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Modeling Rock Fragmentation by Pressure Pulse
A large international energy company is investigating the use of chemically induced pressure pulses to stimulate unconventional hydrocarbon reservoirs. The concept involves pumping a mixture of chemicals into a wellbore, then a rapid exothermic reaction is triggered by the formation temperature or by the introduction of a catalyst. The reaction creates a pulse of pressure that induces tensile hoop stresses in the formation, leading to radial fracturing around the well. It is envisioned that this will be a lower-cost option for stimulation than traditional hydraulic fracturing.
A series of laboratory-scale experiments were conducted by the client to assess the potential of these chemical reactions to fragment rock. A series of tests were conducted on cubes of rock 25 cm in scale. The chemical reaction can be configured to have different rise times and peak pressures. The client wants to know what peak pressure and rise time are optimal for improving permeability but not excessively damaging nearby wellbore rock.
Itasca Consulting Group developed a numerical model of the rock fragmentation laboratory experiments. Itasca's discrete element software PFC3D is used to represent the rock and model the fracturing process. The Itasca material-modeling support package is used to create specimens of spherical-grain, flat-jointed synthetic material, which are subject to explosive loading. Physically, the reaction product gas pressure in the hole is controlled by several interactive processes. As a simplification, the pressure in the hole is assumed to follow a predefined path. Python scripting, which is embedded in PFC, is used to apply the gas pressure and generate the visualization.
Itasca was able to reproduce the qualitative features of the laboratory experiments with PFC3D. The figure to the left shows the fragmentation of a block specimen expected at a range of peak pressure and pressure rise time values. As expected and as observed in the experiments, the number of radial fractures increases as the rise time decreases. The intensity of the rock fracturing and crushing near the hole increase as the peak pressure increases. The permeability change is estimated from the predicted fracturing. Insights gained from numerical modeling have helped the client develop the chemical stimulation methodology.