Abstract
This paper presents a critical insight into evaluation of elastic properties of the Montney Formation siltstone through indentation measurements and log-derived elastic moduli, including Young’s modulus, Poisson’s ratio, and brittleness. Further, we explored the relationship between geomechanical properties and rock fabric, mineralogy, and its role in hydraulic fracturing treatments.
We examined seven wells along a northwest-southeast cross-section, sub-parallel to basin dip. Facies analysis was conducted on four long Montney cores (70 to 250 m). Young’s modulus, Poisson’s ratio, and brittleness were calculated from dipole sonic and density logs. Where shear sonic log was not available, predictions of shear wave velocity were performed from near-by wells. Hardness profiles of core samples, measured by a hand-held indentation device, were compared with rock composition from QEMSCAN (mineralogy) and LECO-TOC (organic matter). A coupled hydro-mechanical code, capable of explicit inclusion of lithofacies variation and bedding discontinuities, was employed to investigate the response of the siltstone to hydraulic fracture propagation in the Montney formation.
A comprehensive facies analysis revealed 16 lithofacies across the basin, with depositional environments ranging from tidal flat to offshore sediments, and deep-water turbidite deposits. The variations of Young’s modulus, Poisson’s ratio, and relative brittleness from well logs were compared against indentation measurements of the four long cores and against rock composition in all wells. Young’s modulus, brittleness, and hardness showed similar trends in each well, while Poisson’s ratio demonstrated a trend with depth opposite to all other elastic parameters. No clear distinction was found between the geomechanical properties of different lithofacies in each well. More importantly, similar lithofacies commonly exhibit significantly different geomechanical properties in different wells. The analysis from coupled numerical simulations also confirmed that effective fracture propagation was not necessarily lithology controlled; rather it was greatly constrained by geomechanical contrasts. Further statistical analysis indicated that clay content, and to a lesser extent organic matter content, had the strongest control on elastic moduli in the Montney Formation, reducing Young’s modulus, brittleness, and hardness, but increasing Poisson’s ratio.
Our study concludes that unlike other unconventional reservoirs, geomechanical properties in the Montney Formation are not lithofacies-dependant. We attribute the weak influence of depositional environments on the sediment to the size and compositional homogeneity of detrital material that entered the basin. Clay minerals and organic matter were identified as controlling factors on elastic moduli -and thus hydraulic fracture propagation- in the Montney Formation.
