(4) Thermogravimetric (TG-DTG) Analysis

The TG-DTG curves of CPC and DPC are shown in Figure 4. From Figure 4, it can be seen that the thermal stability of CPC and DPC is very similar. Within the temperature range of 208℃ to 459℃, both exhibit two essentially identical weight loss intervals.The first weight loss zone is between 208~320 ℃, where the mass rapidly decreases for the first time, losing about one-third of its mass. This phenomenon is caused by partial breakage of the long side chains of the polymer polyethylene glycol;The second weight loss zone is between 320 and 459℃, during which the polymer loses the majority of its mass due to the fragmentation of the entire polymer molecule.The results of thermogravimetric analysis indicate that both CPC and DPC have good thermal stability at temperatures not exceeding 208℃.

2.2 Performance Evaluation of Cement Slurry

(1) Liquidity

The flowability of cement slurry reflects the strength of the yield stress of the slurry. The stronger the network structure between cement particles, the greater the yield stress and the lower the flowability.Figure 5 shows the flowability of cement slurry with different dosages of CPC or DPC. Before reaching saturation dosage, there is a linear growth relationship between dispersant dosage and flowability. The more dosage, the greater the flowability. After reaching saturation dosage, the flowability of the slurry no longer changes.When the dosage is less than 0.40% bwoc (% bwoc is the percentage of cement mass, the same below), the flowability of CPC containing cement slurry is slightly higher than that of DPC containing cement slurry with the same dosage;When the dosage of CPC is greater than 0.25% bwoc, it basically reaches the saturation dosage, while DPC requires slightly more dosage. When the dosage of DPC is greater than 0.30% bwoc, it can only reach the saturation dosage, indicating that DPC has slightly lower dispersion efficiency than CPC, and DPC needs a little more dosage to achieve the same dispersion and drag reduction effect as CPC.

(2) Rheological Properties

The rheological curves of cement slurry at different temperatures are shown in Figure 6. By using the Herschel Bulkley three parameter rheological model to fit the rheological curves, the fitting equation and rheological parameters τ₀, n, and K can be obtained, as shown in Table 2. τ₀ is the static shear stress, which represents the minimum shear stress required for the fluid to start flowing;n is the fluidity index, reflecting the non Newtonian nature of the fluid. 0K is the viscosity coefficient, which reflects the viscosity of the fluid. The greater the internal friction between adjacent liquid layers during fluid flow, the higher the viscosity coefficient K value.At 20℃, the cement paste is a pseudoplastic fluid with high static shear stress τ₀ and viscosity coefficient K. After adding 0.25% bwoc CPC or DPC, the viscosity significantly decreases, with n values close to 1 and τ₀ almost zero. The paste is similar to a Newtonian fluid.Comparing cement slurries at different temperatures, it can be seen that as the temperature increases, the hydration rate accelerates, the overall viscosity of the cement slurry significantly increases, and the τ₀ increases rapidly, resulting in severe deterioration of rheological properties.The cement slurry containing CPC or DPC, although slightly increased in τ₀, still maintains good rheological properties and exhibits good temperature resistance.

(3) Thickening Curve

The thickening curve of oil well cement slurry is a comprehensive reflection of parameters such as thickening time, initial viscosity, transition time (40-100Bc), and thickening line shape, which is of great significance for ensuring cementing safety and improving cementing quality.The thickening curves of cement slurry containing different dispersants (85℃, 46.2MPa, 50min) is shown in Figure 7.The initial viscosity of cement paste without dispersant is 9.2 Bc, which is unstable and increases with temperature. The thickening time is about 71 minutes.The initial consistency of the cement slurry added with DPC is 3.5Bc, and the overall consistency is stable before reaching the thickening time, which is more conducive to cementing construction.Compared with net cement paste, the thickening time of cement paste mixed with 0.25% bwoc DPC is about 79 minutes, slightly longer than that of net cement paste. The ratio of thickening time between the two is 1.11:1.00.The thickening time of cement slurry mixed with 0.25% bwoc CPC is about 168 minutes, and the ratio of thickening time to net slurry is 2.37:1.00, showing a strong retarding effect.The results indicate that conventional anionic polycarboxylates can cause strong retarding effects and are not suitable for oil well cement. The use of quaternary ammonium cations to partially replace carboxylic acids in the synthesis of zwitterionic polycarboxylates provides a solution to this problem.

(4) Compressive Strength

The compressive strength of cement paste containing different dispersants cured at 85℃ for 24 hours, 72 hours, and 168 hours is shown in Figure 8.Whether or not dispersants are added, the compressive strength of cement stone gradually increases with the extension of curing time.The compressive strength of cement paste containing DPC is higher than that of net cement paste with the same curing time, increasing by 7.0%, 9.7%, and 10.7% respectively;The 24-hour compressive strength of cement stone containing CPC decreased significantly, and with the extension of curing time, the compressive strength gradually increased compared to cement stone without dispersants.The results indicate that DPC is beneficial for the strength development of cement paste at different stages, while CPC, due to its strong retarding effect, leads to a decrease in early strength. However, this negative effect on strength will gradually disappear with prolonged curing time.