*2.3. Characterization*

The functional groups of the PCM, WM, and MPCM-1 were analyzed via FT-IR spectroscopy (Perkin Elmer Frontier, Perkin Elmer Co., Ltd., Waltham, USA) using the standard KBr disk method (scanning number: 32; optical range: 400–4000 cm<sup>−</sup>1). The morphology of MPCM-1 without MG (MPCM-2) and MPCM-1 was observed using Zeiss stereoscopic optical microscopy (STEMI 508, Zeiss, Oberkochen, Germany). The NanoVoxel-3502E X-ray three-dimensional scanning system (micro-CT) (Sanying Precision Instrument Co., LT, Tianjin, China) was used to scan MPCM-1 (particle size > 200 mesh) placed in a cylindrical plastic mold. The core-wall structure of MPCM-1 (particle size > 200 mesh) was then assessed by analyzing the reconstructed MPCM-1 data using Voxel Studio Recon software (Sanying Precision Instrument Co., Ltd., Tianjin, China). A schematic diagram and digital photo of the micro-CT are shown in Figure 2.

**Figure 2.** (**a**) Entity devices and (**b**) schematic diagram of micro-CT.

The microstructure of MPCM-1 (particle size < 200 mesh) and the element distribution upon its surface were observed using SEM (Carl Zeiss Sigma 300, Zeiss, Oberkochen, Germany) and energy-dispersive X-ray spectrometry (Smartedx, Zeiss, Oberkochen, Germany) after the MPCM-1 had been dispersed on a conductive adhesive and sprayed with gold powder. Using the transient plane heat source method, the thermal conductivity of WM (mass fractions: 0%, 2%, 4%, 6%, and 8% MG) fabricated into 5 × 5 × 0.05 cm<sup>3</sup> films was analyzed by Hot Disk (TPS 2500S, Kegonas Instrument Trading Co., Ltd., Shanghai, China) at 10 ◦C. Here, three samples in each group were tested; the average of each group was then taken as the final test result. The phase change performances of MPCM-1, PCM, and MPCM-2 (i.e., the phase change temperature and latent heat) were analyzed using DSC (DSC3, ETTLER TOLEDO, Zurich, Switzerland) at a heating/cooling rate of 5 ◦C/min in the range 0–60 ◦C under a nitrogen environment. A thermal analyzer (TGA55, ETTLER TOLEDO, Zurich, Switzerland) was used to analyze the thermal stability of MPCM-1 and PCM at a heating rate of 10 ◦C/min in the range 20–800 ◦C under a nitrogen environment, after which the samples were maintained at 800 ◦C for 30 min. The MPCM-1 (5 g) and PCMs (5 g) were

wrapped with dust-free blotting paper and placed in an electric blast drying oven. The samples were heated and cooled in 20 cycles over a temperature range of 0–60 ◦C. The samples were then weighed, and mass loss rate was calculated to estimate the thermal reliability of MPCM-1.

#### *2.4. Preparation of Cement Slurry*

First, the low-heat and low-density cement slurry applied to GHBS was prepared using the formula provided by the CNOOC Zhanjiang Branch (Sanhe, China); this was taken as the control group and denoted as "CS." Second, 10 wt% slag and 10 wt% fly ash were introduced into CS; the resulting samples were denoted as CSS10% and CSF10%, respectively. Next, the 5 wt% phase change microcapsules (MPCM-3) purchased from Shanghai Feikang Products Factory were added into CS, and the result was denoted as CSM5%. Finally, 3.0 wt%, 5.0 wt%, and 7.0 wt% MPCM-1 were added into CS, and the resulting samples were denoted as CSM13%, CSM15%, and CSM17%, respectively. The function of the additives and the design of the cement slurry system are shown in Tables 3 and 4.

**Table 3.** Functions of cement slurry additives.



**Table 4.** Mixture composition of cement slurry system (g).

The additives in all cement slurry samples were adjusted according to GB/T 19139- 2012, and the density and rheological properties of the cement slurry are shown in Table 5.


