup.2                                    
                                 cP Remarks                               
__________________________________________________________________________
1(*)                                                                      
   30   1    0    0    --  -- 201                                         
                                 36.2                                     
2(*)                                                                      
   30   1    0    30 + --  -- -- -- Rheo.                                 
3(*)                                                                      
   30   0    1    0    --  -- 610                                         
                                 79.3                                     
4(*)                                                                      
   30   0    1    30 + --  -- -- -- Slur                                  
5(*)                                                                      
   30   0.25 0.5  30 + --  --       Rheo.                                 
6(*)                                                                      
   30   0.5  0.25 30 + --  --                                             
7  30   1    1    0    335 280                                            
                              31.9                                        
8  30   1    1    30 + 327 293                                            
                              45.6                                        
9  30   0.5  1    0    243 308   T1                                       
10 30   0.5  1    30 ++                                                   
                       275 260                                            
11 30   1    1    0    371       T2                                       
12 30   1    1    30 +++                                                  
                       340                                                
13 30   1.5  1    0    214       T3                                       
14 30   1.5  1    30 +++                                                  
                       198                                                
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Key to Table                                                              
TT = thickening time                                                      
CS = compressive strength (14-day strength, 221° F. - 105°  
C., 2000                                                                  
psi - 140 kg/cm.sup.2)                                                    
Rheo = rheology impossible                                                
Slur = slurry not pourable                                                
T1 = test at 6000 psi (422kg/cm.sup.2);                                   
150° F. (65° C.); 0.8°/min.                          
T2 = test at 6000 psi (422kg/cm.sup.2);                                   
185° F. (85° C.); 0.8°/min.                          
                    TT measurement                                        
T3 = test at 10,200 psi (717kg/cm.sup.2);                                 
250° F. (121° C.); 1.6°/min.                         
BWOW = by weight of water                                                 
BWOC = by weight of cement                                                
YV = yield value                                                          
PV = plastic viscosity                                                    
 
    
     All tests based on API standard 10, 1984. 
     Slurry composition: 
     Dycherhoff cement, Class G, &#34;Gulf&#34; (tropical grade) 
     Na silicate: 0.19 1/l of fresh water 
     Fresh water: 44% minus the volume of liquid admixtures 
     NaCl, H 3  PO 4 , HEDP (retarders) and contamination by magnesium: see Table. 
     YV and PV: measured at 185° F. (85° C.) 
     Thickening time measured at 185° F. (85° C.) and 6000 psi (422 kg/cm 2 ) 
     heating speed 0.8°/min. except where otherwise indicated. 
     (+)=added after mixing 
     (*)=comparative example (example 5 is a boundary example) 
     (1)=measured after 14 days at 220° F. (105° C.) 
     (++)=added after 40 min. with consistometer at 150° F. (65° C.) 
     (+++)=added after 40 min. with consistometer at 185° F. (85° C.).