CHARACTERIZING SHALE PORE NETWORKS IN SHALES USING FIB-SEM 3D DATA BASED ON A NEW PORE-SPACE SEGMENTATION METHOD

Pore network characterization is an important aspect in shale evaluation as the connectivity of the pore network determines the behavior and characteristics of fluid flow in shales. Despite of the limitation of its small imaging sizes and the lack of overall representativeness, FIB-SEM currently is still the best 3D imaging method to delineate and characterize shale pore structures. Previous researches generally regard all pores within shale reservoirs as a bulk volume without further subdivision into organic pores, inorganic pores or fractures in 3D spaces. This has hampered an in-depth understanding of shale pore connectivity of different types of pores.

A new technique is established to segment the bulk pore space into organic pores, inorganic pores and fractures from an entire FIB-SEM 3D volume by using a organic pore-filling and subtraction procedure and considering pore anisotropy. It is possible to characterize the distribution and contact relationships of different types of pores in 3D spaces, their morphology, individual volume contribution and connectivity. A shale sample from a currently shale gas producing interval in the Sichuan Basin was used to demonstrate the advantage of the technique. Results reveal that the relative contribution of the organic pores, inorganic pores and fractures in the shale are 6.10%, 1.22% and 92.68%, respectively, indicating that fracture is the main pore volume contributor. Using the ratio between organic pore volume and organic matter volume, the occurrence probability of organic pores was determined to be 1.29% in the organic fraction. Based on the same segmentation and calculation method it is possible to get the occurrence probability of organic pores from 2D MAPS images, which would be more representative to compare the degrees of organic pore development among different shale reservoirs over a large area. Connectivity evaluation indicates that fractures have good connectivity and play an artery role in the pore network, while the organic pores and inorganic pores are comparatively poorly connected. The poor connectivity of inorganic pores is due primarily to their sparse distribution. In addition, results show that although many fractures appear to be around the organic pores, there is no direct connection between them.