**Table 5.** Physical properties of samples.

#### *2.5. Temperature and Hydration Heat of Cement Slurry*

First, the cement slurry samples (shown in Table 4) were prepared and stirred at a high speed of 4000 r/min for 20 s and 12,000 r/min for 45 s. Before reaching the GHBS, the cement slurry passes through a low temperature (4 ◦C) region near the mud line. Hence, cement slurry samples were placed in a 4 ◦C curing tank for 15 min and transferred to a self-made semi-adiabatic calorimeter that met the GB/T12959-2008 standard and had a constant temperature of 8 ◦C. Fran bottles filled with the cement slurry were sealed with plasticine at the top, to reduce the effect of air temperature and to prevent water from

entering the cement paste. Finally, the changes in cement slurry temperature and hydration heat release over 48 h were recorded.

#### *2.6. Compressive Strength of Cement Stone*

Following the GB/T 17671-2021 standard, the cement slurry samples were placed into the standard mold (40 × 40 × 40 mm3) and cured at a constant temperature in a humidity curing box (humidity: 90%) and at temperatures of 8 ◦C, 10 ◦C, and 15 ◦C for 24 h. The average compressive strengths of the three samples in each group were then measured using a compression and flexural testing machine (ZCYA-W300C, Star Fire Testing Machine Co., Ltd., Jinan, China).

#### *2.7. Spatial Distribution of Pores and MPCM-1 in Cement Stone*

First, the cement slurry samples were placed into a standard mold (20 × 20 × 20 mm3) and cured in a constant temperature and humidity curing box (humidity: 90%; temperature: 8 ◦C) for 24 h. Micro-CT was then used to scan the spatial distribution of pores and MPCM-1 in cement stone. The dates were recorded after reconstruction using VG Studio Max software (scanning accuracy: 15.63 μm; voltage: 120 kV; current: 110 μA).

#### *2.8. Simulation of GHBS Damage*

The self-made device used to simulate the cementing process in the GHBS is shown in Figure 3. First, the simulated GHBS was formed by injecting methane gas into a formation cavity preloaded with 30 g of tetrahydrofuran, 770 mL of deionized water, and 8 kg of sand particles under increasing pressure and decreasing temperature. The cement slurry samples (800 mL) were injected into the slurry cavity. The damage of the simulated GHBS during cement slurry hydration was estimated using temperature and pressure sensors.

**Figure 3.** (**a**) Structure chart and (**b**) digital photo of device used to simulate cementing in GHBS.

#### **3. Results and Discussion**

#### *3.1. Infrared Spectrum Analysis of MPCM-1*

The infrared spectra of the PCM, WM, and MPCM-1 are shown in Figure 4.

For the FT-IR spectra of PCM, the absorption peak at 1464 cm<sup>−</sup><sup>1</sup> corresponds to the antisymmetric vibration of −CH3, and the absorption peak at 720 cm<sup>−</sup><sup>1</sup> corresponds to the rocking vibration peak of −(CH2)4. In the infrared spectrum of WM, the strong absorption at 3429 cm<sup>−</sup><sup>1</sup> corresponds to the stretching vibration peak of −OH and −COOH dimer OH. The absorption peak at 1731 cm<sup>−</sup><sup>1</sup> corresponds to the umbrella bending vibration peak of −CH3, and the absorption peak at 1270 cm<sup>−</sup><sup>1</sup> corresponds to the stretching vibration band of -O-C(O)-C [32]. Notably, the FT-IR spectrum of MPCM-1 contains all the characteristic absorption peaks of PCM and WM, and no new absorption peaks appear. Therefore, the preliminary results demonstrate that the energy storage density of the PCM has not been lost during microencapsulation, and MPCM-1 consists of PCM and WM.

**Figure 4.** FTIR spectra of core materials (PCM), wall material (WM), and MPCM-1.

#### *3.2. Microstructural Analysis of MPCM-1*

Figure 5 shows the stereomicroscope views of MPCM-1.

**Figure 5.** Stereomicroscope views of (**a**) MPCM-2, (**b**) MPCM-1 section, (**c**) MPCM-1, and (**d**) MPCM-1 of different sizes.

It can be observed from Figure 5 that the PCM was successfully encapsulated by the WM without the addition of MG. The 3D image model and slice images of MPCM-1 are presented in Figure 6. The images show that microcapsules combined with MG also exhibit an excellent core-wall structure; furthermore, the dense surface of WM helped to suppress PCM leakage during the phase transition.

