Abstract

Microseismic events are routinely used to identify overall hydraulic fracture characteristics such as geometry, half width, stage overlap, and estimated stimulated reservoir volume. Engineers utilize this information along with injection data to assess the effectiveness of a stimulation program. However, the potential for microseismics in terms of developing a complete picture of the fracture interactions within the reservoir is generally not being fully exploited. By interpreting microseismic results using advanced seismic signal analysis techniques such as seismic moment tensor inversion (SMTI), it is possible to examine: 1) fracture failure type, such as mixed-mode shear/tensile failure on a rough fracture surface, 2) fracture connectivity as related to the number of intersecting fractures in a volume, 3) fracture intensity based on the developed fracture lengths per volume, 4) fluid flow pathways and enhanced fluid flow volume as related to the relative degree of open fractures, and 5) distribution of fracture lengths (power law distribution). Additionally, by using these techniques, the effectiveness of different stimulation programs can be assessed. For example, examining the degree to which hesitation processes can be used to create a dendritic (branching) fracture network to enhance well productivity and drainage.

In our studies of various naturally fractured shale reservoirs in North America, we have identified that most observed failures are mixed-mode failures, typically shear-tensile with either crack opening or crack closure components of failure, and that the fractures themselves are generally related to the failure of pre-existing fractures. Based on finite sampling (recording bandwidth limitations), fracture sizes are generally limited to joint lengths and follow a power law distribution. By examining the spatial and temporal behavior of opening dominated failures, maps of intersecting zones of potential enhanced fluid flow can be identified. In many ways, stress induced fractures during initial stages appear to prime the reservoir for subsequent stages, improving the interconnectivity and complexity of fractures and thereby enhancing fluid flow opportunities. We also show that fracture control can be achieved using a hesitation process, where the degree of secondary fracturing is related to the duration of the shut-in period and thereby the relaxation of local stresses. Overall, the use of advanced seismic signal analysis techniques allows for the calibration and validation of reservoir simulation workflows, the establishment of surveillance methodologies, the potential to improve drainage and more accurately establish reserve estimates.