Fracture spatial arrangement in tight gas sandstone and shale reservoir rocks

A new statistical analytical method was applied to quantify the spatial arrangement of fractures in sandstones and shales. Results show that spatial arrangements of fractures in the subsurface have a wide range of patterns and that fracture clusters are prevalent. The Upper Cretaceous Frontier Formation is a naturally fractured gas-producing sandstone in Wyoming. East-west-striking regional fractures sampled using image logs and cores from three horizontal wells exhibit clustered patterns, whereas data collected from outcrop have patterns that are indistinguishable from random. Image log data analyzed with the correlation count method shows clusters ~35 m wide and spaced ~ 50 to 90 m apart as well as clusters up to 12 m wide with periodic inter-cluster spacings. A hierarchy of cluster sizes exists; arrangement within clusters is likely fractal. Regionally, random and statistically more clustered than random patterns exist in the same upper to lower shoreface depositional facies. These rocks have markedly different structural and burial histories, so regional differences in degree of clustering are unsurprising. Application to shale reservoirs further link fracture clusters and spatial arrangements with reservoir mechanical stratigraphy: Vaca Muerta Formation shale shows strong control of fracture cluster locality by reservoir mechanical properties; Middle Devonian shales in the Horn River Basin identify spatial arrangement and cluster dimensions associated with preferred wellbore intervals; Marcellus Formation shale shows spatial arrangement controlled by mechanical bed thickness. Our results show that quantifying and identifying patterns as statistically more or less clustered than random delineate differences in fracture patterns that are not otherwise apparent but that may influence petroleum and water production, and therefore may be economically important.