**Figure 6.** 3D image model and slice images of MPCM-1 (particle size > 200 mesh): (**a**) 3D image models, (**b**) XY plane slice, (**c**) YZ plane slice, and (**d**) XZ plane slice.

Besides this, the microcapsules prepared from the same batch with different particle sizes (Figure 5d) indicate that the mechanical strength of cement stone can be enhanced by varying the gradation relationship between the cement particles and MPCM-1, with suitable particle sizes isolated via screening [33–35]. The SEM micrographs of MPCM-1 (particle size < 200 mesh) are presented in Figure 7.

**Figure 7.** SEM micrographs of MPCM-1 (particle size < 200 mesh): (**a**) ×6500 and (**b**) ×5000. (**c**) EDS analysis. (**d**) Surface element content of MPCM-1.

The small-particle MPCM-1 appears easier to agglomerate but exhibits superior sphericity. The surface element content of MPCM-1 (shown in Figure 7c) demonstrates that the mass fractions of C and O are 35.14% and 64.86%, respectively; this conforms to the composition of C and O elements in the wall material (PMMA). Combined with the FT-IR analysis results, this provides strong evidence for the packaging effect of WM on PCMs, which allows them to satisfy design specifications.

#### *3.3. Phase-Change Properties of MPCM-1*

Thermal conductivity is an important factor in heat control; it affects temperature sensitivity and latent heat storage and release rates for MPCM-1 in the cement slurry. However, the thermal conductivity of organic PCMs is generally lower than that of cement slurry. In this study, the thermal conductivity of MPCM-1 was improved by adding microcrystalline graphite (which is inexpensive); furthermore, the lipophilic modification of

graphite was performed to evenly disperse it in MPCM-1. Figure 8 shows the compatibility of MG with water before and after lipophilic modification.

**Figure 8.** Comparison of hydrophobicity between (**a**) MG and (**b**) microcrystalline graphite.

The hydrophobicity of the MG increased markedly compared with that of the graphite, owing to the increase in the number of lipophilic groups on the surface. The improvement of MG lipophilicity was achieved via two stages: chemical modification and physical coating. The surface of the microcrystalline graphite contained hydroxyl groups [36,37] which could be esterified with stearic acid under aluminum chloride catalysis. By ester bonding, the carbon chain of stearic acid was partially distributed on the surface of the microcrystalline graphite, and the chemical modification was completed. During the drying process for microcrystalline graphite, the polymer (after esterification reaction) and stearic acid (without chemical reaction) were gradually precipitated out and coated on the surface of the microcrystalline graphite to produce a physical coating modification. Furthermore, the WM (with different mass fractions of MG) pressed into the films (as shown as Figure 9), and the hydrophobicity of the graphite after modification notably increased, allowing it to be uniformly dispersed in the WM.

**Figure 9.** WM films at different MG dosages.

The thermal conductivity analysis of the WM is presented in Figure 10.

**Figure 10.** Thermal conductivity of WM films with modified MG.

Under the increase in MG mass fraction, the thermal conductivity of WM was considerably improved. However, MG entering the PCM reduced the PCM content per unit volume, lowering the thermal storage efficiency; therefore, a 4 wt% addition of MG was selected to optimally improve thermal conductivity.

The phase-change properties of PCM, MPCM-1, and MPCM-2 were tested, and the results are presented in Figure 11 and Table 6.

**Figure 11.** DSC curves of (**a**) PCM, (**b**) MPCM-1, and (**c**) MPCM-2.

**Table 6.** Phase-change behaviors.


Both PCMs and MPCM-1 had high latent heat and a wide phase transition temperature range, owing to the mixed dissolution of n-tetradecane and n-hexadecane; this indicates that hydration heat was continuously absorbed by MPCM-1 over a large temperature range during the cement slurry hydration process. The phase-change temperature range, peak temperature, and latent heat of MPCM-1 were 8.99–16.74 ◦C, 13.77 ◦C, and 153.58 Jg−1, respectively. Compared to MPCM-2, it is found that the initial phase-change temperature of MPCM-1 is 0.49 ◦C earlier, due to the addition of modified MG, and its temperature sensitivity was significantly enhanced. Moreover, only one phase transition peak appeared in each DSC curve, owing to the eutectic effect of the PCMs composed of n-tetradecane and n-hexadecane in this composition ratio during the phase transition process [20,38]. The formula for calculating the encapsulation efficiency Ee of MPCM-1 is [7,29]:

