Fractal features of the microstructure of the drilling fluid used into the hydrate-bearing sediments under the shearing action

摘要

The microstructures of the drilling fluid changed while circulation due to the different shearing action from pipe or annular to bit water way in the drilling process of hydrate formation, which affects the performance of the drilling fluid and the stability of the wall. Since the dynamical microstructure of the drilling fluid is very complicated, the characteristics of the microstructure of a water-based polymer drilling fluid under the different shearing action (600 and 6000 r/min) were studied in this paper. According to the fractal theory, the fractal models of the microstructures of drilling fluid were established on the basis of the Sierpinski carpet model and Menger sponge model. The microstructure was analyzed quantitatively using the Image Pro Plus software. The results show that the microstructure is comprised of the skeleton structure formed by polymer and pores filled with small molecules such as inorganic salt ions and hydration molecule. The strong shearing action can change the shape of the skeleton structure. The microstructure of drilling fluid has the fractal features, and the fractal models can be described by the fractal model of original skeleton area (volume), the fractal model of area (volume) of pores, and the fractal model of number of pores. The larger the porosity of drilling fluid microstructure, the smaller the fractal dimension of space distribution of the skeleton structure, and the smaller the fractal dimension of number similar to size distribution of pores. The fractal dimension of the skeleton structure is equal to the fractal dimension of number similar to size distribution of pores on condition that ignoring the influence of r(max). The fractal dimension is between 1 and 2 in the planar space. Finally, the process of heat transfer was analyzed in the microstructure of drilling fluid, and the fractal model of heat transfer was set up. All these will support a theoretical basis for establishing the models of mass and heat transfer of the decomposition of natural gas hydrates in the drilling fluid.