Doctor of Philosophy
Part 1:Between August 2010 and June 2011, an intense sequence of induced earthquakes occurred along the Guy-Greenbrier fault in central Arkansas due to fluid injection at nearby waste disposal wells. A previous study by Horton (2010) limited to ~1,000 earthquakes having md > ~2.0 illuminated the ~13km fault. We present an updated catalogue of 17,395 earthquakes that appears complete between 0 <= ml <= 4.4 for the initial part of the sequence between August 2010 and October 20, 2010 located using an updated 1D velocity model for the region. The inclusion of the small magnitude events reveals that seismicity starts below the SRE injection well a month earlier than estimated using only md >2 events. During this period of time, the seismicity migrated from north to south enhancing the resolution of three joined sections that form the northern ~7.3km portion of the fault, which plunges southwards. The seismogenic zone covers the lower portion of the Paleozoic sedimentary layers and extends into the crystalline Precambrian basement (~3km < z < 7.5km). Earthquake size distribution varies along the fault with most ml>3 events constrained within the basement. A b-value of 1.1 was obtained for the updated catalog during this period with the b-value varying between 1.45 and 0.74 for different clusters of events. The seismicity pattern at depth is coincident with structural geologic features observed within the Fayetteville Shale (at ~1500m depth).Part 2: We model pore-pressure diffusion caused by pressurized waste-fluid injection at two nearby wells, and then compare the build-up of pressure with the observed initiation and migration of earthquakes during the early part of the 2010-2011 Guy-Greenbrier earthquake swarm. Pore pressure diffusion is calculated using MODFLOW 2005 that allows the actual injection histories (volume/day) at the two wells to diffuse through a fractured and faulted 3D aquifer system representing the eastern Arkoma basin. The aquifer system is calibrated using the observed well “drawup” following well shut-in at three wells. We estimate that the hydraulic conductivities of the Boone Formation and Arbuckle Group are 2.25×10-2 and 1.47×10-3 m/d respectively, with a hydraulic conductivity of 1.92×10-2 m/d in the Hunton Group when assuming 1.72×10-3 m/d in the Chattanooga Shale. Based on the simulated pressure field, injection near the relatively conductive Enders and Guy-Greenbrier faults (that hydraulically connect the Arbuckle Group with the underlying basement) permits pressure diffusion into the crystalline basement but the effective radius of influence is limited in depth by the vertical anisotropy of the hydraulic diffusivity. Comparing spatial/temporal changes in the simulated pore-pressure field to the observed seismicity suggests that minimum pore pressure changes of ~0.009MPa and ~0.035MPa are sufficient to initiate seismic activity within the basement and sedimentary sections of the Guy-Greenbrier fault respectively. Further, the migration of seismicity appears to follow the ~0.011MPa and ~0.055MPa pore pressure fronts within the basement and sedimentary sections respectively.Part 3: Analysis of the initiation of the Guy-Greenbrier swarm indicates there are more earthquakes during the swarm period than reported in the CERI catalog. We apply the template-matching method to determine relative earthquakes locations using relocated events in the CERI catalog as master events. We locate a total of 387,698 recognizable earthquakes between 07/01/2010 and 09/30/2011, that migrate from north to south as earlier indicated using traditionally located earthquakes. A clear correlation between seismicity rate and well pressure changes shows interaction of pore pressure from the SRE and Edgmon wells. A highly sampled well history provides a powerful tool of analyzing suspected induced seismicity following subsurface injection.
Dissertation or thesis originally submitted to the local University of Memphis Electronic Theses & dissertation (ETD) Repository.
Ogwari, Paul Otieno, "Pore-Pressure Diffusion Based on Analysis and Characterization of Microseismicity in Central Arkansas" (2015). Electronic Theses and Dissertations. 1307.