$$\mathrm{E\_e} = \frac{\Delta \mathrm{H\_m(MPC-1)}}{\Delta \mathrm{H\_m(PCM)}} \times 100\% \tag{1}$$

where To, Tp, and Te are the initial phase-change temperature, the peak phase-change temperature, and the end phase-change temperature in the melting process, respectively; ΔHm is the latent heat during the melting process, for which ΔHm(MPCM-1) and ΔHm(PCM) are the latent heat of MPCM-1 and PCM during the melting process, respectively. Using this formula, the encapsulation efficiency of MPCM-1 is determined as 47.2% which may be lower than that of some other studies (the Ee is 67.8% and 66.5%, respectively) [7,29]; however, the lower encapsulation efficiency within a reasonable range ensures the ability of MPCM to resist external loads, which facilitates the mechanical strength of cement stone incorporated with *MPCM* [39].

#### *3.4. Thermal Reliability of MPCM-1*

Taking oil and gas resources in the South China Sea as an example, GHBS is typically present in the shallow surface of the mud line, with a water depth of 300–2000 m [40,41]. Cement slurry for deep-sea GHBS undergoes the following process: "seawater → GHBS → deeper formation → GHBS", during which the ambient temperature changes continually. In addition, MPCM-1 experiences frequent temperature variations during transport and storage on offshore platforms. The coating integrity of MPCM-1 under temperature change conditions is extremely important for ensuring its heat storage efficiency and the sealing performance of cement sheaths, which may be affected by PCM leakage. Hence, the thermal reliability of MPCM-1 was analyzed via TG and thermal cycles.

TGA and DTG curves of MPCM-1 and PCMs are shown in Figure 12.

**Figure 12.** TGA curve and DTG curve of (**a**) PCM and (**b**) MPCM-1.

It can be observed that the initial and total mass loss temperatures of MPCM-1 were 117 ◦C and 409 ◦C, respectively; furthermore, when the temperature reached 262 ◦C, the mass loss rate was maximized at 0.613/◦C. Compared with PCMs, the initial and maximum mass loss rates of MPCM-1 were higher and slower, respectively, because of the coating effect of the WM on the PCMs [42].

The samples of the thermal cycle experiment are shown in Figure 13, and the test results are shown in Table 7.

**Figure 13.** Digital photographs of (**a**) initial samples and (**b**) samples after thermal cycles.

**Table 7.** Mass loss of MPCM-1.


After five thermal cycling experiments, the mass loss rates of MPCM-1 and PCM were 10% and 100%, respectively. At this stage, the PCMs rapidly melted and leaked during phase change. Meanwhile, the quality of MPCM-1 also decreased significantly because several PCMs were not completely coated inside the microcapsule, and some were only embedded in the grooves on the WM surface. After twenty thermal cycles, the mass loss rate of MPCM-1 was 16%. The mass loss rate of MPCM-1 increased slightly because most of the remaining PCMs at this stage underwent a phase transformation in the WM, and the morphology of the PCMs remained essentially stable. These results further verify that WM can effectively prevent the leakage of PCMs and ensure their heat storage density.

#### *3.5. Heat Control Effect of MPCM-1*

Figures 14 and 15 show the temperature change and hydration heat release of each cement slurry sample, respectively, as obtained using the self-made semi-adiabatic calorimeter.

Table 8 shows the values corresponding to the curves.

