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An anionic polymer P (ANAN) of acrylamide, N-vinyl-2-pyrrolidone and 2-acrylamide-2-methylpropanesulfonic acid was synthesized through free radical polymerization and initiated by 2,2′-Azobis (2-methylpropionamide) dihydrochloride as an anti-high temperature and anti-calcium and magnesium contamination fluid loss control additive for water-based drilling fluid. The P (ANAN) was characterized by 13C-NMR and 1H-NMR, FTIR, elemental analysis and GPC analysis. The fluid loss tests were carried on different WBDFs. The results showed that this polymer can resist 4.5 × 105 ppm of Cl−1, 4 × 105 ppm of Ca2+ contamination at low pH environment caused by acid gas invasion in drilling fluid at 180 °C. The fluid loss control mechanisms of P (ANAN) were investigated by Zeta potential analysis, particle size distribution analysis and mud cake quality analysis. The thermal stability was investigated by TGA analysis. The results illustrate that the P (ANAN) exhibits a stronger interaction with clays and higher-temperature resistance, and it can improve the colloidal dispersion property obviously for drilling fluid under salts, calcium and magnesium contamination and even under circumstances of acid gas invasion conditions. Due to its superior fluid loss control properties, the P (ANAN) can be a promising polymer for water-based drilling fluid in deep and ultra-deep well drilling applications.

In the development of natural gas reservoirs, the water produced in wells will cause a decline in gas well productivity. In this study, a selective frozen gel plugging agent system suitable for water plugging in gas reservoirs under conditions of high temperature and high salinity. The plugging capacity of the gel system was evaluated. The experimental results showed that: (1) the optimal experimental scheme of the frozen gelling system was as follows: 107 °C with 1.0% AM-AMPS +0.6% p-benzenediol +0.6% hexamethylenetetramine +0.2% thiourea. (2) The optimal injection volume of the system for water phase plugging was 0.5 PV, and the system has an excellent plugging effect at a permeability of 356–3118 mD. (3) The system could also effectively improve the degree of heterogeneity, and the smaller the degree of heterogeneity, the better the improvement effect. (4) The gel system had a good effect on different plugging methods, and the gas medium did not affect the plugging performance. This study provides a sufficient theoretical basis for the exploitation of high-temperature and high-salinity water-bearing gas reservoirs through experimental research, which is of great significance for improving production capacity.

A temporary plugging agent is an effective tool for increasing production in old wells. By considering the temporary plugging materials used in the remote WY area, three concentrations and five ratios under different fracture widths were designed and optimized. Thus, the fracture diversion under true triaxial stress was studied. The results showed that when the fracture widths were 2.0, 4.0, and 6.0 mm, the corresponding optimal plugging scheme was that the concentrations of the temporary plugging agent were 12.0, 18.0, and 18.0 kg/m3 and the ratios of 20–70 mesh to 200–300 mesh temporary plugging agent were 4:1, 2:1 and 4:1, respectively. When maintaining the same horizontal stress, an increase in the axial stress was accompanied by an increase in the fracture pressure; the initial fractures almost propagated along the direction of the horizontal maximum principal stress, but the shapes of the turning fractures were different. When the axial stress was the same, an increase in the horizontal stress difference was accompanied by a decrease in the corresponding fracture pressure and a gradual decrease in the degree of fracture turning. This study provides a basis for numerical simulation and field application of temporary plugging fracturing at fracture ends.

During the process of hydrate reservoir drilling, the heat generated by friction in a horizontal well section leads to hydrate decomposition and affects the wellbore stability. Consequently, the low-temperature environment of subsea wellbores can cause hydrate regeneration and flow obstacles. In this study, modified n-alkanes (MNA) with an appropriate phase transition temperature and acceptable latent heat were synthesised, and the thermal conductivity was improved by adding nano-Cu. A composite phase change material (PCM) was prepared using expanded graphite (EG) with adsorbing MNA, and nano-silica modification to improve its hydrophilicity and stability. Structural properties of the composite PCM were characterised via scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and nitrogen sorption. The DSC curves revealed an acceptable latent heat capacity and reliability of the prepared composite PCM. In addition, by simulating the temperature heating and cooling during the drilling process, the composite PCM exhibited a reasonable thermoregulating and hydrate decomposition inhibition performance, which can effectively inhibit hydrate decomposition in reservoirs and hydrate regeneration in wellbores. Moreover, the composite PCM exhibited a reasonable compatibility with the drilling fluid. The study results can be applied to realise intelligent temperature control and reduce the risk of drilling in hydrate reservoirs.

The friction heat generated during the drilling process of natural gas hydrate bearing sediment (HBS) leads to the temperature of drilling fluid higher than the phase equilibrium temperature of hydrate, which induces hydrate decomposition and wellbore instability. In this work, tetrabutylammonium bromide (TBAB) and Nano-silica were microencapsulated by inverse emulsion interfacial polymerization. The effect of different preparation conditions on the encapsulation rate of microcapsules was characterized by differential scanning calorimetry (DSC), and the highest encapsulation rate was 62.2% with the melting enthalpy of 121.4 J/g. Polarizing microscope, scanning electron microscope (SEM) and transmission electron microscope (TEM) images were used to characterize the synthesis process and microstructure of microcapsules, which proved a quasi-spherical structure coated with Nano-silica. Furthermore, it was confirmed that the microcapsules also had good thermal conductivity, reliability and thermoregulating performance. The rheological parameters of drilling fluid with 5 wt% microcapsules increased slightly, and the filtration loss decreased, indicating good compatibility. In addition, it is convenient to adjust the melting temperature of the core materials by adding NaCl with different concentrations. This study is of guiding significance to realize intelligent temperature-control of hydrate bearing sediment during drilling.

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.