Inhibiting hydrate decomposition due to the friction heat generated by the drilling tools is one of the key factors for drilling hydrate formation. Since the existing method based on chemical inhibition technology can only delay the hydrate decomposition rate, a phase-change microcapsule was introduced in this paper to inhibit, by the intelligent control of the drilling fluid temperature, the decomposition of the formation hydrate, which was microencapsulated by modified n-alkane as the core material, and nano-silica was taken as the shell material. Scanning electron microscope (SEM), size distribution, X-ray diffraction (XRD), and Fourier transform infrared spectrometer (FT-IR) were utilized to characterize the structural properties of microcapsules. Differential scanning calorimetry (DSC) spectra displayed that the latent heat was 136.8 J/g in the case of melting enthalpy and 136.4 J/g in the case of solidification enthalpy, with an encapsulation efficiency of 62.6%. In addition, the prepared microcapsules also showed good thermal conductivity and reliability. By comparison, it was also proved that the microcapsules had good compatibility with drilling fluid, which can effectively control the temperature of drilling fluid for the inhibition of hydrate decomposition.

There are abundant natural gas hydrate resources in offshore. However, the development of drilling fluid system is restricted by low ambient temperature, special formation conditions of weak cementation and high environmental protection requirements of marine discharge. In this paper, a new environmental-friendly drilling fluid system in offshore gas hydrate formations has been developed by optimizing the additives. The comprehensive evaluation shows that the freezing point of the drilling fluid can reach -10 °C, and it still has good rheological properties at 4 °C. The apparent viscosity ratio of the drilling fluid is less than 1.3, and the fluid filtration is only 4 mL. With good shale hydration inhibition, it can significantly reduce the swelling ratio and increase the cuttings dispersion recovery radio. There is no hydrate formation in drilling fluid for 24 hours at 4 °C and 10 MPa simulated conditions, which shows a good ability to inhibit gas hydrate formation. At the same time, compared with the experimental fluid without hydrate decomposition inhibitor, the drilling fluid can delay hydrate decomposition. In addition, the drilling fluid has good environmental protection performance, which can meet the requirements of high environmental protection in offshore.