In this case study, a downhole microseismic array was used to monitor a Woodford Shale hydraulic fracture treatment. The initial processing resulted in events being located significantly above the horizontal treatment well and laterally offset from the initiation point, along with a lack of microseismicity at the initiation points. Interpretation of the microseismic image was therefore compromised and undermined the value of information to confidently interpret the fracture geometry. A quality control evaluation was performed to assess the location patterns, including the use of synthetic microseismic signals computed from different origin points. The stages with the suspected processing artifacts were found to suffer from significant signal complexity. The microseismicity was relocated using the synthetic signals to guide phase interpretation, which resulted in locating microseismicity much closer to the fracture initiation points. Discrepancies between the original and reprocessed results were assessed by correlating synthetic waveforms from the corresponding locations with the recorded signals. The reprocessed results were shown to better correlate with the recorded waveforms than the original, disperse locations.
The case study represents an example of critically evaluating differences between various processed results, to quantify the match with the recorded signals. A workflow is presented which allows for quantification of the confidence in the microseismic results through correlation of the match with the recorded data. Although the case study is specific to the Woodford, the workflow could be applied to any dataset with concerns about microseismic location robustness.
Microseismic hydraulic fracture monitoring has grown into a standard technique for imaging hydraulic fracture geometry and is extensively used to understand reservoir contact and improve design of well completions and hydraulic fracturing. Ultimately the quality of the hydraulic fracture image is controlled by the accuracy of individual microseismic event locations. Location accuracy is related to the monitoring array configuration, data quality (e.g. signal-to-noise ratio) and the velocity model. In certain configurations, however, complex microseismic signals can be recorded when significant amplitudes of refracted or reflected waves interfere with direct arrivals. Such signal complexity can lead to substantial inaccuracies in microseismic locations if an inconsistent seismic phase is matched during the microseismic processing. Reprocessing of microseismic data can often result in dramatic location differences if the signal phases are interpreted differently, even in a scenario where similar velocity models are used.
