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In the process of gas hydrate drilling, the drilling fluid is easy to intrude into the formation and change the formation temperature, pressure and chemical conditions of the hydrate formation, leading to the decomposition of hydrates in the surrounding formation and instability of the wellbore. The problem of wellbore instability caused by hydrate decomposition during hydrate drilling in the South China Sea is analyzed and studied. A composite phase change material is prepared by using intelligent temperature control method, adsorbing and modifying n-alkanes with vermicular graphite. The results show that the prepared composite phase change material has good phase change latent heat performance, the phase change temperature is controllable in the range of 17-18 °C, and the particle size can meet the basic requirements for effective recovery of screening particle size on site. It also has good compatibility with drilling fluid and is suitable for water-based drilling fluid system for hydrate reservoir drilling. [ABSTRACT FROM AUTHOR]

The deep Paleogene resources in the eastern part of the South China Sea have huge potential, but weathering crust and coal seams are often developed. The instability risk of coal seam, especially the interbedded coal slime well wall is high, and complex situations occur frequently during drilling and completion, which greatly hindering the exploration and development process. Aiming at the technical problems of borehole stability of complex coal seam of Paleogene in an oilfield in the eastern South China Sea, the mechanism of borehole instability is discussed through the analysis of the microstructure and mechanical properties of coal rock, and the technical countermeasures of "multiple cooperative" borehole stability drilling fluid based on "physical plugging-chemical consolidation-hydration inhibition" are put forward. An anti-collapsing drilling fluid formula suitable for complex coal seam of Paleogene in an oilfield in the eastern South China Sea is constructed. The evaluation results show that the drilling fluid formula has good rheology, fluid loss, lubrication, plugging and inhibition, excellent anti-pollution performance, and can improve the compressive strength of complex coal seam samples to a certain extent, which is conducive to the stability of complex coal seam wall, in line with the needs of field drilling engineering. [ABSTRACT FROM AUTHOR]

The complex features of the compound-salt formation result in wellbore instability during drilling operations. The mechanism of wellbore instability in compound-salt formation is studied by the analysis of mineral compositions, microstructures, physical-chemical properties, and interface mechanical properties of compound salts from different areas. The results show that the existing micro-scale pores and fractures, creep and dissolution of salt, hydration of gypsum-mudstones are the key causes of wellbore instability in compound-salt formation. In order to stabilize the wellbore in compound-salt, we propose the drilling strategies based on the synergism of multiple methods of strengthening the wellbore by the sealing and consolidating effect of drilling fluid, inhibiting the dissolution of salt and the hydration of gypsum-mudstones, balancing the water activity between rocks and drilling fluid, providing effective stress to support the wellbore. Based on the effect of water activity on the shale hydration swelling and dispersion properties, the design basis of the safe window of water activity for drilling fluids is determined. According to the wellbore instability problems when drilling in compound-salt formations in M Oil Field in the Middle East, H-S Oil Field of Uzbekistan, and Xinjiang Uygur Autonomous Region of China, the drilling fluids are designed respectively based on the proposed wellbore stabilizing technology. The field applications show that the drilling fluid technology greatly improves the wellbore stability in compound salt formations and reduces downhole problems, thereby improving drilling efficiency. [ABSTRACT FROM AUTHOR]

The leakage channel in complex fractured formation has multi-scale characteristics and uncertainty, which is difficult for the traditional bridge loss circulation materials achieving effective adaptive bridging plugging. Based on the characteristics of the shape memory materials, the temperature sensitive shape memory lost circulation materials activated by the leakage layer temperature are prepared. According to the requirements of drilling plugging technology, the preparation process and conditions are optimized, and the temperature resistance, expansion performance and drilling fluid compatibility are tested. The fracture plugging effects under different conditions are compared and evaluated, and a set of formula of plugging fluid for 2-4 mm coexisting fractures is optimized. After reaching the activation temperature, the system can play the role of self adaptive bridging plugging, and improve the pressure bearing capacity and adaptability. [ABSTRACT FROM AUTHOR]

To solve the greater deviation of bentonite content measuring result in drilling fluid during complex deep well field operation, the experiment methods for improving measuring accuracy of bentonite content in drilling fluid for complex deep wells was investigated, based on analyzing the affecting factors on bentonite content measuring accuracy, The effect of salt content, pH value and additives on measuring accuracy of bentonite content was analyzed and then the bentonite content measuring method of national standard GB/T16783-1997 was improved. Results showed that the salinity, pH value, additive types and usage all affect the measuring result. With salinity increases, the measuring result is getting less than the actual value, and with the increase of pH value and additive absorbing methyleneblue, the measuring result is getting more than the actual value. According to improved measuring method, the addition of 2.5 N sulfuric acid is 0.5 mL, and that of 5% H2O2 is 15%, while the pH value of testing liquid is 9.0. The improved bentonite content measuring method has high accuracy and good parallelism. [ABSTRACT FROM AUTHOR]