Borehole seismic: using multiple stacked geophones

Paul Boughton
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.

The 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). Therefore 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. This 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.

One of the standard techniques to mapping the fracture growth pattern is through borehole seismic monitoring. The shear slippage during the fracture events generates high frequency microseismicity in the form of both compressional P and shear S energy. The 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 axis) within each borehole receiver satellite, ideally positioned straddling cross-well to the target zone.

The 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.[Page Break]

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.

The response of a coil/magnet geophone 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![Page Break]

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 at the surface, the background electronic noise (En) within a receiver can be enough to mask a microseismic arrival. To improve on this signal to noise ratio, technology has evolved to stack the phones within the sensor pack component of a downhole receiver, with the latest current technology achieving four phones on each component (12in each receiver satellite).

As the thermal noise En output voltage can be expressed as the square root of 4kTBR (B Boltzmann's Constant, Tk Temperature in Kelvin, B Bandwidth and R Geophone Resistance) the sensitivity of a typical geophone is effectively doubled when stacked but the En noise only increases by the square root of the resistance.[Page Break]

Fig. 2 emphasises this with a recorded correlated vibroseis data sample, the recorded controlled source traces show the amplitude of the quad sensors are near double that of the dual sensors.

The advantages of multiple geophones within one sensor pack (Fig.3) goes further in terms of guaranteeing perfect stacking. Standard industry borehole receivers can only accomplish four geophones per axis by mechanically joining two independent 'dual' geophone satellites together within the monitoring well. This can often result in stacking errors, where the incoming signals between the satellites may be different due to inconsistency receiver/casing coupling, either due to mechanical discrepancies between the receiver locking arm drive mechanism, cement irregularities, and increased distance between the joined sensor packs which can be several feet in separation.

There are many bespoke requirements for microseismic monitoring systems, ranging from continuous recording, fast sampling rates and high temperature tolerances; however these are all redundant if the event signal is unheard! By employing multiple phones within a sensor pack, the sensitivity is maximised, signal/noise improves and stacking uncertainty is reduced, all safeguarding the accurate event pick times that hydraulic fracture mapping demands.

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William Wills is a Geoscientist with Avalon Sciences Ltd, Somerset, UK.

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