Formation damage evaluation is a key and basic link in optimizing working fluids. It is widely accepted that formation damage is the reduction of core plugs permeability caused by working fluid invasion. However, the measurement of permeability faces a huge challenge for shale formation, such as overspending, time-consuming and the scarcity of unbroken core plug samples. A new method of fractal analysis derived from Scanning Electron Microscopy (SEM) image of shale pore structure was used to quantify the shale formation damage. This method needs to select optimal magnification and segmentation threshold value of SEM image to obtain exact Fractal Dimension (FD) of pore structure. In this paper, we take the black shale outcrops from Sichuan Basin for an example. The results shows that the optimal magnification for observation of the pore structure using SEM imaging in this area is 1000×, and the optimal threshold value for binary image is 29 (RGB). Microscopic pore structure of the shale follows the fractal law, and the FDs increase with increasing measurement scales. It is evident that the evaluation results of shale formation damage when exposed to 2 wt.% NaOH solution and 2 wt.% brine solution using microstructure fractal are exceptionally in good agreement with permeability reduction results. The microstructure fractal obtained from SEM images provides a new method for evaluation of shale formation damage. And it can be applied to optimize the screening working fluids used in shale formation in real time under the condition of high temperature and high pressure.
Experimental Investigation of Evolution of Pore Structure in Longmaxi Marine Shale Using an Anhydrous Pyrolysis Technique
