Microseismic monitoring in unconventional reservoirs: are natural fractures hiding in plain sight?

Abstract

As the shale revolution is adjusting to the realities of the new market and lower oil prices, the operators’ focus is shifting towards unconventional reservoirs and increasing hydrocarbon production efficiencies. The oil and gas operators’ demands are transforming our paradigm of reservoir characterization, forecasting, and monitoring. As such, heterogeneity of an unconventional reservoir is one of the main factors impacting well productivity. Well performance is driven by the size and efficiency of the interconnected fracture ‘plumbing system’ which is influenced by the presence of natural fractures and the process of multistage hydraulic fracturing. A complex, interconnected natural fracture network can significantly increase the size of the stimulated reservoir volume, provide additional surface area contact and enhance permeability. However, the interaction of the natural fracture networks and the completions-induced fractures is complex and can be affected by multiple factors, such as present-day maximum horizontal stress orientation, character of pre-existing natural fractures, presence of faults, lithology types that the horizontal well is being drilled through, as well as the well completions design itself. With a trend of unconventional resource plays’ development, the Niobrara Formation has become a focus of interest for operators. Recent advances in horizontal drilling and hydraulic reservoir stimulation have allowed for economic development of tight resource plays. In the presence of heterogeneity optimal well placement, orientation, and spacing are especially important. Natural fractures are expected to significantly influence the completions’ efficiency and have the potential to contribute to SRV (stimulated reservoir volume) and drainage volume. Hydraulic fracturing with evenly-spaced frac stages may not be the most successful stimulation method or yield the greatest production. In the given circumstances the process of reservoir monitoring is essential for accurate assessment of the produced reservoir volume (PRV) and for our understanding of how effectively the target formation is being stimulated. Microseismic data is being called upon with ever increasing frequency to satisfy the industry’s reservoir monitoring needs. In this study we take a look at one of the major unconventional resource plays in the US. The research was designed to conduct dynamic reservoir characterization and to test the strengths and limitations of microseismic monitoring. The study integrated 3D seismic data, microseismic data, geophysical well log and formation image log data, and core and outcrop data within the focal area. One of the goals of the study is to analyse factors capable of affecting the distribution of microseismic events. An important question to be answered: ‘Will microseismic data, taken alone, prove to be a sufficiently representative source of information when evaluating the reservoir response and estimating success of completion strategies?’