Abstract:
A self-adaptive cement formulation includes cement, water and thermoplastic block-polymer particles. The set cement demonstrates self-healing properties when exposed to methane, and is particularly suited for well-cementing applications. After placement and curing, the self healing properties help maintain zonal isolation should bonding be disrupted between the set cement and the formation or a casing string, should cracks or defects appear in the set-cement matrix, or both.

Description:
TECHNICAL FIELD 
     The present disclosure relates to self-adaptive cements. In particular, it relates to set cements that are “self-healing,” i.e., formulations that can adapt to compensate for changes or faults in the physical structure of the cement, or which adapt their structure after the setting phase of the cement in the cementing of oil, gas, water or geothermal wells, or the like. 
     BACKGROUND 
     During the construction of wells, cement is used to secure and support casing inside the well and prevent fluid communication between the various underground fluid-containing layers or the production of unwanted fluids into the well. 
     Various approaches have been developed to prevent failure of the cement sheath. One approach is to design the cement sheath to take into account physical stresses that might be encountered during its lifetime. Such an approach is described for example in U.S. Pat. No. 6,296,057. Another approach is to include, in the cement composition, materials that improve the physical properties of the set cement. U.S. Pat. No. 6,458,198 describes the addition of amorphous metal fibers to the cement slurry to improve its strength and resistance to impact damage. EP 1129047 and WO 00/37387 describe the addition of flexible materials (rubber or polymers) to the cement to confer a degree of flexibility on the cement sheath. 
     Nevertheless, the above-described approaches do not allow restoration of the zonal isolation once the cement sheath has actually failed due to the formation of cracks or microannuli. 
     A number of self-healing concretes are known for use in the construction industry. These are described for example in U.S. Pat. No. 5,575,841, U.S. Pat. No. 5,660,624, U.S. Pat. No. 5,989,334, U.S. Pat. No. 6,261,360 and U.S. Pat. No. 6,527,849, and in the document entitled “Three designs for the internal release of sealants, adhesives, and waterproofing chemicals into concrete to reduce permeability”, Dry, C. M., Cement and Concrete Research 30 (2000) 1969-1977. 
     Nevertheless, none of these self-healing concretes are immediately applicable to well cementing operations because of the need for the material to be pumpable during placement. 
     “Self-healing” cements were eventually developed for oil and gas industry applications such as described in U.S. 2007/0204765 A1, WO 2004/101951 and WO 2004/101952 A1. These formulations generally contain additives that react and/or swell upon contact with downhole fluids. When cement-sheath deterioration occurs, exposing the cement matrix or cement-sheath surfaces to downhole fluids, the additives respond and seal cracks or fissures, thereby restoring cement-matrix integrity and zonal isolation. Well cements are potentially exposed to several fluid types during service, including liquid and gaseous hydrocarbons, water, brines and/or carbon dioxide. Thus, depending on the anticipated wellbore environment, it would be desirable to incorporate additives that are able to respond to one or more types of downhole fluids. 
     Despite the many valuable contributions from the art, it would be desirable to have access to a self-healing set cement that responds to formation fluids that contain high concentrations of gaseous hydrocarbons. 
     SUMMARY 
     The present disclosure provides set cements that are self-healing when exposed to hydrocarbons, and methods by which they may be prepared and applied in subterranean wells. 
     In an aspect, embodiments relate to methods for maintaining zonal isolation in a subterranean well that penetrates one or more hydrocarbon-containing formations. 
     In a further aspect, embodiments relate to uses of thermoplastic block-polymer particles to impart self-healing properties to a cement formulation that is placed in a subterranean well penetrating one or more hydrocarbon-containing formations. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plot showing the swelling characteristics of styrene-isoprene-styrene (SIS) and styrene-butadiene-styrene (SBS) particles in the presence of methane at various temperatures and pressures. 
         FIG. 2  is a schematic diagram of an experimental apparatus for measuring the self-healing ability of fractured cement samples. 
         FIG. 3  presents normalized flow-rate reductions for set cements containing SIS and SBS particles exposed to methane. 
         FIG. 4  presents the effect of slurry density on normalized flow-rate reductions for set cements containing SIS and SBS particles exposed to methane. 
         FIG. 5  presents normalized flow-rate reductions for set cements containing SIS and SBS particles exposed to methane at various pressures. 
     
