Microseismic monitoring has proven to be an important tool for optimizing stimulations in many formations. The primary factor limiting more widespread usage of this technology is the availability of offset wells that are sufficiently close so that microseisms can be detected. Microseisms are intrinsically small events that are difficult to see more than a few thousand feet away, but viewing distances vary considerably from formation to formation. Placing multiple geophones at specific depth intervals in a borehole array and stacking the seismic traces improves the microseismic signals. Signals were individually recorded from each sensor and digitally stacked to reduce noise associated with resonances of mechanically clamped sensors and electronic noise associated with digitization. The resulting signal improvement allows better identification of weak signals and more accurate arrival time determination.. The impact of this improvement in signal quality is the ability to create better microseismic images, with more precise event locations and potentially a larger number of recorded events.
Using borehole arrays for microseismic monitoring is a well established technique to map hydraulic fractures and other reservoir stimulation techniques (see for example Albright and Pearson, 1982, Warpinski, et al., 1998, Maxwell et al., 2002, Wolhart et al., 2005). One difficulty of borehole method is finding a borehole in close proximity to the well or area of interest. Microseismic monitoring in the Rocky Mountain region of the US in particular is often hindered by small-amplitude microseismic activity and relatively large well spacing. It is often difficult in the Rocky Mountains to find monitor wells that are close enough to provide an acceptable signal response, and accuracy of the event location accuracy is commonly diminished by small event signal-to-noise ratios (SNR). One of the main concerns in any seismic data recording is to improve the SNR. In this paper, we discuss digitally stacking signals individually recorded from multiple 3C geophones. This results in higher SNR and improved hodogram linearity and overall location accuracy. Stacking of geophones or other sensors is a common and wellknown procedure for improving the SNR of seismic data (Sheriff and Geldart, 1982). The general principle is that two noisy events summed together will have their signals added, but the noise may cancel if it is random. This procedure is the basis for many signal processing applications in both surface and downhole seismology. The unique aspect of this study is that the signals are stacked after being individually digitized from separately clamped sondes. Stacking can potentially improve the vector fidelity of the clamped 3C sensors, by minimizing “noise” from spurious mechanical resonances. Furthermore, in cases of “quiet” background noise where electronic digitizing noise becomes a relatively larger factor, digital stacking can also minimize the incoherent electronic noise between signals.
The project was performed in the Uinta basin in wells drilled and completed by Bill Barrett Corporation in the West Tavaputs field.. The stratigraphy in the field consists of Tertiary and Cretaceous fluvial sediments of the Price River, Dark Canyon and North Horn formations.