Studies have shown that proppant injected into fractures during hydraulic stimulation rapidly increases in packing density as fluids leak off into surrounding rock. Stresses are amplified at proppant grain contacts elevating the potential for stress-corrosion cracking and chemical potential at the contacts. Together, these effects promote immediate mechanical compaction and drive chemical compaction throughout engineering time scales (Lee et al, 2010). Draining the reservoir further enhances stresses leaving the reservoir critically-stressed as fractures close. Injection of fluid induces microseismicity that generally propagates away from injection ports as fluid induces fractures at rates that can be modeled using a pressure-diffusion model (Shapiro, 2009). When the front encounters a depleted reservoir on offset wells, pressures accelerate through the fluid-filled pore network inducing shear failure causing microseisms with observed apparent propagation velocities much higher than typical fracture propagation rates and can be used to delineate depleted fractures (Dohmen, 2013) forming a snapshot of production in time.
Microseismic data were collected during the treatment of a four-well pad in the Williston Basin. After five months of producing hydrocarbons from the first pad, a second pad was also treated and monitored proximal to the first. Microseismic events recorded during the second pad treatment extended toward and accelerated across the first pad, with the majority of offset activity occurring on the well closest to the second pad. By combining hypocenter locations, seismic moments, focal mechanisms, fluid leakoff, treatment volumes, and rock properties, we created a calibrated proppant-filled Discrete Fracture Network (DFN) model for each pad. To further condition the model for the second pad, we extended the methods of Shapiro and Dohmen to define multiple pressure-diffusion fronts to classify events associated with injected fluid, the offset pad, and depleted portions of the reservoir and utilized only fluid related events.
Microseismic event locations monitored during the second pad coincide with the modeled proppant-filled fractures derived from the treatment of the first pad. Furthermore, events consistent with Dohmen's depletion zone coincide with distal producing wells over 6000 ft. away from injection ports. Results suggest that offset well microseismicity is associated with the more conductive offset proppant-pack and can be used to quantify the actual proppant distribution, validate the propped DFN model and identify compacted portions of the depleted reservoir.