    
    
     DETAILED DESCRIPTION 
     At the outset, it should be noted that in the development of any such actual embodiment, numerous implementations—specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system related and business related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. In addition, the composition used/disclosed herein can also comprise some components other than those cited. In the summary and this detailed description, each numerical value should be read once as modified by the term “about” (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Also, in the summary and this detailed description, it should be understood that a concentration range listed or described as being useful, suitable, or the like, is intended that any and every concentration within the range, including the end points, is to be considered as having been stated. For example, “a range of from 1 to 10” is to be read as indicating each and every possible number along the continuum between about 1 and about 10. Thus, even if specific data points within the range, or even no data points within the range, are explicitly identified or refer to only a few specific points, it is to be understood that inventors appreciate and understand that any and all data points within the range are to be considered to have been specified, and that inventors possessed knowledge of the entire range and all points within the range. 
     This disclosure concerns compositions for cementing subterranean wells, comprising a settable material, water and at least one additive that swells in the event of structural failure of or damage to the set material (i.e., the cement sheath). Such behavior restores and maintains a physical and hydraulic barrier in the failure zone. As a result, zonal isolation in the subterranean well is preserved. Such set cements are said to be “self-healing” or “self-repairing.” In this application, both terms are used indifferently, and are to be understood as the capacity of a cement sheath to restore hydraulic isolation after suffering a matrix-permeability increase, structural defects such as cracks or fissures, or debonding from casing or formation surfaces (i.e., microannuli). 
     Examples of settable materials include (but are not limited to) Portland cement, microcement, geopolymers, mixtures of cement and geopolymer, plaster, lime-silica mixtures, resins, phosphomagnesium cements or chemically bonded phosphate ceramics (CBPCs). 
     As stated earlier, there is a need for self-healing set cements that operate in an environment containing high concentrations of gaseous hydrocarbons, methane in particular. Surprisingly, the inventors have discovered that self-healing properties may be achieved in this environment by incorporating thermoplastic block-polymer particles in the cement formulation. Typical block polymers comprise alternating sections of one chemical compound separated by sections of a different chemical compound, or a coupling group of low molecular weight. For example, block polymers can have the structure (A-b-B-b-A), wherein A represents a block that is glassy or semi-crystalline and B is a block that is elastomeric. In principle, A can be any polymer that is normally regarded as thermoplastic (e.g., polystyrene, polymethylmethacrylate, isotactic polypropylene, polyurethane, etc.), and B can be any polymer that is normally regarded as elastomeric (e.g., polyisoprene, polybutadiene, polyethers, polyesters, etc.). 
     Further embodiments relate to methods for maintaining zonal isolation in a subterranean well having a borehole that penetrates one or more hydrocarbon-containing formations. The method comprises pumping a cement slurry comprising thermoplastic block-polymer particles into the well, and allowing the cement slurry to form a cement sheath. Those skilled in the art will recognize that a cement slurry is generally considered to be pumpable when its viscosity is less than or equal to 1000 mPa-s at a shear rate of 100 s −1 , throughout the temperature range the slurry will experience during placement in the well. The cement sheath may be located between the well casing and the borehole wall, or between the casing and another casing string. If microannuli, cracks or defects occur in the cement sheath, the casing-cement interface or the cement-borehole wall interface, the particles will be exposed to formation hydrocarbons, causing them to swell and enabling the cement sheath to have self-healing properties. 
     Yet further embodiments aim at uses of thermoplastic block-polymer particles to impart self-healing properties to a set cement sheath in a subterranean well that penetrates one or more hydrocarbon-containing formations. The particles swell when contacted by hydrocarbons from the formation, in particular gaseous hydrocarbons. 
     For all aspects, the tensile strength of the block polymer may be varied between (but is not limited to) about 1.5 MPa and 40 MPa, preferably between 3.4 to 34 MPa. Even more preferred tensile-strength may be between 2 MPa and 3.45 MPa or between 28 MPa and 34 MPa. 
     Preferred thermoplastic block polymers include styrene-isoprene-styrene (SIS), styrene-butadiene-styrene (SBS) and mixtures thereof. The block-polymer-additive may be in one or more shapes, including (but not limited to) spherical, ovoid, fibrous, ribbon-like and in the form of a mesh. 
