Drilling fluids will affect the hard-brittle shale strength characteristics. Fractal theory has a unique advantage in studying the microstructure of rocks, which can be used to study the influence of drilling fluid on shale strength characteristics. Shale samples obtained from western South China Sea field have been tested using X-Ray Diffraction (XRD) tests, nitrogen adsorption (NA) tests, Scanning Electron Microscope (SEM) tests, uniaxial compressive strength tests to study the impact of drilling fluid on shale. The results indicate the total porosity along with the percentage of micro-pores increased, and the calculated fractal dimension increased as follow, meanwhile uniaxial compressive strength gradually reduced due to immersing shale. At the same time, the SEM results show that the number and size of micro-cracks increased and more corrosion holes appeared. Shown by the experimental results, the fractal dimension is positively correlated with the micro-cracks and inversely correlated with shale uniaxial compressive strength. Accordingly, the drilling fluid formulation is optimized to lower the fractal dimension of shale, by which the decrease in shale rock strength is significantly lowered.

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