**1. Introduction**

In offshore oil and gas well cementing, natural gas hydrate-bearing sediment (GHBS) is often present at the well hole. The hydration heat released during the consolidation of cement slurry changes the stable phase equilibrium state of the hydrate (i.e., to a high pressure and low temperature) [1–3]. When this occurs, the gasified natural gas hydrate enters the cement slurry, which affects the sealing quality of the unhardened cement sheath [4]. In serious cases, the cementing operation may not succeed and a blowout accident could occur [5,6]. Therefore, to prevent accidents and ensure cementing quality in offshore cementing engineering, a cement slurry with a hydration-heat regulation function is urgently required [7,8]. One logical solution to this problem is to introduce phasechange materials (PCMs) as admixtures into the cement slurry, which would adjust its heat of hydration.

**Citation:** Yang, G.; Liu, T.; Zhu, H.; Zhang, Z.; Feng, Y.; Leusheva, E.; Morenov, V. Heat Control Effect of Phase Change Microcapsules upon Cement Slurry Applied to Hydrate-Bearing Sediment. *Energies* **2022**, *15*, 4197. https://doi.org/10.3390/ en15124197

Academic Editor: Andrea Frazzica

Received: 16 May 2022 Accepted: 6 June 2022 Published: 7 June 2022

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Owing to their ability to release or absorb heat during phase changes, and thereby maintain a stable system temperature, PCMs have been implemented in numerous other fields including thermal energy storage [9–11]. PCMs have been introduced as an additive to adjust the heat release rate of concrete with the aim of suppressing temperature cracks and achieved remarkable results [12,13]. However, the increase in fluidity of PCMs during phase change brings hidden danger to the safety of building gel materials [14,15]. Microencapsulated PCMs (MPCMs), which comprise a PCM (as the core material) encapsulated in a dense polymer or insoluble precipitation, are typically applied to address this problem, not only to retain a stable PCM shape during phase transitions but also to avoid incompatibility between the PCM and cement gel materials [16–18]. Usually, the phase change materials in MPCMs can be divided into two categories: inorganic hydrates and organic matter (such as alkanes, fatty alcohols, and fatty acids). Due to the characteristics of hydrated salt removing bound water step by step, when it is used as a PCM, it often has a wide temperature control range. For example, disodium hydrogen phosphate dodecahydrate can store heat in the temperature range of 25–75 ◦C [19]. In the process of offshore cementing, the cement slurry will experience the continuous changing temperature environment of 'seawater— GHBS—deep formation—GHBS', so the cement slurry additive is required to have excellent thermal stability. However, the well-known disadvantage of hydrate salt is that it is easy to precipitate and undercooled, which will damage its heat storage density in complex temperature environments. [20–22]. Compared with hydrated salts, organic PCMs exhibit better thermal stability [23–26]. Sari et al. [27] prepared MPCM by using heptadecane as a PCM, and tested its thermal stability. The results show that the latent heat of the prepared MPCM decreases by only 3.8% after 1000 phase transformations. Yu et al. [28] have also prepared PCMs with good thermal stability using octadecane as a PCM. However, their studies have pointed out that low thermal conductivity of organic compounds may reduce the sensitivity of MPCM to temperature. Furthermore, Liu et al. [29] microencapsulated paraffin with calcium carbonate as a shell to improve the temperature sensitivity of PCM by increasing the heating area of paraffin and the high thermal conductivity of calcium carbonate. Moreover, they tried to use MPCM to reduce the temperature rise rate of oil well cement slurry used in GHBS cementing. The results show that the hydration heat of cement slurry with 12 wt % MPCM decreases by 45.67%. To summarize, it seems feasible to introduce MPCMs into low-heat cement slurry as a more efficient heat-control additive, which might effectively avoid hydrate decomposition caused by high exothermic rates of cement slurry. In addition, to our knowledge, no clear research method has directly verified that the addition of MPCM will inhibit the damage of hydration heat of cement slurry on GHBS, which makes it obviously difficult to confirm the feasibility of MPCM application in this field.