     The concentration of the block-polymer particles is preferably between about 10% and 55% by volume of solids in the cement slurry, also known as percentage by volume of blend (BVOB). A more preferred particle concentration lies between about 20% and 50% BVOB. The particle-size range is preferably between about 100 μm and 900 μm, and more preferably between about 200 μm and 800 μm. 
     One of the current challenge that the industry is facing is the presence in some wells of high concentration of gaseous hydrocarbons such as methane, propane and/or ethane. Such gaseous hydrocarbons being much more volatile than hydrocarbons in liquid form have the tendency to penetrate the failures and/or microannuli that can be present and the cement sheath and thus modifying the pressure and safety conditions of the well as the integrity is diminished. The inventors have determined that the present compositions can solve this problem up to very high concentration of gaseous hydrocarbon. In a preferred embodiment, the gaseous concentration of hydrocarbon fluid is greater than about 91 mol %, and more preferably above about 95 mol %. In addition, the hydrocarbon pressure to which the cement sheath is exposed is preferably above about 3.5 MPa, more preferably above about 6.9 MPa and most preferably above about 13.7 MPa. 
     The block-polymer particles may be further encapsulated by a protective layer. The layer may rupture or degrade upon exposure to one or more triggers, including (but not limited to) contact with a hydrocarbon, propagation of a crack within the set-cement matrix, time and/or temperature. 
     In addition to the block-polymer particles, the cement slurries may also comprise customary additives such as retarders, accelerators, extenders, fluid-loss-control additives, lost-circulation additives, gas-migration additives and antifoam agents. Furthermore, the cement slurries may contain additives that enhance the flexibility and/or toughness of the set cement. Such additives include (but are not limited to) flexible particles having a Young&#39;s modulus below about 5000 MPa and a Poisson&#39;s ratio above about 0.3. Preferably, such particles would have a Young&#39;s modulus below about 2000 MPa. Examples include (but are not limited to) polypropylene, polyethylene, acrylonitrile butadiene, styrene butadiene and polyamide. Such additives may also include fibers selected from the list comprising polyamide, polyethylene and polyvinyl alcohol. Metallic microribbons may also be included. 
     The block-polymer particles may also be used in engineered-particle-size cement formulations involving trimodal or quadrimodal blends of small, medium and coarse particles. Such as formulations exemplified in U.S. Pat. No. 5,518,996 and/or CA 2,117,276. 
     The block-polymer particles may be further associated with one or more compounds from the list comprising an aqueous inverse emulsion of polymer comprising a betaine group, poly-2,2,1-bicyclo heptene (polynorbornene), alkylstyrene, crosslinked substituted vinyl acrylate copolymers, diatomaceous earth, natural rubber, vulcanized rubber, polyisoprene rubber, vinyl acetate rubber, polychloroprene rubber, acrylonitrile butadiene rubber, hydrogenated acrylonitrile butadiene rubber, ethylene propylene diene monomer, ethylene propylene monomer rubber, styrene-butadiene rubber, styrene/propylene/diene monomer, brominated poly(isobutylene-co-4-methylstyrene), butyl rubber, chlorosulphonated polyethylenes, polyacrylate rubber, polyurethane, silicone rubber, brominated butyl rubber, chlorinated butyl rubber, chlorinated polyethylene, epichlorohydrin ethylene oxide copolymer, ethylene acrylate rubber, ethylene propylene diene terpolymer rubber, sulphonated polyethylene, fluoro silicone rubbers, fluoroelastomer and substituted styrene acrylate copolymers. 
     Those skilled in the art will appreciate that the disclosed method and use may not necessarily be applied throughout the entire length of the subterranean interval being cemented. In such cases, more than one cement-slurry composition is placed sequentially. The first slurry is called the “lead,” and the last slurry is called the “tail.” Under these circumstances, it is preferred that the inventive slurry be placed such that it resides in regions where hydrocarbons exist. In most cases, this will be at or near the bottom of the well; therefore, the inventive method and use would preferably apply to the tail. Those skilled in the art will also appreciate that the disclosed method and use would not only be useful for primary cementing, but also for remedial cementing operations such as squeeze cementing and plug cementing. 
     Other and further objects, features and advantages will be readily apparent to those skilled in the art upon a reading of the description of the examples which follows, taken in conjunction with the accompanying drawings. 
     EXAMPLES 
     The following examples serve to further illustrate the disclosure. 
     Table 1 lists the styrene-isoprene-styrene (SIS) polymers and styrene-butadiene-styrene (SBS) polymers that were used in the examples. 
     
