Abstract
The rock physics experiments and fracture toughness tests of shales from the Lower Silurian Longmaxi Formation in the Sichuan Basin in China were carried out. Based on this, the calculation model of the fracture toughness was constructed, thus, the single well evaluation of the fracture toughness in shale formation would be obtained based on the well logging data, which can be used to summarize the spatial distribution characteristics of the fracture toughness in the shale formation. However, it is difficult to obtain transverse distribution characteristics of fracture toughness in shale formation based solely on the well logging data. Therefore, in order to investigate the spatial distribution of the fracture toughness, jointing well logging and seismic method could be adopted to quantitatively predict the fracture toughness in shale formation. The results show that fracture toughness of shales is sensitive to acoustic interval transit time and wave impedance. The prediction model of the fracture toughness of shales was constructed, which had a good prediction effect. The fracture toughness values of shales from the Upper Silurian Wufeng-Longmaxi Formation were larger, whereas those of shales from the Lower Silurian Wufeng-Longmaxi Formation were lower. The fracture toughness is mainly distributed in strips along the vertical direction while the distribution area is continuous in the lateral direction, indicating that it has obvious stratification characteristics.
Highlights
With the adjustment of energy structure and the increasingly prominent environmental problems, natural gas as a clean energy has received more and more attention, especially the unconventional oil and gas resources, of which shale gas has become a hot spot in the exploration and development of the unconventional oil and gas resources around the world
The findings indicate that the fracture toughness in shale gas reservoirs from the Lower Silurian Longmaxi Formation is sensitive to the acoustic interval transit times and wave impedances
The following conclusions are drawn: (1) The fracture toughness is negatively correlated with the acoustic interval transit time and attenuation coefficient, whereas it is positively correlated with the wave impedance
Summary
With the adjustment of energy structure and the increasingly prominent environmental problems, natural gas as a clean energy has received more and more attention, especially the unconventional oil and gas resources, of which shale gas has become a hot spot in the exploration and development of the unconventional oil and gas resources around the world. In the aspect of brittleness evaluation of shale, a large number of researchers have obtained a great deal of understandings from experiment evaluation, prediction through well logging, and its engineering application [4,5,6,7,8,9,10,11]. The results of these studies played an important role in the evaluation of the fracability in shale gas reservoirs. It is recognized that the fracability in shale gas reservoir is related to the brittleness
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