Cement compressive strength

Through the past half century the well cementing industry has considered cement compressive strengths of about 500 psi to be acceptable. However, such low compressive strengths plus some of the past cementing practices may not be adequate for future wells. 

Table 5.13. Effect of fluorides on glass polyalkenoate cement compressive strength, MPa (Crisp, Merson < Wilson, 1980)...

The strength data in Table 7.19 show that using very low amounts of ASTM Type 1 (normal Portland) cement compressive strength of the order of 8 and 35 MPa can be obtained at 1 and 28 days respectively. The concrete s resistance... 

Cement Compressive strength Water (MPa) NaCl (MPa) Strength (NaCl) /Strength (water)... 

FIG. 4—Effect of water-cement ratio on natural cement compressive strength [8]. 

The chemistry of cement slurries is complex. Additives will be used to ensure the slurry remains pumpable long enough at the prevailing downhole pressures and temperatures but sets (hardens) quickly enough to avoid unnecessary delays in the drilling of the next hole section. The cement also has to attain sufficient compressive strength to withstand the forces exerted by the formation over time. A spacer fluid is often pumped ahead of the slurry to clean the borehole of mudcake and thereby achieve a better cement bond between formation and cement. 

In addition to chemical analysis a number of physical and mechanical properties are employed to determine cemented carbide quaUty. Standard test methods employed by the iadustry for abrasive wear resistance, apparent grain size, apparent porosity, coercive force, compressive strength, density, fracture toughness, hardness, linear thermal expansion, magnetic permeabiUty, microstmcture, Poisson s ratio, transverse mpture strength, and Young s modulus are set forth by ASTM/ANSI and the ISO. 

Cemented carbides possess high compressive strength but low ductihty at room temperature, but at temperatures associated with metal-cuttiag these materials exhibit a small but finite amouat of ductihty. Measuremeat of yield strength is therefore more appropriate at higher temperatures. Like hardness, the compressive yield strength of cemented carbide decreases monotonicaHy with increa sing temperatures. 

The high elastic modulus, compressive strength, and wear resistance of cemented carbides make them ideal candidates for use in boring bars, long shafts, and plungers, where reduction in deflection, chatter, and vibration are concerns. Metal, ceramic, and carbide powder-compacting dies and punches are generahy made of 6 wt % and 11 wt % Co ahoys, respectively. Another apphcation area for carbides is the synthetic diamond industry where carbides are used for dies and pistons (see Carbon). 

Mechanical Properties. Measuremeat of the mechanical properties of diamoad is compHcated, and references should be consulted for the vahous qualifications (7,34). Table 1 compares the theoretical and experimental bulk modulus of diamond to that for cubic BN and for SiC (29) and compares the compressive strength of diamond to that for cemented WC, and the values for the modulus of elasticity E to those for cemented WC and cubic BN. 

The compressive strength of polycarboxylate cements at cementing consistency is 55—85 MPa (8,000—12,000 psi). Typical diametral tensile strength ranges from 8—12 MPa (1160 1740 psi). The solubiHty and disintegration in distilled water after 7 days at 37°C is 0.04—0.08 wt %, and is not reflected in clinical performance. 

Resin cements have excellent aesthetic quahties and are essentially insoluble in mouth fluids. Compressive strength is low, but can be increased by the addition of fillers. They have no inherent adhesion to the tooth. Retention is dependent on mechanical locking when the cement flows into irregularities on the surfaces of the substances being cemented. 

The principal applications for furan resins are in chemical plant. Specific uses include the lining of tanks and vats and piping and for alkali-resistant tile cements. The property of moisture resistance is used when paper honeycomb structures are treated with furan resins and subsequently retain a good compression strength even after exposure to damp conditions. 

A properly designed cement slurry will set after it has been placed in its appropriate location within the well. Cement strength is the strength the set cement has obtained. This usually refers to compressive strength, but can also refer to tensile strength. Cement having a compressive strength of 500 psi is considered adequate for most well operations. 

In the compression test, four or five sample cubes of the slurry are allowed to cure for a specified period of time. The cement cubes are placed in a compression testing machine and the compressive strength of each sample cube obtained experimentally. The average value of the samples is obtained and reported as the compressive strength of the set cement. 

Table 4-158 shows the effect of silica flour on the compressive strength of Class G cement cured at 700°F. 

Influence of Time and Temperature on the Compressive Strength of API Class H Cement [162]... 

Effect of Silica Flour on the Compressive Strength of Class G Cement Cured at Pressure... 

Hematite. This additive can be used to increase the specific weight of a cement slurry to as high as 19 Ib/gal. This is an iron oxide ore with a specific gravity of about 5.02. Hematite requires the addition of some water when it is used as an additive. Hematite has minimal effect on thickening time and compressive strength of the cement. 

Ilmenite. This additive has a specific gravity of about 4.67. It is a mineral composed of iron, titanium and oxygen. It requires no additional water to be added to the slurry thus, it can yield slurry specific weights as high as the hematite additive. Ilmenite also has mineral effect on thickening time and compressive strength of the cement. 

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