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Suppliers and Properties of SIS and SBS Polymers Employed in Examples.* 
               
             
          
           
               
                 Property 
                 SIS #1 
                 SIS #2 
                 SBS #1 
                 SBS #2 
                 SBS #3 
                 SBS #4 
               
               
                   
               
               
                 Supplier 
                 ICO 
                 KRATON 
                 ICO 
                 ICO 
                 ICO 
                 KRATON 
               
               
                   
                 POLYMERS 
                   
                 POLYMERS 
                 POLYMERS 
                 POLYMERS 
                   
               
               
                 Product Name 
                 ICO1 
                 D1161 
                 ICO 3 
                 ICO 4 
                 ICO 5 
                 D1192 EM 
               
               
                   
                   
                 PTM 
                   
                   
                   
                   
               
               
                 Melt Index 
                 13 
                 13.5 
                 &lt;1 
                 23-37 
                 &lt;1 
                 &lt;1 
               
               
                 (200° C./5 kg) 
                   
                   
                   
                   
                   
                   
               
               
                 (g/10 min) 
                   
                   
                   
                   
                   
                   
               
               
                 Density (kg/m 3 ) 
                 963 
                 920 
                 940 
                 940 
                 981 
                 940 
               
               
                 Tensile strength 
                 17 
                 21 
                 16 
                 10 
                 33 
                   
               
               
                 at break (MPa) 
                   
                   
                   
                   
                   
                   
               
               
                 Hardness, 
                 24 
                 32 
                   
                   
                 72 
                 70 
               
               
                 Shore A (30 s) 
                   
                   
                   
                   
                   
                   
               
               
                 Elongation at 
                 1400 
                 1300 
                 680 
                 900 
                 880 
                   
               
               
                 break (%) 
               
               
                   
               
               
                 *Test methods: 
               
               
                 ISO 1133 (Melt Index measurement) 
               
               
                 ISO 37 (Tensile Strength at Break and Elongation at Break measurements) 
               
               
                 ISO 2781 (Density measurement) 
               
               
                 ISO 868 (ICO Polymers) and ASTM 2240 (Kraton) (Hardness measurement) 
               
             
          
         
       
     
     Example 1 
     Several polymer particles were placed inside a pressure cell equipped with a window that allows one to observe the behavior of materials within the cell. The cell supplier was Temco Inc., Houston, Tex. (USA). The cell temperature was also adjustable. A camera captured images from inside the pressure cell, and image-analysis software was employed to interpret the behavior of materials inside the cell. For particle-size measurements, the software examined the cross-sectional area of the particles in the cell. 
     After the polymer particles were introduced into the cell, the cell was sealed. The cell was then heated to the desired temperature. The initial particle sizes were measured. 
     A methane-gas line was then connected to the cell, and the methane pressure was raised to 21 MPa over a 3-min period. The cell pressure was maintained for 2 hr, after which the particle sizes were measured again. 
     Tests were performed at 22° C. and 42° C. with an SIS polymer (SIS #1 from Table 1) and an SBS polymer (SBS #3). The results are presented in  FIG. 1 . At both temperatures, both SIS and SBS polymer demonstrated good performance. 
     Example 2 
     The properties of cement slurries containing SIS or SBS particles were measured. The tests conformed to standard methods published by the International Organization for Standards (ISO): “Petroleum and natural gas industries—Cements and materials for well cementing—Part 2: Testing of well cements,” International Organization for Standards Publication No. ISO 10426-2. Two cement slurries were tested—one containing SIS particles (SIS #1), and the other containing SBS particles (SBS #3). The test conditions were as follows—bottomhole static temperature: 53° C.; bottomhole circulating temperature: 44° C.; bottomhole pressure: 21 MPa (3000 psi). 
     The composition of the slurry containing SBS is given in Table 2, and the test results are presented in Tables 3 and 4. The slurry density was 1606 kg/m 3 , and the solid volume fraction (SVF) of the slurry was 51.8%. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Composition of Test Cement Slurry Containing SBS as Self-Healing Particle. 
               
