Cement thickening time

The cement slurry chosen must have a thickening time that is greater than the estimated time obtained for the actual cementing operation using Equation 4-327. Thus, T > T. ... 

Thus the minimum thickening time for the cement slurry to be used this cementing operation is... 

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. 

Select the most appropriate API class of cement that meets the depth, temperature, sulfate resistance and other well limitations. Select the cement class that has a natural thickening time that most nearly meets the cementing operation time requirement, or that will require only small amounts of retarding additives. 

Determine the cementing operation time and thus the minimum thickening time. Assume a cement mixing rate of 25 sacks/min. Also assume an annular displacement rate no greater than 90 ft/min while the spacer is moving through the open-hole section and a flowrate of 300 gal/min thereafter. A safety factor of 1.0 hr is to be used. 

Therefore, the thickening time for the cement slurries to be used must be greater than 4.33 hr. 

The fluid loss and thickening time characteristics of the cement slurry is altered, either by increasing the molecular weight of the lignin by cross-linking with formaldehyde or epichlorohydrin or by adding agents such as sodium sulfite, sodium metasilicate, sodium phosphate, and sodium naphthalene sulfonate. 

Ilmenite has a specific gravity of 4700 kg/m. It requires no addition of water when added to the slurry. Ilmenite has a minimal effect on the thickening time and compressive strength. Barite requires more water then hematite when added to the cement. This results in a decrease of the compressive strength of the set cement. 

Special grades of gypsum hemihydrate are blended with Portland cement for types with reduced thickening time and setting time. Gypsum requires significantly more water addition. The maximal application temperatures are 70° to 80° C. Sodium silicate is used for cement slurries with diatomaceous earth. It can be used up to 7% by weight. 

Another aspect of cement slurry stability is the stability under dynamic conditions. Dynamic conditions are usually more severe than static ones because cement slurries are shear-thinning. This is a problem in the laboratory as the solid particles may settle while the fluid is being sheared (thickening time, rheology), and also in the field especially if the well is deviated from vertical. But there is currently no standard test in the industry to evaluate the stability of cement slurries under dynamic conditions. 

Figure 8. Thickening time curves. (a) Ideal curve corresponding to the beginning of cement setting and (b) increase in cement slurry consistency due to gellation before setting.

Stirring Paddle Devices. In an attempt to overcome these problems as well as the temperature limitation (85 °C) of standard oil field equipment, other devices, similar to the equipment used to measure the thickening time, have been developed (20). The cement slurry contained in a cylindrical cup is usually stirred with a paddle under pressure and temperature. This allows the simulation of the shear history encountered by the fluid during placement. Then, the rotational speed is reduced to a very low value—typically 0.003 rpm—and the torque on the paddle is measured as a function of time. The main advantages of such a technique is that measurements are performed under realistic conditions of pressure, temperature, and shear history. On the other hand, the analysis of the data is not straightforward as the stress distribution in these devices is not known, and it is not clear whether or not measurements are affected by wall slip layers. 

Weathered clinker and low Blaine fineness of cement Problems in thickening time and free water in oil-well cement (Reeves, Bailey, and McNabb, 1984)... 

Small alite crystals Burning of very fine rawfeed at lowtemperature silica feed particles less than 50 pm (Long, 1982a) short thickening time in oil-well cements (Long, 1984a)... 

It is often necessary to either accelerate, or retard the thickening and setting time of a cement slurry. 

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