Activation of natural fractures is generally assumed to be a major contributor to the productivity of unconventional reservoirs completed by creation of many fractures in horizontal wells. These natural fractures are assumed to form an interconnected high permeability network that shortens the length of the flow path and facilitates higher flow rates from the reservoir.
This paper reviews activation of the natural fractures that lie in the formation adjacent to, or in between, hydraulic fractures, but not directly connected to them. The solution uses the Mohr-Coulomb failure criterion and general equations for stress distribution around a hydraulic fracture.
The paper shows that under favorable conditions the stresses induced by the growing hydraulic fracture can activate existing natural fractures by either tensile or shear failure. Natural fractures close to the hydraulic fracture have the best chance for limited tensile activation. This type of activation is insensitive to differences between the original in-situ principal stresses. Activation by shear sliding is more likely to happen in cases of larger difference between the in-situ principal stresses and, again, will be of limited extent.
Hydraulic fracture conditions that increase probability of natural fracture activation are higher extension pressure, and proximity to the natural fracture. Natural fractures farther away from the wellbore have a better chance of activation if they fall within the influence zone of a long hydraulic fracture.
Activation of natural fractures by tensile stresses may create a zone of formation dilation and enhanced permeability around the hydraulic fracture. This increases the productivity of the hydraulic fracture and the reservoir.
Creation of a single hydraulic fracture is more likely to activate natural fractures than two or more concurrent fractures. The zone of activated natural fractures will also be larger. Increased normal stresses caused by creation of multiple concurrent fractures can actually close or narrow natural fractures that have already existed within the formation between these fractures.
Shorter spacing between adjacent hydraulic fractures can increase the percentage of the formation volume between them that contains activated natural fractures.