Incorporating Microseismic Ruptures to Build a Discrete Fracture Network

Abstract

In order to model the production decline of reservoirs after hydraulic fracturing, the optimal workflow includes the collection of microseismic data for the calibration of geomechanical models for assessing long term reservoir behaviour. To incorporate the microseismic data, a critical concept to consider is that the microseismic events are finite-sized shear-tensile ruptures of areas of fractures in the reservoir that occur with given orientations. Geomechanical models require certain parameters of the fracture distribution, including their size scales (power law), orientations, and spacings. As such, there is a significant amount of overlap in these concepts, but still room to further specify the methodology for incorporating microseismicity. Seismic moment tensor inversion (SMTI) is the key to associating the microseismic events with a discrete fracture network (DFN). When events are detected from a number of azimuths, the fracture orientation associated with the rupture can be inferred. Coupled with the estimates of rupture length, a model of the DFN can be constructed by appropriately orientated and sized penny-shaped fractures. To extend this methodology to events that cannot be used for SMTI (due to poor quality, or poor geometry for mechanism resolution) the SMTI fracture orientations can be used to guide an algorithm to assign orientations to these events. In order to provide useful input into geomechanical models, the data need to be further assessed. Generally, geomechanical models require specification of a few joint sets so we use a statistical test to determine which microseismic fracture orientations are most significant. On those dominant fracture sets, we can estimate the power law exponents that describe their fracture lengths, and a scanline approach to describe their spacings. In our paper we discuss not only the methodology for defining a complete DFN based on a subset of known fractures as determined from SMTI, we show the applicability of this approach as applied to hydraulic fracture stimulations in unconventional shale plays. Additionally we show how the obtained DFN can be used to assess the underlying systematic fractures that will allow for the simplification of geomechanical model development. We also consider the effect these observations have on power law determination when incorporating field mapping and core data along with 3D seismic curvature data that is used to define fracture lengths at larger scales.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016