Constraining Geomechanical Models of Hydraulic Fractures using Seismic Moment Tensors
Urbancic, Ted; Baig, Adam; Goldstein, Shoshana
Microseismic monitoring has become an attractive option for tracking hydraulic fractures stimulations because, unlike most other monitoring techniques, it can illuminate the behavior of fractures away from a treatment well. However, in most cases, the potential for microseismics in terms of developing a full picture of the fracture interactions within the reservoir is not fully exploited. Based on the analysis of microseismicity associated with stimulations in naturally fractured shale reservoirs, we illustrate how, using advanced seismic signal analysis techniques, namely seismic moment tensor inversion (SMTI) approaches, we can identify: 1) the failure type, such as mixed-mode shear/tensile failure on a rough fracture surface, 2) the fracture connectivity as related to the number of intersecting fractures in a volume, 3) the fracture intensity based on the developed fracture lengths per volume, 4) the fluid flow pathways and enhanced fluid flow volume as related to the relative degree of open fractures, and 5) the distribution of fracture lengths (power law distribution). Based on our analysis, we identify that most failures observed are mixed-mode failures, typically shear-tensile with either crack opening or crack closure components of failure. The fractures themselves are related to the failure of pre-existing natural fractures rather than in the creation of new fractures. Based on the finite sampling (bandwidth limitations), fracture sizes are limited to joint lengths and follow a power law distribution. By examining the spatial and temporal behavior of opening dominated failures, maps of over-lapping zones of potential enhanced fluid flow were identified. In many ways, stress induced fractures during single stages prepared the reservoir for subsequent stages that overall enhanced the interconnectivity and complexity of fractures thereby enhancing fluid flow opportunities. We further discuss, as outlined in this case study, how, using SMTI, the microseismic data show that the stimulation program as designed achieved its objectives. Overall, we further suggest that these defined seismic parameters can then be used to refine, validate and constrain geomechanical models used as input to reservoir models and further optimize well and stage spacing to effectively drain a reservoir and provide better defined reserve estimates.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013