Rheology of cement slurry

The rheology of fluids is an extremely important factor in the petroleum industry due to the amount of fluid transport happening (mud/cement pumping, crude oil producing, wet gas producing etc). When circulating e.g cement it is of utmost importance to know its rheological properties to be able to accurately calculate needed pump pressure, rate and other parameters.

Performing measurements of the cement slurries are therefore of great importance, and the SiO2+MWCNT-COOH system was tested simultaneously with a zero-additive control slurry to characterize its properties.

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Shear stress, lbf/100sqft

Shear stress

In figure 4.48 the shear stress of the two systems are plotted together, and it shows that the system containing nanoparticles exhibits higher shear rheology compared to the control system for almost all measured points. At very low shear rate (5,1 sec-1) the shear stress of the nano-system is lower than the control, but for all other points it is higher. From the figure one can also observe that the difference in shear stress increases at higher shear rates, and the largest difference is observed at the shear rate of 510,9 sec-1.

Figure 4.49 Casson yield stress of the two tested systems

The figure above depicts the Casson yield stress of the two tested systems. One can see that the system containing nanoparticles is exhibiting a lower yield stress than the control system. In essence, this means that the cement slurry containing nanoparticles would require less force to set in motion which could be beneficial when e.g setting cement plug via wiper darts. Having a lower yield stress could mean allowing more of the available pump power to be used for other actions and is generally a positive property.

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Reference Nanosystem

Casson yield stress, lbf/100sqft

Casson yield stress

Figure 4.50 Casson plastic viscosity of the two tested systems

The figure above showcases the Casson plastic viscosity of the measured systems. One can observe that the PV of the system containing nanoparticles is higher than the control, which means it has a higher resistance to flow. This equates to larger friction in the pipe and thus it requires more pumping power. In addition, it means that the cement is thicker and ensures better solids transport and hole cleaning, which could be beneficial in a cementing job. Usually, washers and spacers clean out the area which is meant to be cemented, but this will not always be completely clean. A high viscosity cement could help further clean the area and properly displace the aforementioned washers to ensure a high-quality tail cement.

Naturally, different wells require different rheological properties of cement, and this can be found through simulation software. To determine whether these rheological properties are good or not for a certain scenario, one would have to run simulations, but due to time constraints this work was not completed in this thesis.

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Reference Nanosystem

Casson plastic viscosity, cP

Casson plastic viscosity

4.3.4 Heat development

When cement is mixed with water, an exothermic reaction occurs which liberates heat. The heat development of Portland cement has several stages and has been studied through this experiment. Two temperature loggers were placed inside the cement and measured the temperature of the cement every 5 minutes for approximately 72 hours during the hydration process. The slurries which were studied were a control slurry containing zero additives, and a slurry which contained the SiO2 and MWCNT-COOH nano-additives. Both systems used a WCR of 200/454=0,44 and were subjected to rheological testing prior to being placed in the insulated box for measurement of heat development. The various stages of heat development for Portland cement is covered in chapter 2.2

Figure 4.51 Heat development as a result of cement hydration

From figure 4.51 one can observe that the whole of the preinduction period is not plotted, as that stage only lasts for a few minutes and the cement slurry was being tested before the temperature logging commenced. As a result of this the logging starts in stage 2 with no heat evolution. The heat buildup starts after approximately 4 hours, and peaks at the end of stage 3/beginning of stage 4. The peak temperature for the zero-additive cement is measured at 48ºC

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Temperature (degrees Celsius)

Time (hours)

Heat development

Reference Nano

and the highest temp for the nano-system is recorded as 50ºC. During the diffusion period (stage 5), very little differences between the systems are observed.

The temperature loggers used could only measure temperatures in 0,5ºC increments, and as a result of this the measured temperatures includes some inaccuracies. Furthermore, the same amount of cement was used for both tests, and the amount of cement should therefore not cause any differences in temperatures as explained by PCA [45].

Previously conducted studies [53] suggest that the addition of SiO2 nanoparticles may shorten the dormant period (stage 2), but this is not evident from the study conducted in this thesis. The results does indicate that the added nanoparticles may act as a nucleation site due to their large reactive surfaces, and thus accelerating the hydration process which results in a more complete hydration after a certain amount of time which can be seen in figure 4.51 as an increased temperature relative to the control.