This paper presents:
a brief review of misconceptions on industry standard brittleness/ductility definitions,
geomechanical aspects & numerical evaluation of pulsed fracturing using an advanced constitutive model implemented within the AUTODYN code, and
fracture network patterns due to pulsed loading in shale that show ductile-brittle transition.
In shale gas, one of the primary goals is to create extensive fracture networks that can remain open during production. Field experience has shown that not all shale formations respond to hydraulic fracturing effectively. It is important to identify and accurately design alternative fracturing techniques that would overcome some of the limitations. In general, hydraulic fracturing involves a relatively slow rate of loading on surrounding rock and results in bi-wing fracture geometries. Explosive fracturing involves very rapid loading of the formation and results in simultaneous propagation of multiple fractures. However, due to extreme stress/heat generated during the explosion, near wellbore region might reach plastic flow and/or compaction limit. Pulsed fracturing rates and peak loads (via high energy gas or propellants) can be customized to lie between hydraulic and explosive fracturing. This technique has the potential to shatter shale, in particular by triggering a ductile to brittle transition at an optimized pulse rate. That is, brittleness or fracture potential is not a " material property" but rather a " material behavior" that can be modified.
Pulsed fracturing is characterized in the field by peak pressures exceeding both the maximum and minimum in-situ stresses. However, operational considerations for success remain qualitative. Recent advances in computational geomechanics help us quantify the effect of key parameters on pulsed fracturing techniques for field applications. Further to this, advanced constitutive models implemented for these analyses have the benefit of simulating ductile to brittle transition, if the stress state & loading conditions dictate that the material should. This study on pulsed fracturing shows that for a certain combination of reservoir, geomechanical and pulse loading parameters: fractures can propagate in multiple directions. This phenomenon might promote a self-propping mechanism for a network of fractures. At the end, favorable conditions when pulsed fracturing technique would work and key parameters that trigger ductile to brittle transition are summarized and presented.