             
          
           
               
                 Component 
                 Type 
                 Quantity (kg/m 3 ) 
               
               
                   
               
             
          
           
               
                 Cement 
                 Class G Portland cement 
                 696 
               
               
                 Self-healing particle 
                 SBS #3 
                 214.5 
               
               
                 Silica 
                 200 mesh (74 μm) 
                 200.5 
               
               
                 Water 
                 Fresh 
                 395 
               
               
                 Lightweight particle 
                 Acrylonitrile-butadiene copolymer 
                 5 
               
               
                 Antifoam 
                 Polypropylene glycol 
                 4 
               
               
                 Dispersant 
                 Polymelamine sulfonate 
                 9 
               
               
                 Antisettling 
                 90% crystalline silica; 10% 
                 1 
               
               
                   
                 polysaccharide biopolymer 
                   
               
               
                 Fluid-loss additive 
                 RHODOFLAC ™, available from 
                 72 
               
               
                   
                 Rhodia Nederland 
                   
               
               
                 Retarder 
                 Calcium Lignosulfonate 
                 2.5 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Rheological Properties of Test Cement Slurry Containing  
               
               
                 SBS as Self-Healing Particle. 
               
             
          
           
               
                   
                 Mixing 
                 20-min Conditioning 
               
               
                   
                   
               
               
                   
                 PV: 233 cP 
                 PV: 219 cP 
               
               
                   
                 Ty: 4.3 kPa (9 lbf/100 ft 2 ) 
                 Ty: 8.1 kPa (17 lbf/100 ft 2 ) 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Additional Properties of Test Cement Slurry Containing  
               
               
                 SBS as Self-Healing Particle. 
               
             
          
           
               
                   
                 Measurement 
                 Results 
               
               
                   
                   
               
               
                   
                 Free fluid 
                 0.8% 
               
               
                   
                 Fluid loss 
                 13 mL 
               
               
                   
                 Thickening time 
                 8:53 (to 70 Bearden consistency, Bc) 
               
               
                   
                 Compressive strength  
                 500 psi [3.4 MPa] (UCA) after 23:42 
               
               
                   
                 development 
                 783 psi [5.4 MPa] (crush); 512 psi  
               
               
                   
                   
                 [3.5 MPa] (UCA) after 24:00 
               
               
                   
                   
                 1316 psi [9 MPa] crush (996 psi  
               
               
                   
                   
                 [6.9 MPa] (UCA) after 72:00 
               
               
                   
                   
                 1000 psi [7 MPa] (UCA) after 72:58 
               
               
                   
                 Tensile strength* 
                 1.9 MPa 
               
               
                   
                   
               
               
                   
                 *Cement was cured for 7 days at 53° C. and 20 MPa before measuring tensile strength. 
               
             
          
         
       
     
     The composition of the slurry containing SIS is given in Table 5, and the test results are presented in Tables 6 and 7. The slurry density was 1606 kg/m 3 , and the solid volume fraction (SVF) of the slurry was 51.7%. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Composition of Test Cement Slurry Containing SIS  
               
               
                 as Self-Healing Particle. 
               
