Analysis of stress and geomechanical properties in the Niobrara Formation of Wattenberg field, Colorado, USA

Abstract

In Wattenberg Field the Niobrara Formation is the primary productive zone for horizontal drilling and completions. It is an unconventional reservoir made up of alternating chalk and marl layers which require hydraulic fracturing for completion. The main study area for this project is a four square mile region where time-lapse multicomponent seismic surveys have been acquired. This area includes the Wishbone section, where 11 horizontal wells have been drilled, and is the focus of dynamic reservoir characterization. The primary goal of this research study is to investigate relationships between geomechanics, stress and fractures. Determining the geomechanical properties of the reservoir is essential for better reservoir management. Geology is the main driver controlling production, due to the presence of fault compartmentalization in the field. The central graben, within the Wishbone section, causes geologic heterogeneity and displays signs of high net pressure. This is due to a larger increase in pore pressure, ultimately decreasing effective stress. Outside of the graben, naturally fractured areas, displaying decreasing net pressure trends, will maximize fracture network surface area during completions. This allows for a larger volume of rock to be stimulated, and a greater chance of opening pre-existing fractures. As far as re-fracturing efforts are concerned, areas outside of the graben which are brittle and have low stress anisotropy should be targeted to create complex fracture networks. Geomechanical and stress information about the reservoir is vital for predicting fracture propagation. After investigation of fracture characterization trends, we have a better understanding of stimulated areas within the Wishbone section. Specific completion techniques can be applied to stages based on geomechanical properties and geologic location. Fracture networks defined through the integrated dynamic reservoir characterization process provide targets for future re-fracturing efforts.