Integrated microseismic analysis: From relocation to advanced geomechanical interpretation

• Author / Creator

  Barthwal, Himanshu

• Microseismic events are often recorded during hydraulic fracturing stimulations of unconventional hydrocarbon reservoirs like tight sands and shales, Enhanced Geothermal Systems, mine development and carbon dioxide sequestration. Most applications involve locating and tracking these events over time to monitor subsurface deformation. However, event locations give information only in the seismically active regions. One of the objectives of this thesis is to use the microseismic data for imaging the medium through which the seismic waves propagate. We first test the feasibility of seismic tomography using microseismic data. The data were recorded during an underground mine development in January 2011 by a network of 7 boreholes each having 4 three-component geophones. We perform P-wave double-difference tomography using the waveform cross-correlation derived differential arrival times. The relocated events tightly cluster in the space near the main working level and the access shaft and show preferential alignment along a planar surface such as a geological fault. The 3D velocity model obtained from tomographic inversion correlates well with the known geotechnical zones in the mine. Some of the mapped geological faults appear to delineate the high and low-velocity regions in the velocity model. Thus, passive seismic tomography gives information beyond the excavation damaged zones in terms of the seismic velocity which can be an effective tool in complimenting geological and geotechnical interpretations. Next, we try to understand the origin of the microseismic events recorded during an underground mine development. We compare the spatiotemporal distribution of microseismicity with various mining activities like blasting and rock removal to identify the main cause of seismicity. The microseismic events do not occur immediately following the mine blasts but show some correlation with the daily rate of rock volume removed. Furthermore, the events are located far from the actual construction sites at the main working level. Therefore, the large stress concentrations near the walls of the newly excavated cavities are not responsible for triggering microseismicity. We then model the stress perturbations due to the extensive horizontal tunnel network at the main working level and the vertical shafts. Based on the geometry of the microseismic event cluster, we propose a hypothesis that the events are triggered due to fault reactivation. Using the Coulomb stress change, we investigate the likelihood of fault reactivation based on their orientations and spatial locations with respect to the mine layout. Furthermore, we show that the dynamic stresses generated by the vibrations due to a rock crusher near the access shaft may be responsible for triggering the observed microseismicity along an unmapped fault. We then focus on the origin of the microseismic events observed during hydraulic fracturing of unconventional hydrocarbon reservoirs like tight sands and shales. We model the effect of the opening of the fracture cavity on the in-situ elastic stresses and the pore pressure diffusion profiles. We show that for a fixed length of a hydraulic fracture cavity, the first events in time occur near the crack tip region where the Coulomb stress changes are positive due to the elastic stress perturbations. The pore pressure diffusion subsequently leads to microseismic events near the fracture face where elastic stress perturbations have a stabilizing effect. Furthermore, the shape of the pore pressure diffusion front depends upon the shape of the hydraulic fracture cavity. Thus, the elastic stress changes ahead of the crack tip due to fracture opening facilitate failure and this process affects the spatiotemporal distributions of microseismicity. Finally, we study the effect of stress changes on seismic velocities and anisotropy of rocks in a hydraulic fracturing environment. We propose a forward model to compute the stress-induced seismic anisotropy due to hydraulic fracturing using third-order elasticity. We present a methodology to model the shear wave splitting delay times by simulating a real hydraulic fracturing job and acquisition set up. We then compare the measured and the modeled splitting delay times for the microseismic data acquired during a single stage of the hydraulic fracturing stimulation. Thus, forward modeling of stress-dependent stiffness tensors and splitting delay times combined with other information such as lithology and well-constrained stress measurements can be used to get insights into the potential source of anisotropy.

• Subjects / Keywords

  • Stress modeling
  • Microseismic
  • Hydraulic fracturing
  • Underground mining
  • Shear wave splitting
  • Double-difference tomography

• Graduation date

  Spring 2018

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.