SEISMIC SURVEYING
Borehole seismic: using multiple stacked geophones
William Wills discusses some of the advantages of employing multiple stacked geophones within a borehole seismic monitoring system in order to accurately map fracture induced microseismic events.
T
he fundamental principles of hydraulic fracturing are broadly understood throughout the
hydrocarbon exploration industry. ‘Unconventional’ reservoir rocks possess the desired ultra-high porosity (density of microscopic pore spaces hosting hydrocarbon molecules) but also feature the undesirable property of ultra-low permeability (pathway potential for a substrate to migrate through the lithology). Terefore to attain hydrocarbon migration to a well driven by overburden pressure, the ‘tight’ host rock must be artificially fractured in order to provide a flow pathway. Tis is achieved by the high pressure pumping of a liquid proppant (often a sand water gel matrix) through a well at a perforated cased point into the tight rock.
axis) within each borehole receiver satellite, ideally positioned straddling cross-well to the target zone.
Te quality of the geophone receiver
therefore plays a fundamental part in how accurately the fracture progression is mapped. Not only do these borehole receiver require downhole gain electronics that may have to function at temperatures >175˚C, but the properties of the internal sensor itself must be optimised for microseismic recording. Most seismic geophone sensors
have historically been passive analogue devices typically comprising a spring- mounted magnetic mass moving within a wire coil to induce an electrical signal. Recent designs have also been based on Microelectromechanical systems (MEMS) technology which generates an electrical response to ground motion through an active feedback circuit to maintain the position of a small piece of silicon. Te response of a coil/magnet geophone
Fig. 1. Hodogram display (left) of recorded microseismic traces for each 3D component. By establishing the receiver orientation and azimuth within a velocity model, the particle motion of the geophone shown in the hodogram, and picked P and S wave arrival time differential, allow s the fracture event can be pinpointed on a 3D grid in real time.
One of the standard techniques to mapping the fracture growth pattern is through borehole seismic monitoring. Te shear slippage during the fracture events generates high frequency microseismicity in the form of both compressional P and shear S energy. Te direction to a microseismic event is found by examining the particle motion of the P and S arrivals at a geophone array in a neighbouring well where background noise conditions are minimal. While many seismic techniques are
available to determine the direction, the simplest representation is a hodogram (Fig. 1), which is achieved by having 3 component geophones (one on X,Y and Z
13 IHSS
is proportional to ground velocity, while MEMS devices usually respond proportional to acceleration. Although an inevitable next step in geophone technology due to the potential to record vast high end broadbands (KHz) MEMS systems have so far demonstrated a much higher noise level (below 5Hz) than coil/magnet phones, and are further limited by temperature conditions of the borehole as well as requiring a downhole power supply. If the borehole geophone response has
electronic downhole gain applied (54dB) in conjunction with a fast sample rate (one sample every 1/4ms) it will provide a recording bandwidth of up to 4KHz which is perfectly suited to comprehensively measure all the energy content generated by the high frequency microseism. However, it does not matter how fast you sample the high frequency data if you cannot hear the event! Sensitivity is everything when trying to pick such low amplitude seismicity compared to controlled active sources. Even with the luxury of relative noiseless downhole environments compared to that experienced
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