AGU Meeting-2014 San Francisco

• Laboratory Experiments on Wave Emissions Generated by the Variable Viscosity of Fracturing Fluids
  

Arash Dahi Taleghani¹; J. M. Lorenzo²

1. Petroleum engineering, Louisiana State University, Baton Rouge, LA, United States.
2. Geology and Geophysics, Louisiana State University, Baton Rouge, LA, United States.

Microseismic analysis is recognized as the main method for estimating hydraulic fracture geometry. However, because of limited access to the subsurface and usually high levels of environmental noise it becomes crucial to verify assumed fracture propagation models under more controlled laboratory conditions. Considering the fact that fluid driven fractures may grow under different regimes i.e., toughness-dominated or viscous-dominated, scaling is necessary to reproduce the corresponding fracture growth regime. Scaling is achieved by constraining material deformational parameters, fluid flow rates, and fracturing-fluid viscosity for the appropriate value of the non-dimensional toughness.

Hence, we implemented hydraulic fracturing tests on translucent plexiglass samples, at room temperature with contrasting fracturing fluid viscosities. A modest, biaxial loading frame creates relatively low directed principal stresses (< 1000 psi, or less < 1 km overburden pressure). A sealed fluid conduit generates fluid pressures (< 3000 psi) created by a positive displacement pump. We record microseismic events on the upper and lower faces of a thermally annealed, sample block (13 cm x 13 cm x 10 cm) with 3-component, broadband sensors (10¹-10⁶). Preliminary results indicate that the dominant frequency band of the microseismic events appears similar for both toughness-dominated and viscous-dominated regimes (10¹-10² Hz). The experiments in both regimes show rippled crack surfaces although in the toughness-dominated regime, ‘ripples’ are more closely spaced (mm cf. cm). The fracture surfaces show bifurcating, “wish-bone” structures only in the viscous regime.

• Moscone South
  Poster Hall

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  Microseismic mapping is a passive seismic technique used extensively for assessment of hydraulic fracturing treatments during the last two decades. Basically, microseisms are microearthquakes induced by the changes in stress and pore-fluid pressure associated with the hydraulic fracturing treatment. Current practice to locate events and determine the source mechanism of microseismic events associated with hydraulic fracture treatments only includes far-field terms for the moment tensor inversion. The intermediate-field terms and near-field term are normally ignored, perhaps simply following the tradition in locating distant earthquakes. However, source-receiver distances in hydraulic fracturing are usually 1000 ft (~300m), which is much less than the typical distances in earthquakes; therefore the effect of near and intermediate-field effects are not yet known.

  We try to include these near-field effects to improve the precision of locating the events and consequently determining the source mechanism. We find that the intermediate-field term may contribute up to 1/3 of the signal amplitude when the source-receiver distance is about 300 m. The intermediate-field term contributes ~1/20 of the signal amplitude when the source-receiver distance is ~ 2000 m . When the source-receiver distance exceeds ~ 2000 m, the intermediate-field terms can be ignored in our inversion. In our case, we also confirm that the near-field term can be ignored in microseismic analysis. Our results indicate that the intermediate-field terms can improve moment tensor inversion between 2% to 40% at source-receiver ranges less than 300 m. However for the case of large distances, the improvement using this technique is limited to 1%. In the presence of strong environmental noise, intermediate-field terms help to effectively improve the moment tensor inversion: i.e., 15% improvement with noise present vs 3% improvement without noise.