Predictions of fracture growth in Walloon coals using a layer fracture model

Abstract

Individual low-permeability Walloon coal seams are separated by the stiffer beds of carbonaceous shales, mudstone, siltstone and sandstone in the Surat basin. Considering the thin nature of Walloon coals, the fracture growth across the stiffer beds becomes a key issue in assessing the fracture treatment efficiency and in determining the impacts of fracture stimulation on groundwater resources. A hydraulic fracture model considering the elastic property difference across layers is developed to obtain the fracture growth behaviours in a multiple layer formation. A case study was performed where the in situ stress across beds is of similar magnitudes to highlight the effect of property difference. The fracture initiates at the targeted lower coal seam and the complex footprints generated by fracture growth are obtained. In contrast to the assumed constant fracture height and the large fracture height to length ratio obtained by other layer models, the vertical fracture growth is limited in the propagation speed by the alternating stiff and compliant rocks. The alternating growth in lateral and vertical directions results in an oscillating pressure, which is an indicative for fracture height growth.