Locating massive syneresis fractures in shale: An experimental study

Natural fractures play an important role in the storage, migration, accumulation, and escape of hydrocarbons in shale reservoirs. They can also interact with hydraulic fractures to create an interconnected fracture network, thereby enhancing the productivity of the reservoir. Among various types of natural fractures in shale, subaqueous syneresis fractures are commonly observed. The identification of syneresis fractures can aid greatly in pinpointing the most favorable areas in shale reservoirs. In our study, we conducted experimental research to investigate the occurrence of syneresis fractures in a subaqueous environment, where the only influencing factors were the natural processes of mud deposition. Through analyzing fracture parameters, we found that subaqueous syneresis fractures can develop rapidly and extensively during mud subsidence, exhibiting a preference for specific regions. The key factor governing the formation of syneresis fractures is the dip angle of the underlying structure. Steeper dip angles tend to generate fractures with higher density, intensity, and fracturing degree. Furthermore, among underlying structures with the same dip angles, those with longer strike lines tend to form longer and wider fractures. In addition, the sediment composition plays a crucial role in generating more fractures. Although environmental temperature has a minor controlling influence, it leads to limited variations in subaqueous fracture development. Our findings provide efficient guidance for locating significant natural fractures in shale formation. Specifically, we propose that calcareous shale layers deposited on moderate to steeply dipping slopes with longer strike lines, under high depositional temperature, hold promise for developing extensive syneresis fractures. Such areas could serve as favorable zones for hydrocarbon accumulation, representing potential sweet spots in shale reservoirs.