Microseismic event analysis is a valuable source of information that can play a pivotal role in optimizing well completion and spacing. This analysis can be taken a step further with the generation of discrete fracture networks (DFNs) from microseismic events. While DFNs can be modeled with microseismic event locations only, source mechanisms inverted from near surface-acquired microseismic data provide greater constraints for the DFN model so that the orientation of failure planes responsible for events can be explicitly assigned. The differences between such DFN realizations based on event locations only and source-mechanism constrained DFN realizations are evident in areas with significant geological complexity.Three iterations of a DFN model were produced from a microsesimic monitoring project in the Barnett shale. The fracture network of the first iteration is modeled stochastically using only basic geologic assumptions for the area and microseismic event locations and the orientations of trends formed by the events. The second iteration is refined by deterministically locating fractures in the model and defining the fracture orientations using a source mechanism determined from the microseismic point set. The third iteration uses the results from a mechanism scan on an event per event basis to determine the best source mechanism that fits the polarity reversal signature observed on the surface array.Refining the model by determining the mechanism of individual events can identify multiple fracture orientations within the point set. In this data set two distinct mechanisms were identified, further analysis of which identified separate event energy distributions for the two mechanisms.The changes in the model can be quantitatively evaluated with analysis of flow properties generated from the DFN and output to the stimulated reservoir volume (SRV). While changes in the SRV and total fracture volume for models presented in this study are most significant between the first two iterations, the total permeability change across the geocellular volume is significant between all three iterations.
The mapping of microseismic events induced by hydraulic fracturing plays an important role in well completion and design. This is especially true in a newly developing area of gas producing shales. In this case study, we will show how the microseismic monitoring of a hydraulic fracture treatment in the Marcellus Shale identified a pre-existing natural fault which intersected the wellbore. The data from nearby wells indicated several possibilities of structural evolution affecting the producing formation. These range from regional reverse or strike-slip faulting to small displacement local reverse faulting. The hydraulic fracture stimulation was monitored using a 10 line, radial surface array composed of 1000 vertical component geophone stations. The treatment consisted of seven perforated stages stimulated with slickwater and proppant. Microseismic activity mapped during the early stages of the treatment is consistent with the regional stress direction and indicates that stages 1-4 activated natural fractures oriented along the maximum horizontal stress direction. During stages 5 and 6, the hydraulic fracture encountered a pre-existing natural fault. A source mechanism was determined for events occurring along the fault, identifying oblique failure with strike-slip and reverse faulting along the steeply dipping fault with SSE strike. This indicates that the regional strike-slip fault, with a strike similar to the break we observed at other offset wells, is most likely responsible for the geological evolution of this formation.
Microseismic monitoring of hydraulic fracture stimulation is used to map the extent of fracture growth during the completion of unconventional resource wells. Usually the geometry of the event distributions is used to infer fracture plane orientations, for instance when microseismic events are concentrated along a particular azimuth. Often the induced microseismicity is the result of reactivation of existing fractures in the reservoir. Source mechanism analysis that allows identification of the specific fracturing behavior of individual microseismic events can aid differentiation between reactivation of existing fractures and the creation of new fractures. This paper presents the results from the microseismic monitoring of a Mid-Continent horizontal gas shale well where failure planes of source mechanisms from the microseismic events are compared with fractures identified in a resistivity image log. The source mechanisms originate from failure on existing fracture planes, many of which the image log show to be partially or completely healed. The reactivation of these fracture planes are the dominant failure mechanism detected by the monitoring, but additional fracture planes were also likely stimulated by the treatment but seismicity associated with other fractures has a signal to noise ratio below that required to invert for source mechanisms. Enhanced production resulting from the stimulation is expected to result from a combination of fractures in the natural fracture network; those related to the source mechanisms and other fractures that may be opened aseismically. The result is a well-connected fracture network because of contributions of flow from multiple fracture orientations.
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