Understanding the Influence of Subsurface Bedding-Parallel Fractures in Shale Gas Enrichment

Abstract

Abstract Bedding-parallel fractures are common to see in "productive sweet spot" of Wufeng-Longmaxi gas-shale, Sichuan basin. They are commonly showing slickensides on core and own rough fracture surface. Therefore, they can serve as flow channels and storage spaces in gas shale. However, their size and spatial distribution in subsurface were rarely investigated, and little was known about their permeability behavior responding to the reservoir buried depth. And the relationship between the fracture intensity and reservoir porosity is also rarely reported. Knowing these relationships will help to understanding the influence of subsurface bedding-parallel fractures in shale gas enrichment. 30 wells of Wufeng-Longmaxi gas-shale were studied, they are spanning in 100km west-east across the southern Sichuan basin and have buried depths ranging from 1000m to 3000m. Core validated borehole image logs are used to characterize the spatial distribution of the bedding-parallel fractures in subsurface. Stress-dependent permeability experiments are designed to investigate the fracture permeability responding to the change of confining stress. The effect of the fractures on reservoir porosity is examined by the NMR T2 relaxation well log and Ar-ion-milled scanning electron microscope (SEM) images. The study reveals that the bedding-parallel fractures are calcite-filled and are much more intense in organic-rich intervals. The aperture size of bedding-parallel fractures ranges from 1cm to 50cm. The permeability experiments suggest that under the same confining pressure, the permeability in samples with the fractures is two to three orders of magnitude larger than in samples without fractures. The fracture permeability decreases exponentially until the confining pressure reached 25MPa. NMR log analysis indicates that for the shale buried shallower than 1000m, the NMR log component with T2 relaxation time greater than 30ms has obvious inverse relationship with the intensity of bedding-parallel fractures, but for the shale buried deeper than 1000m, this relationship is not clear. The scanning electron microscope (SEM) images show the macropore are hardly found nearby the fractures. The results reveal that bedding-parallel fractures are products of shear movement between shale beddings, and they act as a flow channel to enhance lateral migration of gas. The lateral migration is much more active when the effective stress is less than 25MPa which is equivalent to 1000m reservoir buried depth in southern Sichuan basin. Connection of bedding-parallel fractures to faults extending to the surface, common in southern SiChuan basin, boosts the rate of gas emission. Gas emission dropped down the pore pressure and made the shale macropore collapsed, the reservoir porosity was decreased accordingly. The finding suggests that structures with fewer bedding-parallel fractures or reservoir buried depth deeper than 1000m in southern Sichuan basin may be more favorable for preservation of gas in shale.