Hydraulic fracturing in shale formations induces microseismic events in a region we refer to as the microseismic volume. Many of these microseismic events are signatures of failure in the formation that are believer to be a result of induced fractures, beyond the primary propped fracture. Aerially extensive microseismicity may be evidence that these induced, unpropped fractures occur and extend spatially beyond the propped fracture in many unconventional reservoirs.
To illustrate these effects, microseismic and radioactive tracer data is presented for four laterals drilled and fractured from a single pad. Our simulations show that the opening of these induced, unpropped fractures results in significant temporary changes to the stress field in the rock. One consequence of this is that later fracture stages tend to propagate into the open fracture networks of induced, unpropped fractures created earlier due to stress reorientation. This can lead to inefficient usage of time, fluid, proppant, and capital since the region being stimulated has already been stimulated by the previous stage. By analyzing the net pressure, radioactive tracer and microseismic data from the four-well pad, we show that these induced, unpropped fractures close over time (over a period of hours) as the fracture fluid leaks-off. This relaxes the stresses and subsequent induced fractures are no longer subjected to the significantly altered stresses, allowing for more efficient fracture network coverage by subsequent fractures in a horizontal well. Based on the data presented and computer simulations, we propose the idea of establishing a minimum time between fracturing in the microseismic volume of a recently fractured region. The time required for the induced unpropped fractures to close can be calculated from our models and varies based on the reservoir and fluid properties but is typically on the order of hours. One example of how this is accomplished in practice is zipper fracs. However, our work suggests that there may be other fracture sequencing strategies for accomplishing this as well.