This paper discusses the determination of in situ stress regime and its resulting impact on hydraulic fracture geometry through an analysis of two datasets that were acquired in a vertical well and a nearby horizontal well in a Canadian reservoir. The hydraulic fracture geometry monitored using microseismic was available for comparison.
When solving the poro-elastic horizontal strain model for closure stress, using analogous Hooke's Law for continuous media, one needs to resolve a stiffness tensor which requires determination of multi-dimensional shear. Measurements done in vertical wells using acoustic data are challenged by the limitation of not being able to directly record a horizontal shear. This limitation is typically overcome by modeling a horizontal shear using Stoneley mode firing. This well-developed technique works better when data is acquired using tools that allow simplified modeling of the tool wave, such as those tools that do not have slots cut in the housing to reduce direct signal coupling through the tool housing and mandrel. With the acquisition of simultaneous horizontal and vertical well data in the study, complementary information from the horizontal well provides a unique opportunity to physically confirm the validity of horizontal shear determined in the Vertical well, thereby providing further confidence in the modeling technique.
The analysis showed approximately 7% acoustic wave anisotropy for both compressional and shear wave in this part of the reservoir. It also indicated both vertical and horizontal transverse isotropic anisotropy presence. This enabled confirming a particular stress regime.
As fracures usually open up against minimum stress, once the three stresses σh, σv and σH are known, a comparison of the three stresses can predict the geometry of the induced fracture. In this case, the comparison of stresses revealed that the induced fractures should have been vertical. However that was not the case in reality. This suggested that additional factors could be impacting the outcome of the fracture. One possible factor is the presence of fine layering and lamination in the rock. When lamination is present induced fracture geometry is governed not only by stress but also by the frequency of lamination.
It was seen that the zone of interest was higly laminated and layered. As such this might have contributed to the observed fracture complexity