UC Berkeley

Multistage fracturing technique, together with horizontal drilling, make production from organic-rich shale possible. Microseismic monitoring of hydraulic fractures has been an important technology for far-field fracture diagnostics. It can provide us hydraulic fracture geometry and its growth behavior vs. time. Getting accurate microseismic event location is important to interpretation. Various methods originally developed for earthquake location have been used for microseismic event location.

The main objective of this dissertation is to make use of multiple arrivals of microseismic data to improve microseismic event location accuracy. The improvement can be achieved from two aspect: (1) simultaneous inversion of multiple microseismic data for event locations and velocity model and (2) improving microseismic event location accuracy with head wave arrival time. We begin this dissertation by laying out the inverse problem theory as the basis of the simultaneous inversion. Then, we built a Bayesian framework to simultaneously invert for microseismic event locations and the velocity model. We developed a software package, BayesTomo, based on the simultaneous inversion framework.

The first application is the simultaneous inversion of microseismic event locations and the velocity in a microseismic survey at Newberry Enhanced Geothermal System (EGS). We successfully applied the developed method on both synthetic examples and real data from the Newberry EGS. Comparisons with location results based on a traditional predetermined velocity model method demonstrated that we can construct a reliable effective velocity model using only microseismic data and determine microseismic event locations without prior knowledge of the velocity model.

The second application is on the microseismic data acquired from a geophone array deployed in the horizontal section of a well drilled in the Marcellus Shale near Susquehanna County, Pennsylvania. We identified the existence of prominent head waves in some of the microseismic data. The head waves are refractions from the interface between the Marcellus and the underlying Onondaga Formation. The source locations of microseismic events can be significantly improved by using both the P-, S-wave direct arrival times and the head wave arrival times in place the traditional method of using direct arrival times and P-wave polarizations. The traditional method had substantially greater uncertainty in our data due to the large uncertainty in P-wave polarization direction estimation. Our method was applied to estimate the locations of perforation shots as well as microseismic events. Comparison with traditional location results shows improved location accuracy thanks to head wave arrival times.