             
          
           
               
                 Component 
                 Type 
                 Quantity (kg/m 3 ) 
               
               
                   
               
             
          
           
               
                 Cement 
                 Class G Portland cement 
                 694 
               
               
                 Self-healing additive 
                 SIS #1 
                 208 
               
               
                 Antifoam 
                 Polypropylene Glycol 
                 5 
               
               
                 Silica 
                 200 mesh (74 μm) 
                 219 
               
               
                 Water 
                 Fresh 
                 393 
               
               
                 Dispersant 
                 Polymelamine Sulfonate 
                 8 
               
               
                 Antisettling 
                 Biopolymer 
                 1 
               
               
                 Fluid loss 
                 RHODOFLAC ™, available from 
                 81 
               
               
                   
                 Rhodia Nederland 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Rheological Properties of Test Cement Slurry Containing  
               
               
                 SIS as Self-Healing Particle. 
               
             
          
           
               
                   
                 Mixing 
                 20-min Conditioning 
               
               
                   
                   
               
               
                   
                 PV: 119 cP 
                 PV: 107 cP 
               
               
                   
                 Ty: 6.7 KPa (14 lbf/100 ft 2 ) 
                 Ty: 9.1 KPa (19 lbf/100 ft 2 ) 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Additional Properties of Test Cement Slurry Containing  
               
               
                 SIS as Self-Healing Particle. 
               
             
          
           
               
                 Measurement 
                 Results 
               
               
                   
               
               
                 Free water 
                 0.3% 
               
               
                 Thickening time 
                 4:13 (to 70 Bearden consistency) 
               
               
                 Compressive strength development  
                 500 psi after 11:52 
               
               
                 (measured by UCA) 
                 867 psi after 24:00 
               
               
                   
                 1000 psi after 32:00 
               
               
                   
                 1260 psi after 72:00 
               
               
                   
               
             
          
         
       
     
     Example 3 
     Various cement formulations containing SIS or SBS were evaluated for their self-healing properties. The slurry compositions are presented in Table 8. The formulation that contains acrylonitrile-butadiene copolymer rubber (ABCR) was included as a control with no self-healing capability. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 8 
               
             
             
               
                   
               
               
                 Slurry Compositions for Self-Healing Tests. 
               
             
          
           
               
                   
                   
                   
                   
                   
                   
                   
                   
                 SBS 
               
               
                 Particle type 
                 Unit 
                 ABCR 
                 SIS #1 
                 SIS #2 
                 SBS #1 
                 SBS #2 
                 SBS #3 
                 #4 
               
               
                   
               
             
          
           
               
                 Density 
                 (kg/m 3 ) 
                 1571 
                 1498 
                 1606 
                 1498 
                 1498 
                 1498 
                 1606 
               
               
                 SVF 
                 (%) 
                 55 
                 50.3 
                 52.3 
                 50 
                 50.6 
                 50 
                 52 
               
               
                 Particle 
                 (kg/m 3 ) 
                 286 
                 240 
                 210 
                 243 
                 239 
                 243 
                 213 
               
               
                 Cement 
                   
                 616 
                 560 
                 645 
                 555 
                 563 
                 553 
                 641 
               
               
                 Silica 
                   
                 219 
                 199 
                 281 
                 197 
                 200 
                 196 
                 279 
               
               
                 Water 
                   
                 436 
                 494 
                 459 
                 498 
                 491 
                 497 
                 463 
               
               
                 Antifoam* 
                   
                 3 
                 4 
                 3 
                 4 
                 4 
                 6 
                 3 
               
               
                 Dispersant* 
                   
                 5 
                 0 
                 5 
                 0 
                 0 
                 3 
                 5 
               
               
                 Antisettling* 
                   
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                 Retarder* 
                   
                 5 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
               
               
                   
               
               
                 *Antifoam Agent: polypropylene glycol; Dispersant: polymelamine sulfonate; Antisettling Agent: 90% crystalline silica, 10% polysaccharide biopolymer; Retarder: calcium lignosulfonate. 
               
             
          
         
       
     
     Each cement slurry was prepared according to the method described in ISO Publication 10426-2, and samples were prepared in the manner required to perform a Brazilian tensile-strength test. This test is also described in ISO Publication 10426-2. The cement-core samples were 66 mm long and 22 mm in diameter. The samples were cured at room temperature and atmospheric pressure. The curing times are presented in Table 9. Columns with two numbers indicate that two tests were performed. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 9 
               
             
             
               
                   
               
               
                 Curing times. 
               
