Micro seismic data and coring studies suggest that hydraulic fractures interact heavily with natural fractures creating complex fracture networks in naturally fractured reservoirs such as the Barnett shale, the Eagle Ford shale, and the Marcellus shale. However, since direct observations of subsurface hydraulic fracture geometries are incomplete or nonexistent, we look to properly scaled experimental research and computer modeling based on realistic assumptions to help us understand fracture intersection geometries. Most experimental analysis of this problem has focused on natural fractures with frictional interfaces. However, core observations from the Barnett and other shale plays suggest that natural fractures are largely cemented. To examine hydraulic fracture interactions with cemented natural fractures, we performed 9 hydraulic fracturing experiments in gypsum cement blocks that contained embedded planar glass, sandstone, and plaster discontinuities which acted as proxies for cemented natural fractures. There were three main fracture intersection geometries observed in our experimental program. 1) A hydraulic fracture is diverted into a different propagation path(s) by a natural fracture. 2) A taller hydraulic fracture bypasses a shorter natural fracture by propagating around it via height growth while also separating the weakly bonded interface between the natural fracture and the host rock. 3) A hydraulic fracture bypasses a natural fracture and also diverts down it to form separate fractures. The three main factors that seemed to have the strongest influence on fracture intersection geometry were the angle of intersection, the ratio of hydraulic fracture height to natural fracture height, and the differential stress. Simply put, the most significant finding of this research is that fracture intersection geometries are complex. Our results show that bypass, separation of weakly bonded interfaces, diversion, and mixed mode propagation are likely in hydraulic fracture intersections with cemented natural fractures. The impact of this finding is that we need fully 3D computer models capable of accounting for bypass and mixed mode I-III fracture propagation in order to realistically simulate subsurface hydraulic fracture geometries.

Field experience in shale gas and some tight gas sandstones suggests significant interaction between hydraulic fractures and natural fractures. Experimental analysis of this problem has mostly focused on frictional interfaces with regard to the natural fractures. We have performed tests to examine the effect of cemented natural fractures on hydraulic fracture propagation. The motivation for this type of work is that core observations from the Barnett and some other shale gas plays suggest that natural fractures are largely cemented (or healed) and trend obliquely or orthogonally to the present day hydraulic fracture direction. We embedded planar glass discontinuities into a cast hydrostone block as proxies for cemented natural fractures. Consistent with theoretical predictions, our results show that oblique embedded fractures are more likely to divert a fluid-driven hydraulic fracture than those occurring orthogonal to the induced fracture path. Hydraulic fracture – natural fracture interaction took three forms – 1) the hydraulic fracture bypassing the natural fracture by propagating around it (via height growth, not curving), 2) the hydraulic fracture arresting into the natural fracture and then diverting along it and sometimes kinking off the end of it, and 3) a combination of bypass and diversion. We also saw some leakage of fracture fluid along the interfaces of multi-layer blocks, even though such interfaces were perpendicular to the maximum compressive stress, suggesting an analogue for laminated and shaly sedimentary sequences.