Microseismic Monitoring in Oil or Gas Reservoirs
by Leo Eisner, Ph.D., MSc.
Duration: Two days
Intended Audience: Entry and Intermediate levels
Prerequisites (Knowledge/Experience/Education Required): The course is designed to be followed by anyone with a broad geoscience background: no specific detailed foreknowledge is required, although a familiarity with geophysical terminology will be useful.
This class focuses on understanding the measurements made in passive seismic and their uncertainties. I explain principles of microseismic monitoring ranging from acquisition in a single monitoring borehole to surface and near surface networks to engineering applications of microseismicity. The case studies are used when they illustrate the conceptual point, however they are not the main focus of this class. At the end of this class, attendees should be able to design, select the right kind of processing and understand the uncertainties in the microseismicity. Attendees will be able to understand and avoid interpretation of uncertain observations and gain insight in true information provided by microseismicity. No requirement on prior class is needed, although knowledge of hydraulic fracturing and seismology helps. Social and scientific aspects related to felt seismicity in the vicinity of oil and gas reservoirs will be discussed including recent case studies. The course will also discuss the latest development in microseismic applications from source mechanisms, though anisotropy to engineering applications. The class will focus on what a potential geoscience or engineering user of microseismic information needs to know to make good decisions procuring, understanding and applying microseismic results and information in your work.
- Introduction: Why do we perform microseismic monitoring? Definition of microseismicity, induced/triggered seismicity, a brief review of microseismicity outside of oil industry: water reservoirs, mining, geothermal. Oil reservoir production induced seismicity. Historical review of microseismicity in oil industry with focus on hydraulic fracturing (M-site, Cotton Valley, Barnett, etc). Principles of the hydraulic fracturing and geomechanics.
- Earthquakes: number of unknowns, differences from active seismic. Instruments suitable for measuring earthquakes and their optimal parametrization. Earthquake location techniques. Relative locations. Location techniques.
- Downhole location technique: single well monitoring acqusition. S-P wave time and P-wave polarization technique location, P-wave and S-wave polarization. P-wave or S-wave only location from a single monitoring borehole. Horizontal monitoring boreholes. Picking strategies for downhole microseismic data. Optimal design of downhole monitoring array. Magnitude and detection. Orientation of downhole geophones/deviation surveys/velocity model calibration. Inclined/dual and multi well monitoring techniques. Check list for downhole monitoring.
- Surface monitoring technique: Why do surface and near-surface microseismic monitoring? Imaging of microseismic event using P-wave migration. Uncertainty associated with P-wave only locations: depth vs. origin time. Detection uncertainty and signal-to-noise ratio. Frequency content, attenuation and detection. Design of surface monitoring array. Calibration shots/velocity model building: isotropic vs. anisotropic velocity. Relative locations through cross-correlations and using S-wave from surface monitoring. Downhole and surface location case study. Near surface amplification. Check list for surface monitoring.
- Source mechanisms: concept of source mechanism and why do we care about source mechanisms. Definition of the dip, the strike and the rake for a shear source. Description of shear, tensile, volumetric, CLVD source through moment tensor. Inversion for source mechanisms from single monitoring borehole/ multiple monitoring boreholes/ surface P-only data. Radiation pattern of typical source mechanisms.
- Advanced source characterization: Vp/Vs ratio and amplitudes from point source, Seismic moment and Magnitude, Energy of seismic events and moment, Intensity, relative magnitudes. What is the b-value and how can we use it. Stress drop and source dimensions.
- Anisotropy: Introduction to anisotropy. Effect of anisotropic media on S-waves: shear wave splitting. Shear wave splitting observed in microseismic data. Inversion of anisotropic media from P and S-waves using microseismic events. P-wave anisotropy in surface monitoring data. Time-lapse changes in anisotropy.
- Engineering applications of microseismicity: Current use of microseismicity in oil industry and implementation of microseismicity into modeling. Microseismic based completions evaluation. Diffusion model for pressure triggering of microseismic events. Discrete Fracture Networks constrained by microseismicity. Stimulated reservoir volume. Microseismic and production, reservoir simulations and history matching.
- Seismicity in the vicinity of oil or gas reservoirs. Theory and history of induced felt seismicity. Seismic moment and total injected volume. Blackpool case study as an example of induced seismicity. DFW seismicity case study. Oklahoma seismicity triggered by salt water disposal. Hazard assesment and mitigation.
- Review of recent important case histories. Summary of microseismic pros and cons, applications pros and cons. Business considerations – microseismic timelines, deliverables, minimum standard. Relationship between microseismicity and hydraulic fracturing. The road ahead. Most important things to remember about microseismicity.