Analysis of Microseismic Events during a Multi-stage Hydraulic Stimulation Experiment at a Shale Gas Reservoir

English Abstract:

Microseismic events are a useful indicator of fluid migration associated with hydraulic fracturing and thus monitoring of microseismic events has made significant contributions in understanding the geometry of fractures associated with hydraulic stimulation. The key is to track the creation of fractures during and after the stimulation. One of the most important steps in determining the geometry is to detect and accurately locate microseismic events. In this study, we process and analyze 176 events generated during a multi-stage stimulation experiment at a shale gas reservoir in Saudi Arabia. The monitoring array includes a downhole array of 12- 3C sensors that were deployed in a vertical well with a 100 ft level spacing. There were 12 stages of stimulation and we only focus on the events located during the first 6 stages. Our aim was to analyze microseismic events to better understand fracture growth and fracture directions. This work was conducted as part of a larger study in developing a methodology for generating dynamic, high-resolution seismic and geomechanical models of shale reservoirs before, during, and after stimulation, and interpreting the models in terms of fracture susceptibility and fracture dynamics. Preliminary results show that the data signal to noise ratio is satisfactory for locating local magnitude (M_L) events up to -4.0 during the early stages. We observed events with M_L >+1.0 to a maximum of +1.97 near the injection well in the latter stages. During the initial stages, most events are scattered near the injection well, however, in later stages, they migrated westerly towards the monitoring well and show increased vertical distribution. Additional processing is currently underway to precisely relocate some selected events and estimate the focal mechanism as well as to carry out a magnitude analysis. We believe that this study will provide additional information needed to identify fluid migration associated with the stimulation.