A Novel Evaluation Technique for Mudcake of Drilling Fluid Based on the Nuclear Magnetic Resonance Method

In drilling engineering, mudcake is formed when the drilling fluid invades the near-wellbore zone, which can reduce drilling fluid leakage and reservoir pollution and maintain wellbore stability. Exploring the method that can best represent the field working conditions to evaluate the mudcake is an urgent problem. At present, the macro-evaluation method cannot describe the characteristics of heterogeneity, particle accumulation, and porous media of mudcake. The micro-evaluation method needs local sampling, drying, curing, slicing, and other tedious disturbance treatments, which cannot reflect the overall characteristics of mudcake. To solve these problems, a novel technique for evaluating the pore structure of mudcake was established by taking nuclear magnetic resonance (NMR) T2 tests as the key mean and integrating high-pressure mercury injection tests, fluid isotope tracing, dynamic/static filtration experiment of drilling fluid, pore permeability parameter tests of core, and particle-size distribution tests of drilling fluid. The evaluation results of mudcake formed by drilling fluid static and dynamic filtration show that this technology can study the pore structure characteristics of the outer mudcake and intruded core and the distribution characteristics of the inner mudcake and filtrate in the intruded core. The novel evaluation technique has strong operability and less demand for experimental samples, which can study the micron-scale pore structure of mudcake and provide practical methods for drilling fluid system optimization and application effect evaluation, reservoir damage evaluation, and development or verification of the filtration model.