2019 AAPG Annual Convention and Exhibition:

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What Drives the Formation of Natural Fractures in Unconventional Reservoirs?


Natural fractures have the potential to affect completion practices and production outcomes in unconventional reservoirs in complex ways, in some cases enhancing production, in other cases interfering with completion or causing enhanced water production. Although fractures are widespread in unconventional reservoirs, their distribution in terms of clustering or fracture prone versus non-fractured intervals, is highly heterogeneous. Moreover, whether or not fractures are fully open, partially cemented, or sealed varies. Even in extensively cemented fractures, nano-scale residual porosity may provide preferred flow pathways in some ultra-tight shale reservoirs. Not clear, however, are the processes that lead to the formation of natural fractures in the deep subsurface. A proper understanding of these processes is needed to fully evaluate the contribution of natural fractures for hydrocarbon charge of the reservoir, and to parameterize predictive fracture network models for well completion and production planning.

In this presentation we review existing data on the timing of natural fracture growth in several unconventional sandstone and shale reservoirs including the Travis Peak Formation of East Texas, the Mesaverde Sandstone in Colorado, the Barnett Shale of West Texas, and the Vaca Muerta Formation in Argentina. Based on fluid inclusion studies we find that, while some fractures form during early prograde burial, all reservoirs contain fractures that formed during maximum burial, coinciding with hydrocarbon generation, and during early stages of exhumation. While compaction disequilibrium can account for elevated pore fluid pressures that promote fracture growth during early prograde burial, we show that hydrocarbon maturation is likely the primary driver for fracture growth under peak burial conditions. Tectonic processes and thermal stresses provide secondary drivers. Thermal contraction with exhumation and cooling of the rock mass can promote fracture growth depending on the thermal expansion coefficient of the fluid phase. The possible contribution of hydrocarbon generation after peak burial as a driver for fracture growth during incipient exhumation is discussed.