Effect of shearing actions on the rheological properties and mesostructures of CMC, PVP and CMC + PVP aqueous solutions as simple water-based drilling fluids for gas hydrate drilling

The mesostructures of aqueous solutions (one simple type of water-based drilling fluids), with a kinetic hydrate inhibitor (polyvinyl pyrrolidone, PVP) or/and drilling fluid additive (sodium carboxymethyl cellulose, CMC), and their rheological properties after two shearing actions (600 r/min and 6000 r/min) were respectively investigated using a scanning electronic microscope (SEM) and a six-speed rotation viscometer, considering the different shearing actions imposed on drilling fluids during their circulation in the well. The results show (1) aqueous solutions with polymers (CMC, PVP and CMC + PVP) exhibit three different types of network framework (normally in the size of several to tens of micrometers), thin films + thin rods, globular particles + thin rods, and thin films + thin slices. Upon increasing the concentration of CMC or/and PVP, the thickness of the backbones and branches of the network framework increased, causing the volume of the pore space to decrease and the apparent viscosity and shear stress to increase. The tackifying effect of CMC was stronger than that of PVP, and the synergistic effect of CMC and PVP apparently increased the apparent viscosity and the shear stress. (2) With the shear rate increasing from 600 r/min to 6000 r/min, the apparent viscosities and shear stresses of these aqueous solutions decreased to some degree, and the four aqueous solutions of 0.75 wt% CMC, 1.5 wt% PVP, 0.75 wt% CMC + 1 wt% PVP, and 0.75 wt% CMC + 1.5 wt% PVP had relatively larger decreases in the apparent viscosity and the shear stress, which might result from changes in the spatial morphology of the network framework and the pore space, the spatial distribution and contents of the three different states of water, and the mobility of free water. The changes in the mesostructures might affect the local conditions of the heat and mass transfer in hydrate dissociation and formation in the annular space.