Fault, fracture, and geomechanical analysis of the Niobrara Formation, Wattenberg field, Colorado

Abstract

The Wattenberg Field in the Denver Basin covers approximately 3,200 square miles northeast of Denver, CO. One of the most prolific hydrocarbon producing formations of the Wattenberg Field is the Niobrara Formation. Production in the Niobrara began with vertical wells in 1986, and horizontal drilling began in 2009. Since 2009, there has been extensive horizontal drilling throughout the Wattenberg Field. With such an increase in horizontal drilling in the Niobrara Formation, it is important to address the complex lateral and horizontal reservoir heterogeneity. In conjunction with the CSM Niobrara Consortium and Reservoir Characterization Project (RCP), this study uses detailed core observations, fracture characterizations through borehole image logs, and reservoir geomechanics to investigate the complex reservoir heterogeneity of the Niobrara Formation in a fifty square mile study area within the Wattenberg Field. It is the goal of the RCP to conduct a multidisciplinary study to better understand the static and dynamic aspects of the Niobrara petroleum system. The goal of this work is to provide a geologic framework that can provide insights into the characteristics of the reservoir heterogeneity and fracture networks in order to optimize production within the study area. Four cores are described in this study and their facies distributions reveal important trends about the heterogeneity throughout the study area. Borehole image logs for two wells are interpreted to investigate the distribution and density of fractures within the complete B and C benches of the Niobrara Formation. Dipole sonic logs are also used to study the mechanical characteristics of the complete Niobrara Formation and the facies determined from the core observations. When combined together, these techniques identify the most fracture prone benches, facies, and their distributions throughout the study area. Core observations and trend mapping indicated an increase in chalk content from east to west across the study area. The borehole image log study indicated a primary, natural fracturing direction in the B and C benches (chalks and marls) of north-west/south-east with the B marl bench having the highest fracture density (0.029 fractures/foot). Investigations into the brittleness of the Niobrara Formation revealed that the C chalk is the most brittle and of the six facies, the chalk facies is the most brittle and the marl facies is the least brittle.