Tomographic fracture imaging: Examples of induced fracture and reservoir-scale observations during wellbore stimulations, Niobrara and Bakken plays, USA

Abstract

Classic microseismic techniques have been unable to adequately resolve many of the questions that have been asked with respect to reservoir stimulation and production in the Bakken and Niobrara unconventional resource plays as they have matured over the last few years. This has become evident especially in relation to the infill drilling and restimulation phases of field development. In support of efforts to design efficient continued development of operations in these areas, Whiting Oil and Gas has utilized a different, but related, technology to enhance understanding of the subsurface environment during and after reservoir stimulation treatments. We present two separate field examples of a novel reservoir monitoring technology that utilizes microseismic surface geophone arrays and seismic emission tomography (SET) methodologies to directly image both natural and hydraulically induced fracture systems. Tomographic fracture imaging (TFI) was developed with the goal of directly identifying and mapping active induced or natural fracture systems, as opposed to inferring them from a population of microseismic hypocenters as is done in classical microseismic data acquisition and analysis. Examples shown here consist of results from a purpose-designed array intended to monitor the stimulation efficiencies of three wellbores on a single Niobrara drill pad that was completed within a very short timeframe, and the results of a much larger reservoir-scale study showing the 4D reservoir effects of field production over a nine-month period by the TFI reprocessing of legacy microseismic data in the Bakken. These examples represent clear empirical evidence that the TFI technique is effective in detecting and localizing active fracture networks that are highly correlative to other wellbore data, even though much remains to be researched about the exact mechanisms involved in the generation of seismic energy within fracture networks.