**Table 8.** Temperature and heat of hydration data for cement slurry.


**Figure 14.** Hydration temperature of cement slurry samples.

**Figure 15.** Hydration heat of cement slurry samples.

Compared to slag and fly ash, MPCM-1 has a more notable inhibitory effect on cement slurry temperature increase; this is reflected in the fact that the Tmax of CSM17% was 6.5 ◦C and 4.9 ◦C lower than those of CSS10% and CSF10%, respectively. Simultaneously, the Tmax of CSM17% appeared later than those of CSS10% and CSF10%. Similar results were also obtained in the research of Huo et al. [30]. They suggested that MPCM-1 not only replaced the active component of cement particles (similar to the function of slag and fly ash) but also absorbed heat released by the cement slurry during phase transformation. Compared to CSM, the maximum temperature and hydration heat of CSM1 were both lower, owing to the higher storage density of MPCM-1. In Figure 15 and Table 8, a significant positive correlation can be identified between the retarding effect of each low-heat material on the hydration heat release of the cement slurry; the effect of this on the temperature increase can also be seen. Moreover, the rate of increase in hydration heat for CSM1 was significantly faster than that for CS when the cement slurry temperature fell and approached the water bath temperature; this resulted from heat absorption during the liquid–solid phase transition of MPCM-1. Furthermore, compared with CS, the Tm and Qm of CSM17% decreased by 9.7 ◦C and 25,509 J (41.3%), respectively, and Tm was extended by 5.9 h. The results show that MPCM-1 can effectively control the temperature increase and heat release rate of cement slurry, indicating that MPCM-1 has excellent prospects as a new additive for low-heat

cement slurry. Huo et al. [43] conducted a similar study. The results showed that the peak temperature of cement slurry decreased by 6 ◦C with 7.5% MPCM (prepared from urea-formaldehyde resin coated paraffin) dosage. The reasons why the ability of MPCM in the study of Huo et al. to regulate the temperature of cement slurry was worse than that of this study, is that MPCM-1 can quickly sense the temperature change in cement due to the incorporation of high thermal conductivity materials, and that PCMs' phase transition occurs in time to complete the endothermic process [44]. Here, Ti, Tm, and ΔT represent the initial temperature, maximum temperature, and maximum temperature difference for the cement slurry, respectively; furthermore, tm denotes the time taken for the cement slurry to reach the maximum temperature. Q20h and Qm denote the heat release in 20 h and the final heat release of the cement slurry.

#### *3.6. Comprehensive Strength of Cement Stone*

In this section, the effects of MPCM-1, low-heat inorganic materials, and as-purchased microcapsules upon the comprehensive properties of cement stone are studied. As GHBS is soft and the pressure window between fracture and formation-pore pressure gradients is narrow, a low-density cement slurry system is usually used to cement the well to prevent loss of circulation [45–47]. As shown in Table 5, the reduction effect of MPCM-1 upon the initial density of cement slurry was more notable than other low-heat materials. Thus, the MPCM-1 cementing system conforms to this design philosophy [48]. Compared with CS, the fluidity of CSM1 decreased slightly within the allowable range. Wang et al. [39] found that the reason for the decrease in fluidity of cement slurry is that the particle size of MPCM is generally larger than that of cement particles, which enhances the friction between particles in cement slurry. The compressive strength of cement increased under the increase in MPCM-1 dosages within a certain range (see Figure 16).

**Figure 16.** Compressive strength of cement stone after 24 h at 8 ◦C, 10 ◦C, and 15 ◦C.

Compared with CS, the compressive strength of CSM13%, CSM15%, and CSM17% cured at 15 ◦C for 24 h increased by 5.5%, 9.7%, and 13.8%, respectively. This occurs because PMMA (the shell of MPCM-1) is rigid; thus, it plays the role of an aggregate, which allows the cement stone of CSM1 to withstand higher external loads [49,50]. However, the results of Aguayo et al. [51] showed that a 5% dosage of microcapsules increased the number of pores in the cement stone, reducing its compressive strength by 12.5%. Ghods et al. [52,53] classified the pores inside the cement stone in terms of size and shape; they suggested that the macropores were the main factor affecting its compressive strength. Therefore, the reduction of large volume pores in cement stone may also represent an important factor when improving the compressive strength of CSM1. The spatial distribution of pores in the cement stone is studied in Section 3.7.

#### *3.7. Distribution Analysis of MPCM-1 and Pores in Cement Stone*

Figures 17 and 18 and Table 9 show the distribution of pores and MPCM-1 in cement stone, as obtained via micro-CT analysis.