             
          
           
               
                 Particle 
                   
                   
                   
                   
                   
                   
                 SBS 
               
               
                 name 
                 ABCR 
                 SIS #1 
                 SIS #2 
                 SBS #1 
                 SBS #2 
                 SBS #3 
                 #4 
               
               
                   
               
               
                 Curing 
                 40/121 
                 48 
                 104 
                 101 
                 79/77 
                 78/105 
                 100 
               
               
                 time 
                   
                   
                   
                   
                   
                   
                   
               
               
                 (days) 
               
               
                   
               
             
          
         
       
     
     The samples were fractured by the Brazilian method, then transferred to a steel tube and secured by a sealing cement. As shown in  FIG. 2 , the steel tube  101  is 180 mm long. There are two 90-mm sections—one with an internal diameter of 31.5 mm in diameter, the other with an internal diameter of 29.5 mm. The fractured cement sample  102  is placed inside the tube and the sealing cement  103  is applied around the sample. Midway along the cement sample, owing to the different tube diameters, there is an edge  104  to prevent the cement sample from sliding. 
     The composition of the sealing cement was a 1.88-kg/m 3  Portland cement slurry containing 2.7 mL/kg polynaphthalene sulfonate dispersant, 2.7 mL/kg polysiloxane antifoam agent, 178 mL/kg styrene butadiene latex and 2.1% by weight of cement calcium chloride accelerator. 
     Pure methane was then injected through the fractured samples for 24 hours at 21 MPa backpressure and at ambient temperature (20°-23° C.). Flow-rate and pressure variations were recorded, and normalized flow rates were calculated. The results are shown in  FIG. 3 . 
     The cement matrices incorporating SIS particles demonstrated normalized flow-rate reductions greater than 98%. The performance of cement matrices incorporating SBS particles demonstrated flow-rate reductions between 49% and 97%. The control did not show a flow-rate reduction. 
     Example 4 
     Using the methods described in Example 3, the effect of slurry density on the performance of set cements containing SIS#1 or SBS#3 was investigated. The slurry compositions are shown in Table 10. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 10 
               
             
             
               
                   
               
               
                 Slurry Compositions for Self-Healing Tests 
               
             
          
           
               
                 Density 
                 (kg/m 3 ) 
                 1606 
                 1606 
                 1498 
                 1498 
               
               
                   
               
             
          
           
               
                 SVF 
                 (%) 
                 52 
                 51.5 
                 50.3 
                 50.7 
               
               
                 Particle type 
                   
                 SIS#1 
                 SBS#3 
                 SIS#1 
                 SBS#3 
               
               
                 Particle 
                 (kg/m 3 ) 
                 213 
                 216 
                 240 
                 242.5 
               
               
                 Class G cement 
                   
                 641.5 
                 635.5 
                 560 
                 554.3 
               
               
                 Silica 
                   
                 280 
                 277 
                 199 
                 196 
               
               
                 Water 
                   
                 462.5 
                 467.5 
                 494 
                 496.5 
               
               
                 Antifoam 
                   
                 5 
                 5 
                 4 
                 4 
               
               
                 Dispersant 
                   
                 3 
                 3 
                 0 
                 3 
               
               
                 Antisettling 
                   
                 1 
                 1 
                 1 
                 1 
               
               
                   
               
             
          
         
       
     
     The cement slurries were cured for 7 days at 53° C. and 20 MPa. The self-healing test results are presented in  FIG. 4 . For both cement matrices, density variation does not affect performance in terms of flow-rate reduction. 
     Example 5 
     Using the methods described in Example 3, the effect of pressure on the performance of set cements containing SIS#1 or SBS#3 was investigated. The 1606-kg/m 3  formulations from Table 9 were tested. 
     The samples were cured for 7 days at 53° C. and 20 MPa. Flow-rate-reduction measurements were performed at four methane pressures: 3.5 MPa, 7 MPa, 13.7 MPa and 20 MPa. The results, presented in  FIG. 5 , indicate that flow-rate reduction was achieved at 3.5 MPa for the set cement containing SIS, and at 7 MPa for the set cement containing SBS.