Methods for in-situ borehole seismic surveys using downhole sources
Methods for in-situ reservoir investigation by borehole seismic methods are provided using receiver(s) and a downhole source. The downhole source may be a microseismic event, and may be located relative to the receiver(s) in any configuration. The downhole source may also be a controlled source that is positioned in a reverse vertical seismic profile (RVSP) geometry with respect to the receiver(s). The methods may involve locating the receiver(s) in a first well (which may have any orientation, including vertical or horizontal), and locating the source in a monitoring well (which may have any orientation, including vertical or horizontal), such that the source in the monitoring well is positioned at a greater depth in the formation than the receivers in the first well.
1. A method comprising:
using microseismic events as seismic sources by inducing, via a lateral wellbore of a treatment well, at least one first microseismic event having a first spectral frequency content and inducing at least one second microseismic event having a second spectral frequency content;
recording data relating to seismic waves generated by the microseismic events at at least one receiver positioned in a wellbore of a monitor well at a depth that is less than corresponding depths of the microseismic events wherein the at least one first microseismic event is closer to the at least one receiver than the at least one second microseismic event; and
interpreting at least a portion of the seismic data for spectral power of the seismic waves generated by the microseismic events as located between the microseismic events and the at least one receiver, the interpreting comprising comparing the spectral power of the first spectral frequency content with the spectral power of the second spectral frequency content for qualitative information relating to hydrocarbon content based on hydrocarbon content diminishing spectral power.
2. A method according to claim 1 wherein comparing the spectral power of the first spectral frequency content with the spectral power of the second spectral frequency content for qualitative information relating to hydrocarbon content based on hydrocarbon content diminishing spectral power comprises comparing at frequencies greater than approximately 325 Hz.
3. A method according to claim 1 wherein comparing the spectral power of the first spectral frequency content with the spectral power of the second spectral frequency content for qualitative information relating to hydrocarbon content based on hydrocarbon content diminishing spectral power comprises plotting power versus frequency.
4. A method according to claim 1 comprising, based at least in part on the comparing, identifying a fractured zone that comprises hydrocarbons.
5. A method according to claim 1 wherein the inducing at least one second microseismic event having a second spectral frequency content is via the lateral wellbore of the treatment well.