Mesoscale Mechanisms of Drilling Fluid Invasion on Hydrate Reservoir Stability: Effects of Drilling Fluid and Reservoir Properties

The disturbance around the well caused by drilling fluid invasion is the main reason for wellbore instability during drilling in hydrate reservoirs. Current research mainly focuses on the macro scale. This study explores the mechanism between invading drilling fluid and hydrate reservoir from a mesoscopic scale. The morphological characteristics of hydrate reservoirs in the invasion zone were observed by using a mesoscale experimental system. The influence mechanisms of different disturbance factors and reservoir properties on drilling fluid invasion and reservoir strength were analyzed through changes in pressure, temperature, and invasion front velocity during the invasion process. The results indicate that the local high pore pressure generated by hydrate decomposition has a dual mechanism for reservoir stability: weakening reservoir mechanical strength and hindering drilling fluid deep invasion. When drilling fluid invasion does not cause hydrate decomposition, the reservoir does not show significant strength-weakening characteristics, and the increase in pore pressure is relatively small. When the hydrate decomposes, the peak pore pressure increases by 0.13 MPa under an invasion pressure difference of 0.3 MPa, and the loose and weak cementation characteristics of the reservoir in the decomposition zone are observed. However, the high pore pressure reduced the drilling fluid invasion depth by 8.14 mm. During the invasion process of drilling fluid, local high pressure is manifested as a package-like protrusion, and the dissipation rate of the package-like protrusion depends on the decomposition rate of the upper hydrate and the permeability of the reservoir, which is more likely to occur in low-permeability reservoirs. Because of the slow heat and mass transfer rates in low-permeability reservoirs, the decomposition rates of upper hydrates and the release rates of decomposed gases are both relatively low. This study provides valuable insights for the control of wellbore stability during drilling in hydrate reservoirs.