HR: 11:05h
AN: T42C-04    [Abstracts]
TI: Formation of Network Fractures During Hydraulic Fracturing of the Barnett Shale, a Tight Gas Shale with Preexisting Fractures
AU: * Busetti, S
EM: sbusetti@ou.edu
AF: School of Geology and Geophysics, U of Oklahoma, Norman, OK 73019, United States
AU: Reches, Z
EM: reches@ou.edu
AF: School of Geology and Geophysics, U of Oklahoma, Norman, OK 73019, United States
AB: Hydraulic fracturing operations generate new fractures as well as dilate preexisting fractures, creating networks of fractures. Here we model the complexity of the created network fractures and apply the results to wellbore log data and hydrofracture operations in the Barnett Shale, a tight gas-shale that requires artificial fracture stimulation to produce. It is shown that the resulting fracture geometry is related to the state of local stress, orientations of the existing fractures, and qualities of the hydraulic fracturing operation. The model assumes that preexisting fractures dilate when the hydrofracture pressure is larger than the normal stress across them. The orientations of the dilating fractures can be presented as 2θ_w and 2θ_L (width and length of the dilating fracture population on a stereographic projection). The model indicates that sin²θ_w /sin²θ_L = (σ₂ - σ₃)/(σ₁ - σ₃) and sin²θ_w = (P_m - σ₃)/(σ₁ - σ₃), where σ₁, σ₂, and σ₃, are the principal stresses of the local stress field and P_m is the hydrofracture fluid pressure. We expect the shape of the stimulated rock volume to vary (e.g., spheroidal, elliptical, penny- shaped) under different local conditions corresponding to 2θ_w and 2θ_L. We apply the above relations to hydrofracturing data from the Barnett Shale, Fort Worth Basin, Texas. We combined the pressure data of the hydrofracture operations with pre- and post-treatment fracture orientations determined on down-hole image logs to characterize the local stress magnitude and orientation. It was found that in the studied wells: (1) σ₁ =S_v, calculated from overburden at the depth of fluid penetration; (2) σ₂ =S_H from the inversion of borehole sonic data (run as a post-drilling logging tool) or from solving the stress state around a borehole; and (3) σ₃ =S_h, calculated as the instantaneous shut-in pressure from pressure-time curves. The direction of S_H corresponds to the orientation of drilling-induced fractures, interpreted from formation multi-imaging logs (FMI). We use the spatial distribution of micro-seismic events recorded during the hydrofracture operations as a proxy for the shape of the stimulated rock volume. We use these observed volumes to test our model predictions based on monitored hydrofracture fluid pressure and the calculated local stresses. It is shown that at sites where near-field stresses are highly anisotropic, S_H ≫ S_h, stimulation is restricted to preferentially oriented fractures and the volumetric zone of stimulation is a prolate spheroid, elongated in the direction of the far-field maximum horizontal stress. Under conditions of low near-field stress anisotropy, S_H=S_h, dilation of multiple fracture orientations is possible and the volumetric zone of stimulation is an oblate spheroid. Additionally, we show that when detailed fracture orientation data is available, the stimulation model can be used to back-calculate the stress state.
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
DE: 8020 Mechanics, theory, and modeling
DE: 8094 Instruments and techniques
DE: 8168 Stresses: general
SC: Tectonophysics [T]
MN: 2007 Fall Meeting