In this study, a heat-control microcapsule (MPCM-1) with a mixed alkane core and polymethyl methacrylate (PMMA) shell was developed via in situ polymerization. The structures diagram of MPCM-1 is shown in Figure 1.

**Figure 1.** Structure diagram of MPCM-1.

Owing to the low thermal conductivity of alkanes and PMMA, lipophilic-modified graphite was introduced into MPCM-1 to improve its temperature sensitivity. Furthermore, the prepared MPCM-1 and other low-heat additive materials (e.g., fly ash, slag, and as-purchased MPCM) were added into the oil well cement slurry system to investigate their effects upon hydration temperature increase rates and maximum hydration heat. Based on the characteristic that gas channeling occurs easily during cementing in GHBS, the compressive strength and pore distribution of cement stone are studied, which are rarely focused on by other studies [29–31]. Finally, one of the innovations of this work is that a device to simulate the cementing process was designed, to intuitively evaluate the impact of low-heat cement slurry hydration upon the stability of hydrate-bearing sediments. This study verifies the feasibility of low-heat cement slurries containing phasechange microcapsules for safe cementing in hydrate-bearing sediments, which represents an important step forward in this area of research.

#### **2. Experimental Section**

## *2.1. Materials*

N-tetradecane [analytical reagen<sup>t</sup> grade (AR), 99%], n-hexadecane (AR, 98%), benzoyl peroxide (AR, 99%), polyvinyl alcohol (AR, 99%), and stearic acid (AR, 98%) were purchased from Aladdin Reagents (Shanghai, China) Co., Ltd. Aluminum chloride hexahydrate (AR, 97%) and absolute alcohol (AR, 99%) were purchased from Shanghai Meirel Chemical Technology Co., Ltd. (Shanghai, China). Microcrystalline graphite was purchased from Dingsheng Xin Chemical Co., Ltd. (Tianjin, China) Deionized water was self-made.

Class G oil-well cement and additives were provided by CNOOC (Sanhe, China) Co. Ltd. Fly ash and slag were purchased from Shanghai Weishen New Building Materials Co., Ltd. (Shanghai, China). The chemical compositions of Class G oil-well cement, fly ash, and slag are listed in Table 1.

**Table 1.** Chemical composition of cement, fly ash, and slag (%).


In addition, several typical particle sizes are listed in Table 2.



#### *2.2. Synthesis of MPCM-1*

2.2.1. Lipophilic Modification of Microcrystalline Graphite

First, 0.2 g stearic acid was dissolved in 20 mL absolute alcohol, and 0.22 g aluminum chloride hexahydrate was dissolved in 100 mL deionized water. The ethanol solution was then mixed with aqueous solution to form an emulsion, and 4 g microcrystalline graphite was added. The mixture was placed in a water bath with a temperature of 60 ◦C and stirred continuously at a rate of 100 r/min for 1.5 h. Finally, the modified microcrystalline graphite (with a rich lipophilic group on the surface) was obtained via suction filtration and drying. During this period, the filter material was not washed in order to prevent removal of the polymer material and suppression of its physical coating effect on the microcrystalline graphite during drying.

#### 2.2.2. Synthesis of MPCM-1

First, 3 g polyvinyl alcohol was dissolved in 100 mL deionized water at 90 ◦C. Subsequently, 0.2 g benzoyl peroxide was dissolved in 10 mL methyl methacrylate (the inhibitor having been washed away by 1 wt% NaOH solution), and 0–0.8 g of lipophilic modified graphite (MG) was added and stirred evenly to produce a wall material solution (WM). Next, n-tetradecane and n-hexadecane were mixed and fused in a mass ratio of 1:4 to form 10 g of the phase change core material (i.e., the PCM). To prevent early polymerization of the methyl methacrylate (caused by high temperature or low temperature-induced solidification of PCM), the polyvinyl alcohol solution (100 mL), WM mixture (10 mL), and PCM solution (10 g) were kept at 50 ◦C for 10 min before being added together in a three-pot flask (500 mL). The water bath environment was then adjusted to 60 ◦C for 15 min (prepolymer process) and 80 ◦C for 1.5 h; the stirring speed was kept at 200 r/min. Finally, MPCM-1 was obtained by suction filtration, washing, and drying.
