Introduction to January Focus: Unconventional Reservoirs

The promise of unconventional reservoirs is sustainable hydrocarbon production from previously inaccessible reservoirs. The high capital expenditure required to develop this resource and the depressed commodity prices makes for narrow profit margins and increases the focus on efficient and optimal project execution. Most of the focus is on lowering costs of drilling and completions but many gains can be made through accurate mapping of the subsurface. This includes mapping the so-called “sweet spots”, associated to an increase in production, as well as geohazards and production obstacles, such as hydro-frac barriers. Seismic data can be quite useful in determining where “sweet spots” and geohazards occur through the use of specialized processes and workflows. More than ever, unlocking the subtle variations in seismic data and how they relate to reservoir performance relies on sophisticated interpretation of prestack and poststack seismic attributes. Geophysicists now want to relate the elastic properties estimated from seismic data and relate them to lithology, failure properties, (as they relate to hydraulic stimulation) and even estimates of in-situ stress. Structural interpretation and horizon slices to identify geomorphological seismic bodies are necessary but insufficient when attempting to increase efficiency and optimization.

The three focus articles in this month’s Recorder investigate advanced methods for using seismic estimates of elastic properties to determine frac length, mapping lithologic variations and potential hydrofrac barriers and finally, assessing the fidelity of elastic property estimates in the presence of multiples.

The first article “Hydraulic fracturing as a global cascade in networked systems” by David Cho of Qeye Labs, uses a networked system to model the propagation of fractures. He proposes that how the components of the system interact is more important than the properties of themselves. His numerical simulation yields interesting results that can be used to model recorded microseismic data induced from hydraulic simulation of the reservoir. Important takeaways can be made without dealing with the complicated fluid flow and fracture mechanics.

The second article “Unconventional shale gas reservoir characterization using the hitcube approach – the mapping of marl rich mudflows in the Horn River Basin” by Claire Pierard and co-authors, endeavour to map marl that can act as potential hydro frac barriers. The claim is that these marls are often below seismic resolution and stochastic methods are required to detect subtle seismic variations indicative of the presence of these marls. To this end, a large set of synthetic wells, derived from real wells, is constructed to model the wide array of potential seismic responses. The synthetic wells produce synthetic seismic data which are compared to actual seismic data. The synthetic trace with the highest similarity score has the properties of that pseudo well associated to the trace of the actual seismic data. It is in this way that facies, one of which is the marls, can be mapped through out the extent of a 3D survey. These maps can then be used to optimize development of the reservoir.

The final article, “The impact of interbed multiples on the inversion and interpretation of pre-stack data” by Andrew Iverson, assess the error associated with coherent linear noise in the estimates of elastic properties from seismic data. Through numerical modelling of synthetic reflection coefficients that mimic unconventional reservoirs, he investigates how the amplitude variation with offset changes with long and short period multiples. The amount of residual moveout on the multiple, where the multiple interferes with the primary reflection and the angle range available all effect the inversion result to various degrees. Iverson shows that the inversion process is quite robust though advocates modelling to determine the impact for each specific case.