Hostile wells: the borehole seismic challenge

Paul Boughton

William Wills discusses some of the basic exploration challenges faced with characterising deep reservoirs in such a high risk, high cost environment, and the requirements of downhole seismic tools to overcome such issues.

The use of borehole seismic recording systems to acquire high resolution data has become common practice over the last two decades. The advantages of noise reduction and immunity from the distorting effects of near-surface rocks, especially sediments, have become well known. Downhole vertical seismic profiling (VSP) eliminates both the inferred depth uncertainty presented by surface seismic and time uncertainty (such as from sonic logs) giving an enhanced velocity model around a well.

Borehole recording can be extremely complex; involving gimballed or multiple fixed sensors, special cables, and high gain downhole electronics. Such installations, however, require robust mechanical housing to withstand the extreme pressures, temperatures and damaging borehole fluids encountered at such depths.

Deep reservoir exploration often has the challenge of operating in environments such as within subsalt zones. Such complications often manifest as poor seismic resolution in conventional 3D seismic surveys. Recent seismic wide-azimuth (WAZ) towed streamer acquisition and processing has enhanced the ability to map the subsalt environment. However, certain reservoirs remain below seismic resolution (such as at the deep water Wilcox Trend, Gulf of Mexico). These deep wells, which include exploration challenges of well depths often up to 35 000ft, water depths ranging from 8-10 000 ft, and allochthonous salt canopies of the magnitude of 10-20 000 ft thick complicating regional reconstructions and resolution of individual structures, all present a real challenge to determine reservoir quality and integrity.

Surface seismic technology is not solely sufficient for such deep reservoir characterisation, even when incorporating WAZ. The modest angle of incidence from the distal surface (mid 20° range) renders reservoir amplitude analysis as an inappropriate technique, especially within poorly/unevenly illuminated sub-salt environments. Pre-stack depth migrated data (PSDM) can also be limited by velocity error and poor vertical resolution due to the geometry of a salt layer. Such deep velocities models rarely incorporate localised low velocity zones or take into account anisotropy, resulting in velocity models which may be too deep and too steep.

Within deeper and hotter wells, resolving low amplitude primary arrivals out of the noise is increasingly challenging. This is made more difficult due to the contamination of primary energy at reservoir level by multiple signals generated above the top salt and water bottom and can often be a serious issue when migrated multiples resemble faulted geometry.

By having geophones located deep downhole the up wave travel time to the receiver generated from a reflection is reduced. This minimises the amount of energy attenuation, improving the signal strength. These geophone tools need to be designed to function within very high borehole temperatures of 180°C plus and pressures reaching 25 000PSI.

To enhance oil and gas recovery from ultra low permeability reservoirs (eg, Haynesville Shale, Texas) the geology often requires fracturing. This is achieved by the pumping of a proppant through the tight rock to fracture hydrocarbon pathways to the well. As these unconventional rocks fracture, they generate microseismicity. This energy will be both as Primary compressional energy and as Secondary shear energy. By employing three component geophones (HX, HY and VZ axis) in a neighbouring monitoring well and recording the time gap between the different velocity P and S waves it is possible, when combined with a velocity model, to calculate the azimuth and distance of the microseism ray path, and so pinpoint the event location (Fig 1). This will give crucial information such as fracture height, azimuth and half length along planes of weakness, allowing a more accurate characterisation of the fracture pathway.

These microseisms are small amplitude and high frequency, and as such receiver distance from the event needs to be close, and geophone sensitivity needs to be high. When fracturing unconventional geology it is important that the downhole receivers have electronics capable of delivering a high gain, achieve continuous recording and are able to maximise sensitivity by doubling or even quadrupling the number of geophones within a sensor pack, whilst still maintaining a slim tool housing that can be accommodated within deep boreholes.

For such surveys to be successful it is critical to establish the orientation of all the downhole geophone components as the tools can rotate as they travel through the well. This is achieved by an offset surface orientation shot triggered from the surface through the geology to the receiver, where the signals recorded on each component can be compared and so orientation relative to source can be known. Within deep wells and in sub-salt environments, standard surface source orientation shots may not deliver easy to pick, high signal to noise traces. By using a downhole impulsive source such as high energy sparker tool in a proximal well, the shot can be repeated and so can be stacked multiple times to improve the signal to noise ratio. This use of a downhole source will also provide a clear time break on the recorded trace and so can be used for QC as any poor signals would suggest bad receiver coupling to the casing and so the operator could simply move the tool string and lock into a better cemented zone (Fig 2).

Key uncertainties such as faulting and other structural geometry play prominent roles in determining the drilling programme. Seismic data quality in such deep environments is often of a low frequency and is usually contaminated with multiple energy. This makes characterisation of key uncertainties such as faulting a real struggle. The data quality can be achieved if high gain, high sensitivity downhole receivers are used. These tools must be capable of withstanding the pressures, temperatures and reactive proppant/fluids within the borehole, and should be deployed close to the seismic event with good coupling to the casing. This can be augmented with repeatable downhole sources giving clear stackable signals for QC. As this downhole technology continues to advance, the challenges of accurately characterising and monitoring deep unconventional reservoir geology will be met with increasing certainty.

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

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