Tight formations often have an ultra-low permeability that prohibits fluid migration from the reservoir. To counter this issue, large fracture networks are created to connect pore space and increase permeability. The total volume created by hydraulic fracturing is the stimulated reservoir volume (SRV) and is used as a correlation parameter for well performance and to quantify the fracture network (Mayerhofer et al., 2010). The uncertainty that arises in the dimensions of SRV is caused mainly by the complexity of the fracture network. On a field scale, fractures are modeled using methods such as PKN C and P-3D-C, but these models only take the primary fractures into consideration, which dismisses the secondary fracture network entirely. This research illustrates that secondary microfractures in the elastic zone can triple the amount of stimulated area and pore connections within a reservoir. This is made possible by a constant supply of energy pulsating around the primary fracture during propagation. This pressure wave has enough energy to create a large fracture network that acts to connect pores to the primary fracture. As the demand for reservoir stimulation increases, efforts are being directed towards quantifying microfractures to see how they impact reservoir production. In this work, scanning electron microscopy (SEM) analysis is used to study hydraulically fractured Tennessee sandstone, Marcellus shale, and pyrophyllite. A series of high-resolution images were taken to investigate microfractures and their contribution to SRV. A total of 3 cores, 9 plugs, and 25 samples were prepared for SEM analysis. Over 75,000 high-resolution images were recorded from the primary and secondary fracture networks to extract statistics such as fracture density, distribution, orientation, symmetry, length, width, mechanical twinning, crystal orientations, and stimulated reservoir area xxv (SRA). In addition to SEM analysis, X-ray, confocal, and petrophysical measurements were performed as well.
Results show that microfractures have a large impact on the reservoir by increasing the total fractured volume by 25-fold and tripling the connected pore space.
By propagating in a direction perpendicular to the primary fracture, secondary fractures act as a means of connecting micropores that were originally isolated. The fracture tip poses interesting findings by increasing the SRV, which suggests a change in physics in the elastic/plastic transition of a terminating fracture. It was further found that primary fractures with low velocities create larger SRV’s while the termination zone creates the largest SRV. This indicates that a start-stop pumping process during hydraulic fracturing would be most beneficial. Hydraulic fracturing was further found to induce Dauphiné twins in quartz crystals while also changing the orientation of grains during fracture propagation. These crystallographic alterations aid in fracture propagation by causing slip and changing plane orientations. Shale and pyrophyllite analysis provide insight on fracture morphology in unconventional formations. It was found that secondary fracture networks significantly decrease in unconventional reservoirs, but the complexity of each secondary fracture is far greater. A high number of tertiary nanofractures are stimulated around the primary and secondary fractures creating micro damage zones. Furthermore, the fracture length and width consistently increase from the Tennessee sandstone, to Marcellus shale, to pyrophyllite.