Multiscale factors that control hydrocarbon storage capacity, and successful hydrofracturing and refracturing in mudrocks

Abstract

Hydrocarbon production from mudrocks (“shales”) is vital to global economic growth and smooth transition to a clean energy infrastructure. The commercial development prospect of a shale play depends on its evolution history over millions of years. Economic hydrocarbon production from shale starts after hydraulic fracturing, that creates a multiscale fracture network leading to an increased overall permeability. The properties of the stimulated rock can be assessed via parameters at different scales (nano-, micro- and macro-scale). Better understanding of these parameters is the key to predicting well productivity and profitability.

This work aims to deepen the understanding of the multiscale parameters that define effective hydraulic fracturing. To investigate permeability increase in shales, we start with a model of micro-capillary in contact with nanopores . We show that the nanopores that discharge gas into a fracture network in the source rock significantly increase and extend gas flow into the hydrofractured horizontal wells. We then use a fractal stimulated reservoir volume model to match production histories of 45 Barnett gas wells and to quantify connectivity between the nanopores and the fracture network. This model relies on a source term, ${s}$, and fracture permeability $k_f$ . Our analysis shows that the different degrees of coupling between ${s}$ and $k_f$ create distinctly different types of fracture networks after rock stimulation and impact the well production profiles. We then couple the fractal SRV model with universal scaling $τ − M$ model to simulate production history of 1000 wells each in the Barnett, Marcellus, Haynesville and Eagle Ford shale plays. The analysis shows the coupled effect of stimulated surface area $A$, fracture half-distance, $d$, and the fractal dimension ,$D$, on production and economics of gas production. These parameters define the key differences between different shale plays in the US. Finally, we simulate microfracturing associated with hydrocarbon expulsion in the Tuwaiq Mountain source rock, Saudi Arabia, and propose the pore/microchannel blocking by bitumen/pyrobitumen as a viable mechanism of sustaining the high pore pressure in the source rock for millions of years.