Study of microseismic events: their source mechanism, spatial and temporal distribution, and the evolution of transmitting medium

Open Access
Author:
Tan, Yunhui
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 21, 2015
Committee Members:
  • Terry Engelder, Dissertation Advisor
  • Charles James Ammon, Committee Member
  • Andrew Arnold Nyblade, Committee Member
  • Yilin Wang, Special Member
Keywords:
  • microseismic
  • hydraulic fracturing
Abstract:
Microseismic is commonly used to monitor the hydraulic fracturing process. Seismic signals are recorded using either surface or downhole geophones to invert for hypocenters of rock movements (moment magnitude usually between -3 and 0) happened during hydraulic fracturing. The location of these microseismic events are used to estimate the stimulated reservoir volume. This thesis focuses on further understanding of microseismic events beyond the cloud of locations. To test whether the lineament of microseismic event cloud represent fluid filled open fractures, we looked at the change of S wave from perforations when it passes microseismic event cloud (Chapter 2). Strong attenuation of S wave suggests that the microseismic event cloud indicates fluid-filled open fractures. We also studied the evolution of medium velocity structure during hydraulic fracturing. By inverting the arrival time of perforation shots recorded by downhole geophones, we found that the velocity of stimulated region is higher than the unstimulated region (Chapter 3). A new model (Rutledge-Eisner model) is introduced to describe the mechanism of microseismic event generation. This model states that the microseismic events are a results of shearing on bedding planes caused by opening of hydraulic fractures. Moment tensor inversion results from surface microseismic monitoring of the Marcellus shale is presented to validate this model (Chapter 4). Based on this model, we explained some phenomena in the temporal and spatial distribution of microseismic events under different geological and geomechanical conditions (Chapter 5). We found that microseismic events concentrate behind the fracture tip in a single planar fracture propagation. Low in-situ stress environment leads to early microseismic events while high in-situ stress lead to late microseismic events during fracturing. Natural fracture networks show unique microseismic temporal patterns with the increased sand concentration. Higher in-situ stress variation between layers lead to more microseismic events and larger magnitude. In summary, microseismic monitoring is an effective method to reveal details about hydraulic fracturing.