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In this paper, shale gas cores from the Lower Silurian Longmaxi Formation in the Zhaotong National Shale Gas Demonstration Area were selected to study the hydration effect of different fluids on the fracture morphology inside the shale, the rock tensile strength and the Brazilian tensile failure mode. Fresh water and slick water were adopted for hydration pretreatments and the CT technique was used to compare the changes of the fabric in the shale. Then, Brazilian tensile tests were carried out to study the tensile strength and tensile failure modes of the shale specimen after hydration pretreatment. Finally, two horizontal wells in the study area were selected to perform pilot tests of hydration pretreatment in their fracturing operation sites. And the following research results were obtained. First, in the process of spontaneous imbibition, the hydration effect of fresh water is superior to that of slick water in promoting the fracture complexity of marine shale in the same hydration duration. Second, fresh water has greater surface tension and lower viscosity and its hydration effect can not only promote the propagation of original fractures but induce new micro-fractures or branches, while the hydration effect of slick water mainly promotes the propagation of original fractures. Third, due to hydration effect, marine shale is damaged and its tensile strength is reduced. After the hydration pretreatment by fresh water and slick water, the tensile strength of the shale specimens are reduced by 35.6% and 18.1%, respectively. Fourth, according to the propagation paths of main fractures, the Brazilian tensile failure modes of hydrated shale can be divided into four types (i.e., step shaped, dog-leg shaped, branching shaped and arc shaped) or their combinations, while the tensile failure mode of unhydrated shale is only a straight line. Fifth, hydration effect can effectively increase the complexity of the hydraulic fractures in marine shale, so if the conditions permit, it is recommended to inject a certain amount of fresh water at a high pumping rate within the limited pressure after perforation and to shut in the well until the hydraulic fracturing operation. And in order to reduce the difficulties of pumping the proppant during the hydraulic fracturing operation, a pumping strategy of “low proppant concentration, large volume of slurries” can be adopted to reach the expected proppant volume.

Complex tectonic movements and high thermal maturity of marine shale dominate South China, where preservation conditions are critical for shale gas enrichment and productivity. Based on the exploration practices of the Silurian shale gas at the southeastern margin of the Sichuan Basin in recent years, conventional gas and shale gas were compared in terms of their preservation conditions. The results revealed that superior roof and floor conditions are indispensable to shale gas preservation. Moreover, the self-sealing ability and the huge gap of up to 2–8 times between vertical and lateral permeability of shale gas reservoirs determine the lateral diffusion as the basic pattern of shale gas migration. The unconformity at the bottom of Lower Cambrian leads to worse preservation conditions in the system, and cutting by faults may accelerate the diffusion of shale gas. Major controlling factors for shale gas preservation and their criteria of discrimination were also investigated. It is suggested that: (1) the strength of tectonic modification is the major factor controlling shale gas preservation. Broad and gentle structures with continuous seals and an anticlinal setting are more favorable for the enrichment of shale gas, and a closed evolutionary environment with late uplifting is more favorable for the preservation of shale gas; (2) shale gas can be preserved well in downdip areas without faults or effectively closed or shielded by faults and areas far away from outcrops or zones with stratigraphic hiatus; (3) pressure coefficient is a comprehensive indicator for discriminating preservation conditions. In the study area, the pressure coefficient is in positive correlation with shale gas production and the high or super-high pressure of reservoir is a signal of good preservation condition for shale gas; and (4) in the areas within the southeastern Sichuan Basin, other than those close to erosion zones or hiatus, the Wufeng Fm. of Upper Ordovician and the Longmaxi Fm. of Lower Silurian present high pressure coefficient (up to 2.25) generally, demonstrating good preservation conditions for shale gas, while the pressure coefficient reduces progressively toward or outside the margin of the basin, corresponding to downgrading preservation conditions.