Observations of broad‐band micro‐seisms during reservoir stimulation

G. E. Sleefe, N. R. Warpinski, BP Engler
1993 SEG Technical Program Expanded Abstracts 1993   unpublished
the use of novel clamping-p-a_ges or geophones cemented directly to the formation. The second limitation in using the polarization During hydrocarbon reservoir stimulations, such as hydraulic method is that the geophone i,teelf often does not accurately fracturing, the cracking and slippage ofthe formation results in the measure particle motion over a wide frequency range. emission of seismic energy. The objective of this study was to Conventional geophones exhibit spurious modes which are due
more » ... odes which are due to determine the properties of these induced miero-seisms. A off-axis excitation of the 8eophone springs. The spurious mode hydraulic fracture experiment was performed in the Piceance Basin manifests itself as a resonance effect which occurs at a frequency of Western Colorado to induce and record micro-seismic events, which is approximately 25 times higher than the natural frequency The formation was subjected to four processes, break-of the geophone. For example, a I0 Hz 8eophone can exhibit down/bailout, step-rate test, KCL mini-fracture, and linear-gel mini-spurious modes at and above 250 Hz, thus limiting its usefulness fracture. Micro-seisms were acquired with an advanced three-above 250 Hz. Additional geophone limitations include phase shi/_s component wall-locked seismicaccelerometer package,placedin an within the first few octaves above the naturalfrequencyand highobservationwell 211 ft offset from the fracture weil. During the frequencyself-noiseabove approximately200 Hz [4]. two hoursof formation treatment, more than 1200 mioro-seisms Due to the limitations presentedby the conventional wallwith signal-to-noise ratios in excessof 20 dB were observed. The locking geophone, an advanced borehole seismic receiver was observedmicro-seismshad a nominallyflat frequency spectrum utilized to extendthe useablefrequency rangeoutto 1500 HZ. The from 100 Hz to 1500 Hz and lack the spurioustool-resonance advancedreceiver utilizes a novel clamp mechanismwhich enables effectsevidentin previousattemptsto measuremicro-seisms. Both its resonantfrequency to be above 2000 HZ. Additionally, solidp-wave and s-wave arrivalsare clearlyevident in the data set, and state accelerometers are used as the sensingdevice, thereby hodogram analysis yielded coherent estimates of the event eliminatingspuriousmodes, phaseerrors,and highfrequency noise locations. This paper describesthe characteristics of the observed limitations. The resulting seismicreceiver is potentiallycapable of micro-seismicevents(event occurrence, signal-to-noise ratios, and accurateparticlemotionmeasurementto frequenciesnearlyartorder bandwidth)and illustratesthat the newacquisitionapproachresults of magnitudehigher than previously reported. This suggeststhat in enhanced detectabilityand eventlocationresolution, muchimproveddetection andlocation of micro-seismiceventscan beobtainedto map a hydraulicfracture. The remainderof this paper INTRODUCTION will describe an experiment in which a broadband accelerometerbased seismic receiver was used to measure micro-seisms generated lt has long been recognized that reservoir stimulations such by hydraulic fracture treatments. The resulting data described below as hydraulic fracturing result in the emission of seismic energy [1]. depict the advantages of broadband micro-seism measurements. A popular methodology is to place one or more seismic sensors in either the stimulation well or an adjacent monitoring well to DATA ACQUISITION APPROACH determine the origin of the micro-seismic events. Typically, a walllocking three-component geophone instrument is utilized to sample The borehole seismic receiver utilized in the experiments is the the vector wave-field produced by the micro-seism. The data from accelerometer-based Advanced Borehole Receiver, available from the three geophones is then processed such that the particle motion OYO Geospace Corp., Houston, TX [4]. The receiver consists of polarization is used to infer the orientation ofthe fracture plane [2]. two pressure housings fitted with standard Gearhart-Owens seven In order to utilize the polarization approach, however, the seismic conductor cable-heads, one on either end of the clamping section. receiver instrument must faithfully record the particle motion of the One housing contains the td-axially arranged accelerometers and the seismic wave-field that is incident on the borehole. The other housing contains the electric gear-motor assembly. This gearconventional wall-locking geophone instrument generally does not motor drives the rectangular piston perpendicular to the longitudinal enable the accurate measurement of particle motion over a wide axis ofthetool through the use of a 1'1 fight angle gearbox to clamp frequency range [3 ]. Two instrument limitations can cause the wall-the tool into the borehole. locked geophone to record inaccurate particle motions. The first Finite element analysis of the receiver was performed to assure limitation, known as locking resonance, results from inadequate that the clamping mechanism has a potential flat frequency response coupling of the geophones to the borehole over a wide frequency out to 2.0 KHz. The analysis resulted in a clamping piston design band. The locking arm of the instrument provides good coupling of with 1.5 inches of travel, and accommodation of adapters to allow the geophones to the borehole only at relatively low frequencies, for clamping into boreholes ranging from 4.25 inches to 9 inches in At some higher frequency, the motions of the clamping unit do not diameter. The clamp force to tool weight ratio developed by this follow the motions of the borehole wall. In conventional VSP design is a function of the gearset selected to mate with the electric geophone receivers, the resonant frequency of the clamped receiver motor, and can vary from 5:1 to 20:1. is typically in the 200 Hz to 400 Hz range. Therefore, conventional The low-noise piezo-electric accelerometers utilized in the VSP instruments can only be used for accurate polaxization receiver offer significant advantages over conventional geophones. ' measurements for seismic excitations below about 200 Hz. These accelerometers do not exhibit the spurious resonance problem Methods for extending the frequency range above this limit include common to geophones, are not sensitive to their mounting
doi:10.1190/1.1822456 fatcat:s64gihf4pjc4di24dlprbl2bdu