Abstract:

In order to understand the ways and intensity of fugitive emission of shale gas from the Wufeng-Longmaxi Formation in Sichuan Basin and its periphery, we analyzed the time and features of late fugitive emission of shale gas and established shale gas leak mode by integrating various data inclduingthe burial history, structural features, pressure coeficient, gas contents and poroperm characteristics of cores from typical wells.It is believed that late fugitive emission of shale gas occurred in the whole period of late continuous uplifting of the shale gas strata.From the periphery to the interior of the basin, the initiation time of fugitive emission changed from the Early Cretaceous to Late Cretaceous.The matrix pores of shale reservoirs are dominated by nanoscale and the permeability is at nano-darcy level(averaging at 0.000 215× 10^-3 μm²), and the sensitivity of permeability to stress is strong(descending two order of amplitude when effective overburden pressure of core ascending from 3.5 MPa to 40 MPa).The fugitive emission of shale gas from deep strata in which faults are poorly developed is the weak diffusion driven by hydrocarbon concentration difference.The late tectonic movements led to the uplifting of shale reservoirs or development of open faults.As the stress released, the permeability of shale reservoirs increased and horizontal permeability became far greater than vertical permeability(the average horizontal permeability of whole cores on surface is 0.567 8× 10^-3 μm², while the average perpendicular permeability is 0.153 9× 10^-3 μm²).The fugitive emission of shale gas is the most intensive near the eroded outcrop area, in areas where the bu-rial depth of shale is shallow or near the open faults belt, and the intensity of emission along strata direction is far greater than that along the direction perpendicular to strata.As the stress, permeability and hydrocarbon concentration change from deep area to shallow area and then to outcrop area as well as the area near open faults belt, the bedding-parallel fugitive emission mode of shale gas is featured by gradual changes from weak diffusion to relative strong diffusion to strong diffusion and finally to seepage flow and the intensity of fugitive emission increase gradually.

Key words: Wufeng Formation, Longmaxi Formation, shale gas, Sichuan Basin