**Figure 17.** Slice images and 3D image model of the pores in CS.

**Figure 18.** Slice images and 3D image model of the pores and MPCM-1 in CSM15%.

**Table 9.** Total volume of MPCM-1, all pores, and the pores in MPCM-1.


MPCM-1 is evenly distributed in the cement stone without agglomeration, which is amenable to the uniform heat control effect of MPCM-1 upon the cement slurry hydration process. Moreover, no clear arc-shaped pores (resulting from the incompatibility of MPCM-1 with cement slurry) were found around MPCM-1, and the porosity of cement in CSM1 was approximately equal to that of CS. Further, through reconstruction analyses, the volumes of MPCM-1 and its internal porosities were found to be 269.4997 mm<sup>3</sup> and 36.2646 mm3, respectively. Calculated to be 13.4%, the porosity in MPCM-1 was already high, though it was lower than the true value, owing to accuracy limitations of the micro-CT scan.

Therefore, reducing the internal porosity of MPCM-1 should be considered in subsequent studies to improve its heat storage density and mechanical strength.

The voxel is the smallest volume unit that can be represented by a pixel after micro-CT reconstruction. The cumulative probability of voxel numbers in the pores inside the cement stone of CS and CSM15% is shown in Figure 19.

**Figure 19.** Cumulative probability of pore voxels in CS and CSM15% cement.

The voxel of the largest pore in CSM15% cement was 10.61 (i.e., 0.155 mm3). Furthermore, it was calculated that 27 pores with a volume exceeding 0.155 mm<sup>3</sup> were present in CS, among which the maximum pore volume was 15.005 mm3. Compared with CS, more small volume pores and fewer large volume pores were present in the cement stone of CSM15%. Two factors may contribute to the reduction in the number of large volume pores: (1) the MPCM-1 increases the number of nucleation sites for the precipitation hydration products [39], and (2) the MPCM-1 (with a small particle size) plays a role in filling the pores. Therefore, MPCM-1 can (similar to defoamers) reduce the occurrence of large volume pores in the cement stone; this improves the mechanical strength of the cement stone [54,55]. Here, the side length of the unit voxel is 15.63 μm. Furthermore, micro-CT analysis at this precision shows that 447,743 and 501,657 pores are present in the CS and CSM1 cement stones, respectively.

#### *3.8. Simulation of GHBS Damage*

To further evaluate the protective function of a low-heat cement slurry system with MPCM-1 (to function as the key temperature control for GHBS stability), a device simulating the cementing operation of GHBS was designed and prepared (as shown in Figure 3). As the change in pressure during hydrate decomposition was considerably more drastic than the temperature change, the large pressure increase was selected as the standard by which to quantify the damage to GHBS stability.

The influence of the CS hydration heat on GHBS was observed, as shown in Figure 20.

**Figure 20.** Damage of GHBS caused by hydration heat of CS.

In the first 7.5 h, owing to the slow increase of CS temperature in the initial hydration reaction [56], GHBS did not decompose under the thermal interaction between CS and GHBS. During this time, the temperature and pressure of GHBS remained essentially unchanged. Subsequently, the hydration process of CS entered an accelerated period [56], and the temperature increase in CS became more rapid. Owing to the enhanced heat exchange between CS and GHBS, the phase equilibrium condition for GHBS was destroyed, and the hydrate began to decompose. GHBS pressure increased significantly at 9.5 h, which continued until the end of the experiment, indicating that the stability of GHBS was completely damaged by the hydration heat release of CS.

As shown in Figure 21, under the hydration exothermic process of CSM15%, the GHBS temperature and pressure began to increase significantly after 7 h via heat exchange between the CS and GHBS.

**Figure 21.** Damage of GHBS caused by hydration heat of CSM15%.

When GHBS pressure reached the peak at 11.5 h, the ambient temperature and pressure were restored to the phase equilibrium via the pressure increase and temperature decrease caused by hydrate dissociation, after which the hydrate stopped decomposing. Meanwhile, the hydration exothermic rate of CSM15% gradually began to fall below the cooling rate, and the temperature of the cement slurry dropped. From 14.5 h, GHBS pressure began to decline until the GHBS reached a stable state at 28 h. At this stage, the GHBS pressure drop was attributable to re-formation of the lattice, in which the cavities that were trapped previously decomposed gas molecules and formed hydrate crystals. Compared to CS, the overall temperature increase rate of CSM15% was slower, and the temperature increased

less, which was conducive to the phase equilibrium recovery within the GHBS [2]. These results indicate that the addition of MPCM-1 can successfully reduce the damage to GHBS during heat release in cement slurry.
