| dc.description.abstract | Downhole microseismic monitoring is a widely used tool for the assessment of hydraulic
fracturing job effectiveness. During the process of fluid injection into the reservoir, new fractures develop due to the induced pressure, which gives rise to microseismic events. Therefore,
the knowledge of an accurate velocity model is necessary in order to locate the induced microseismic events. Subsurface complexity is often raised by a horizontal layering, an intrinsic anisotropy of shales, and aligned fracture sets. That introduces anisotropic effects into the velocity
field. In such a case, the anisotropy should be taken into account during the velocity model
building. Otherwise, some errors will be introduced into the microseismic event locations, and
hence, the interpretation of treatment effects will be biased. Therefore, this thesis is devoted to
the anisotropy estimation using downhole microseismic data. It examines possible location errors caused when the anisotropy effect is not considered and proposes a technique of anisotropic
velocity model inversion. It also presents field data examples of anisotropic model building and
fractures characterization.
In this thesis, I introduce a new technique for anisotropic (VTI) velocity model inversion
based on traveltimes of the P-, SH-, and SV-waves onsets and probabilistic event location algorithm. This is followed by synthetic studies showcasing errors expected in microseismic event
locations when anisotropy is neglected. In addition, a feasibility study of performing quasi-realtime anisotropic velocity model inversion during an ongoing hydraulic fracturing job is included.
Then, I present two different applications of the developed methodologies to the field data
from a downhole microseismic survey that was carried out to monitor hydraulic fracturing in
the Lower Paleozoic gas-bearing shales in Lubocino well, Northern Poland. In the first application, the VTI anisotropic velocity model inversion using the traveltimes of perforation shots
is applied. The accuracy of the model provides high-quality locations of microseismic events
induced during the hydraulic treatment. Then, the locations become a basis for a detailed stageby-stage evaluation of the stimulation performance and provide information about geological
units that were successfully fractured.
In the second application, I utilize shear-wave splitting (SWS) measurements to reveal
weak azimuthal (HTI) anisotropy caused by aligned fractures. The HTI is dominated by
stronger VTI fabric produced by the alignment of anisotropic platy clay minerals and by thin
horizontal layering. I perform the rock-physics model inversion based on SWS measurements
to finally obtain an orthorhombic stiffness tensor, which links the dominant VTI fabric with
HTI anisotropy produced by the presence of aligned vertical natural fracture sets in the shalegas reservoir.
Finally, based on both synthetic and real data examples, it is concluded that taking the
anisotropy into account during the velocity model building in downhole applications always
enhances the accuracy of microseismic event locations, and hence, raises the quality of the final
assessment of hydraulic fracturing operation. It is also demonstrated that the proposed VTI
anisotropic velocity model inversion can be implemented on-site during an ongoing industry
operation. | en_US |
