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Hydro-Fracture Monitoring Using Vector Scanning With Surface Microseismic Data

Abstract

Since 2006, we have monitored hydro-fracturing (HF) processes of over 100 oil/gas wells in sandstone, carbonatite, shale, and coal bed, etc. for developing a vector scanning method with surface microseismic data. We have achieved a cost-effective detection routine for HF through surface network with sparsely distributed 3-component seismic stations, and near real time 3D imaging of the geometry of the fractures in an acceptable precision. Two critical features in HF should be noticed: 1) they consist of numerous micro-fractures, so that the events, in general, cannot be located by picking P and S wave arrivals from surface records; and 2) besides the tensile fractures, shear rupturing is the most commonly encountered fractures. We have to, thereafter, solve three key problems: A) More than 20 seismic stations should be about 1–2 km far from the fracturing vehicles, and each station must be in a quiet point determined by a seismometer instead of a man. Such kind of data acquisition is to avoid any strong noise from the vehicles, etc., and the length of a seismic trace emitted from HF location to surface is now almost the same as that with stations near the vehicles. B) All seismic stations should be equipped with 3-component sensor, which should be screwed into ground for being well coupled with land. Each of station should be independently controlled by GPS, and can transmit numerous of real time data quickly and wirelessly; and C) A vector stacking should be used instead of scalar one for 4D imaging S(k)=(∑_i▒〖[∑_j▒,〖,(±) 〖(f_ij)]〗^2 〗〗)/F, where S(k) is seismic energy emitting from kth point in a target volume;f_ij is a vector at jth record sample of ith station; F is a chosen normalization factor; the sign ± before f_ij is determined by the feature of shear rupturing, or the correlation between a reference station and the stacked station in a time window. The applications of this technique to a synthetic data set generated by numerical modeling and the real-world field data show that it is able to trace the 4D development of hydro-fractures even when the signal to noise ratio is lower. The data acquisition and scanning technique significantly improve the fracture imaging quality, and provide a cost-effective approach for monitoring HF processes.