Physical Mechanisms Related to Microseismic-Depletion-Delineation Field Tests With Application to Reservoir Surveillance

In unconventional hydrocarbon resources, determining appropriate infill spacing for horizontal wells is a difficult reservoir-engineering task that can have major economic implications. Infill-well spacing should be dense enough to maximize contact with the reservoir, but the wells should not overlap into regions that were depleted previously. Similarly, refracturing operations depend heavily on the reservoir response during production. Techniques must be developed that one can apply in the field to identify the extent of the depleted zones near production wells. Microseismic depletion delineation (MDD) was proposed as one such reservoir-surveillance technique. MDD takes advantage of a poroelastic-stress change that occurs as a result of reservoir depletion. After a significant period of production, fluid is reinjected into the well at low rates for a short duration to cause pressure to rise in the network of fractures connected to the well. One can determine appropriate injection pressures to ensure that fractures that fail in shear during reinjection must exist within the depleted region. One could infer the shape and the extent of the depleted zone by monitoring for microseismic activity during the injection test. In this study, we used numerical modeling to investigate the physical mechanisms that underpin the MDD technique. We first modeled a multistage hydraulic-fracture treatment of a horizontal well in a naturally fractured reservoir. After the stimulation treatment, we modeled 1 year of production to cause pressure draw-down and an associated poroelastic response. The field test was then modeled by reinjecting water into the well at low rates and pressures for a short time. Shear-slip events on fractures that occurred during reinjection were considered proxies for microseismic events. The heterogeneous distribution of pressure draw-down was demarcated clearly with the locations of the microseismic events. The results indicate that one could apply MDD in the field as an effective means of reservoir surveillance. One could use the modeling framework that we developed to design field tests and to interpret field data.