Ulrich Zimmer

Impact of distance-dependent location dispersion error on interpretation of microseismic event distributions

Along with horizontal drilling and hydraulic fracturing technology, monitoring-induced microseismic activity during hydraulic fracturing has played a significant role in the economic development of todays prolific tight-gas and shale resource plays. Many microseismic fracture monitoring papers in the current literature discuss the equipment, how to acquire and process microseismic data, and how to determine an appropriate velocity model. Unfortunately, for an operating companys completion engineers and geophysicists charged with interpreting these data and integrating them with other data sets, only a few papers discuss the impact of artifacts and location uncertainties on the interpretation. While microseismic data have been very useful in the economic development of resource plays, there are a number of significant pitfalls and interpretational issues that, if not understood, will potentially lead to erroneous interpretations. This is especially important for any real-time decisions during a hydraulic...

Evidence of a Horizontal Hydraulic Fracture From Stress Rotations Across a Thrust Fault

Microseismic imaging of a hydraulic-fracture stimulation showed significant fracture reorientation across a thrust fault. Fracture orientations were identified through a combination of alignment of event locations, polarization of the seismic waves, and injection details. Stimulation below the fault indicated a near-horizontal fracture geometry. Above the fault, a near-vertical fracture geometry was observed. A change in fault orientation was supported by differences in the microseismic-signal characteristics and the treatment-injection data. This difference in fracture geometry was attributed to rotations in the direction of minimum principal stress, which is consistent with observed differences in the injection pressures. To order the full paper, visit https://doi.org/10.2118/110696-PA

Hydraulic Fracture Reorientation Across a Thrust Fault

Microseismic imaging of a hydraulic fracture stimulation showed significant fracture reorientation across a thrust fault. Fracture orientations were identified through a combination of alignment of event locations, polarization of the seismic waves and injection details. Stimulation below the fault indicated a near horizontal fracture geometry. Above the fault, a near vertical fracture geometry was observed. This difference in fracture geometry was attributed to rotations in the direction of minimum principle stress, which is consistent with observed differences in the injection pressures. The fracture reorientation suggests a significant difference in the stimulated permeable pathways.

Realistic uncertainty space for microseismic event locations from multiple well recordings

Microseismic event locations are increasingly used as input parameters for additional calculations, e.g. stimulated reservoir volume (SRV), moment tensor inversions. Uncertainties in the event locations often have a direct effect on the derived parameters and therefore it is important to quantify the location uncertainty. Although companies generally report location uncertainties in form of error bars, often along Cartesian coordinates, the calculation of these error bars and their interpretation is not standardized and sometimes obscure. By assigning each point in the localization grid a probability for the event location based on the available input data it is possible to use input from very different sources, e.g. traveltime residuals, hodogram analysis, velocity model uncertainties, and combine them in a probability grid. The distribution of the overall probability can then be used to define the uncertainty space based on the desired level of accuracy, e.g. the 95% confidence uncertainty space will be larger than the 90% confidence space. This method has the advantage that very different types of uncertainty, e.g. sensor position, can be included in the calculation and that an interpretation of the uncertainty space in terms of confidence is possible. The derived uncertainty space clearly represents the space where the event is with 90%, 95% or any other degree of probability.

Quality Control/Assurance Reporting of Passive Microseismic Data

Effective interpretation of microseismic images requires fundamental quality and confidence information about the seismogram data and final image. A comprehensive report is described which effectively and concisely documents the critical parameters for quality control and assurance. The report includes documentation of the signal-to-noise ratio and arrival time and direction residuals, which is summarized with a unique confidence factor for each event. Background noise and magnitude distribution is documented to examine the image completeness, and can be used to assess whether the entire seismic deformation has been detected. Finally, images of location uncertainties from both the arrival time and direction components and the velocity model are also included. The report allows an interpreter to define the confidence in either regions or through the entire microseismic image, and also for the recorded trace data and computed seismic parameters.