Brittleness estimation from seismic measurements in unconventional reservoirs: Application to the Barnett Shale

Abstract

We estimate the brittleness index based on the rock mineralogy composition from log measurements from a fully cored well, located just outside the area of our seismic survey. Using density logs and Pand S-wave sonic logs in the well we map the brittleness index against Young's modulus and Poisson's ratio as well as against λρ and μρ. We use this template to predict brittleness from surface seismic inversion, calibrating my predictions using microseismic event locations. Prestack simultaneous inversion was performed to estimate λρ and μρ seismic volumes in an effort to detect and highlight brittle and ductile regions in an unconventional reservoir. 2D colorbars and interactive 2D crossploting technology are used to estimate geomechanical behavior from λρ and μρ estimated from surface seismic inversion. Wells are used with microseismic experiments and surface seismic estimates of λρ-μρ to quantify damaged rock. At each microseismic location, we extract the corresponding λρ and μρ values and crossplot the results using a 2D colorbar, providing a link between discrete interactive crossplotting and the continuous variability of the data. Neither of these seismic estimates are direct measures of reservoir completion quality. We therefore use production logs and extract surface seismic estimates at microseismic event locations to analyze the completion effectiveness along several horizontal wellbores in the reservoir. We define four petro-types in λρ and μρ space depending on their brittleness and gas saturation and find that the majority of the microseismic events fall into the zone described as brittle in the λρ-μρ crossplots. Our results show that the majority of the microseismic events occur in the area that we define as brittle, avoiding more ductile shale layers and the ductile limestone fracture barriers. From these observations we conclude that regardless of where the well is perforated, microseismic events appear to preferentially grow towards the more brittle areas, suggesting the growth of hydraulic fractures into the brittle petro-type.