Nanoseismological Monitoring of Hydraulic Fracturing

Abstract

The nanoseismic monitoring combines the methods focused on the weaker seismic sources than the micro-earthquakes. The localization of the latter is carried out on seismic records using hypocentry methods with the preliminary picaping of the arrival times of seismic phases. An emission seismic tomography allows to conduct the localization of extremely weak noise-like endogenous sources, signals from which are completely buried in the noise on single records. If nanomaterials often exhibit properties that are significantly different from the behavior of similar materials in a massive state, then nanoscale monitoring makes it possible to investigate the fine energetics of geophysical processes that is substantially different from the relatively higher energy manifestations. The nanoseismological methods allow not only to localize the seismic events associated with the rock fragmentation, but also to monitor the processes of the preparation of micro-events and the processes of the relaxation of the stressed state of the environment after them. These methods also allow us to study the slow changes in the stress-strain state of the geological environment, under which the dissipation occurs in the form of the noise-like continuous emission radiation and is not accompanied by micro-earthquakes. The emission seismic tomography allows to extract information about the structure of the rock medium and the processes occurring in it from the spatially coherent seismic emission signals, which are a weak additive component of the seismic noise. The activation of sources of seismic emission within the geo-environment occurs with various kinds of the external natural and technogenic impacts. An important experimental fact of our research studies is that the configuration of the active emission clusters in a large volume of the rock medium, up to several kilometers, changes significantly with the local technogenic impact on the natural rock mass during the hydraulic fracturing and is not uniform in different frequency ranges. According to the records of the seismic background prior to the hydraulic fracturing, the stationary emission zones through which the dissipation of the background effects (tidal, tectonic, technogenic) occurs are "highlighted". As the pressure increases during the injection of the fracturing working fluid, the rock mass adapts to the change in the energy flux by increasing the number of emitters and changing the position of the clusters of emission sources. The change in the stress-strain state of the rock media leads to the "illumination" of the new emission sources not previously detected in the background state. A sharp change in the spatial distribution of the emission sources can be considered as a bifurcation with the emergence of a new spatio-temporal dissipative radiating structure. A day after the hydraulic fracturing process is over, the rock media returns to the background distribution of the sources. The experimental results show that the nanoseismological monitoring of the technogenic impact in the form of the hydraulic fracturing allows to identify the dynamic areas in the field (shifts of blocks, zones of natural fracturing, filtration of fluid into permeable zones, the position of the fronts of displacement, temperature anomalies). The nanoseismological monitoring allows a prompt local appraisal of residual hydrocarbon reserves in a radius of several kilometers from the area of the hydraulic fracturing.