Reconstructing 3D digital model without distortion for poorly conductive porous rock by nanoprobe-assisted FIB-SEM tomography.

Abstract

Oil and natural gas prospecting requires precise pore characterisation of insulating rock samples, which involves severe charging problems in the state-of-art FIB-SEM tomography, such as overexposure, drift and distortion. For weak cemented samples with very poor conductivity, the conventional ways such as decreasing accelerating voltage or current as well as coating a thin layer of carbon or gold fail to eliminate all the detrimental effect, leading to image distortion in the form of lateral shift and longitudinal stretching. A new nanoprobe-assisted method is explored in FIB-SEM tomography to address this problem and improve image quality. To be specific, a metallic nanoprobe is induced and attached on the sample surface to create an express path for the export of excess electrons near the region of interest, which effectively removes distortion and drift when imaging. Two adjacent areas were characterised and reconstructed into 3D digital models by FIB-SEM tomography with nanoprobe-assisted method applied to one region only. The lateral shift creates zigzag feature for distorted region and the longitudinal stretching of undistorted object can reach 14%. Average pore size of distorted region is larger than that of the undistorted region, however considering the longitudinal stretching, the average pore size of distorted region can be corrected to the same level as the undistorted region. The systematic error caused by distortion for poorly conductive porous rock is hazardous for digital rock physics analysis. Therefore, the nanoprobe-assisted FIB-SEM tomography should be regarded as a one of the optional and feasible procedures in case decreasing accelerating voltage or current as well as coating a thin layer of conductive material does not work.