ABSTRACT: Polarized shear wave travel times and spectral changes are used to determine natural fracture intensity and orientation. The study develops this concept for fracture density mapping associated with laboratory hydraulic fracturing experiments in pyrophyllite, a fine grained monomineralic metamorphic rock comprised of the mineral pyrophyllite, and hence an analogy for natural shale. A 6” long horizontal cylindrical pyrophyllite sample (wellbore parallel to bedding plane) is hydraulically fractured using water under uniaxial conditions with an effective maximum stress of 830 psi applied perpendicular to the bedding plane (breakdown pressure – 1914 psi). The pyrophyllite sample exhibits a P-wave anisotropy of 18% 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. Post-fracturing shear wave velocity measurements were conducted using an array of seven pairs of polarized shear wave transducers which were systematically stepped across the end faces of the cylinder producing 931 discrete shear wave velocity measurements for every polarization. These arrays are used to record shear wave travel time with polarizations parallel and perpendicular to the direction of maximum stress before and after hydraulically fracturing the sample. Fourier analysis of the post-failure recorded shear waveforms mapped attenuation associated with the SRV which was consistent with the shear wave velocity analysis. The geometry of experiment reflects hydraulic stimulation in a horizontal wellbore condition. Orthogonally polarized shear velocities show measurable differences which reflect a preferred fracture orientation, with more than 32% post fracture reduction in shear velocity, in the fractured plane. The polarized shear wave map is consistent with the AE event locations recorded during the fracturing process. Secondary microfractures appear normal to the primary fractures in the horizontal plane.
Hydraulic fracturing in combination with horizontal drilling has made the extraction of hydrocarbon from certain geologic formations economically feasible, thereby boosting the available energy resources in US. It has induced an oil and gas “boom” in various parts of the country.
There are arguments that state that the physical laws governing fractures are known and fracture models are accurate, but the emergence of ‘new mechanisms’ every few years suggests that the basic physics incorporated into models has not been as comprehensive as required to model a fracture fully (Warpinsky, 1996).
It has become critical to understand the location of fracture and the extent to which it stimulates a reservoir to plan future drilling and completions. Thus, mapping hydraulic fracture is essential. There have been various methods that have been used to map the fracture propagation. Commonly used method in the field is to record the microseismic events generated because of release of elastic energy during the fracturing process, as established by Albright and Pearson, 1982, Rutledge and Phillips, 2003 and Warpinski et al., 2004. Acoustic emission (AE) techniques had been 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). Third common method is to use Scanning Electron Microscope (SEM) to map the stimulated reservoir volume of the fracture generated by taking out a chip out of the fractured sample for analysis (Damani et al., 2012).