Development and Working Mechanism Analysis of a New Weak Gel Water Base Drilling Fluid

A new weak gel water base drilling fluid was formulated with a new developed weak gel agent as the main additive and other additives such as filter loss reducer, shale inhibitor and sealing and plugging agent selected through experiment. Laboratory experiment shows that this drilling fluid has unique rheological behavior, i.e., it has high viscosity at low shear rates, and weak gel is formed quickly within the fluid. This drilling fluid has very good filtration property and superior performance in shale inhibition. The properties of this drilling fluid remain stable at 150℃ or when contaminated by calcium of 4 000 mg/L. Study with SEM and Zeiss microscope shows that sol particles are encapsulated and embedded at the nodal points of a 3D network developed within the drilling fluid. This drilling fluid can form dense sealing layers with good compressibility in formations full of micro fractures or fissures, preventing water from entering into the formations, thus inhibiting the hydration and swelling of shale or formations with high content of clays.