Sondergeld, CarlBhoumick, Pritesh2018-12-132018-12-132018-12https://hdl.handle.net/11244/316756It has become critical to understand the location of hydraulic fracture and the extent to which it stimulates a reservoir to plan future drilling and completions. Various methods have been used to map the fracture propagation. Microseismic event mapping is a common field technique which uses the elastic energy generated during the fracturing process (see Albright and Pearson, 1982, Rutledge and Phillips, 2003 and Warpinski et al., 2004). Acoustic emission are utilized in mapping hydraulic fractures and assessing fracture mechanisms in laboratory studies as well (Matsunaga et al., 1993, Masuda et al., 2003, Damani et al., 2012). Other methods include using temperature sensors to monitor the fracture propagation in real time (Holley et al., 2010). Scanning Electron Microscopy (SEM) is used to image and map the Stimulated Reservoir Volume (SRV) of the fracture generated in laboratory experiments (Damani et al., 2012). With the advancements in Computed Tomography, it is possible to acquire artificially created 3-D fracture structures as well as the fracture aperture maps (Karpyn et al., 2003). Electromagnetic geophysical principles also offer a method to determine the location of the proppant in the far-field of fractured wells (Palisch et al., 2017). Electromagnetic transmitters are lowered to the fracture zone in the well and response is measured by surface receivers. Electrically conducting proppant is introduced into the fractured zone (Cannan 2015; Aldridge 2016). Measurement is taken before fracturing and post-fracture to estimate the difference due to the proppant introduction and estimate the SRV (Rassenfoss, 2016). Digital Image Correlation techniques helps to analyze the strain development over the surface of the sample in real time providing fracture initiation and propagation, however, it is also limited to laboratory scale measurement (Mokhtari et al., 2017). Methods, such as SEM provide detailed fracture imaging; they are incapable of capturing the macroscale of the fracture system or the near-real-time fracture development, and suitable only for laboratory observation. Even with the extensive use of hydraulic fracturing, a fundamental understanding in the micro-scale is lacking. This experimental investigation is aimed at understanding the complexity of hydraulic fracturing using polarized shear wave attributes. We studied the shear wave response in one 6” diameter, 6” in length cylindrical Tennessee sandstone (vertical core) and 2 pyrophyllite samples (horizontal cores) before and after fracturing and analysed the change in the shear wave travel time, and signal attenuation to map fracture density and morphology. Each sample is hydraulically fractured under uniaxial conditions (Tennessee sandstone fractured using water; one pyrophyllite sample fractured using water, other one with oil) with an effective maximum stress of ≈830 psi applied perpendicular to the natura foliation. Tennessee sandstone sample is isotropic while the pyrophyllite samples exhibit a P-wave anisotropy of 20% and displays transverse anisotropy. Acoustic emissions (AE) were recorded using sixteen 1-MHz piezoelectric P-wave transducers; the spatial acoustic emission density was mapped. Berryman’s strong anisotropy model was used to build an anisotropic velocity model for AE event locations for pyrophyllite samples. Post-fracturing polarized shear wave velocity measurements were conducted using an array of seven pairs of polarized shear wave transducers to record discrete shear wave velocity measurements. Fourier analysis of the post-failure recorded shear waveforms mapped attenuation associated with the stimulated reservoir volume which was consistent with the shear wave velocity analysis. Using O’Connell and Budiansky’s Self-Consistent model (O’ Connell and Budiansky, 1974), crack density is also mapped for the fractured sample. Maximum shear wave velocity reduction observed (for both shear wave polarizations) in Tennessee sandstone is 24% post fracture, while it is as high as 30% in pyrophyllite fractured using water and is around 25% in pyrophyllite fractured using oil. Shear wave frequency map is consistent with the physically observed fracture for Tennessee sandstone as well as both the pyrophyllite samples. Crack density is observed to be more than twice in Tennessee sandstone (0.18 – 0.28) compared to in pyrophyllite (0.09 – 0.12). Secondary microfractures appear normal to the primary fractures in the horizontal plane in all three samples.Hydraulic fracturePolarized shear waveMicroseismicCrack Density