Borehole seismic acquisition gives 'high definition' results

Paul Boughton

Nicholas Brooks reports on specialised borehole seismic data acquisition and processing solutions.

Understanding complex completions in unconventional reservoirs is less mysterious now than it was just a decade ago. Downhole microseismic monitoring has matured in to a proven technique for mapping fracture characteristics in real-time. In the early days, availability of a second well to deploy a wireline conveyed array tool was a must have, but not any more.

TABS is a technology licensed from ExxonMobil Upstream Research Company. The TABS system negates the need for a second well to perform the monitoring and is now being offered globally through borehole seismic technology specialists at Weatherford.

After many successful surveys with the prototypes, we are now manufacturing these tools in US for use domestically and overseas. TABS consists of an array of three tri-axial geophone receivers, a telemetry sub to transmit the data to surface in real time on conventional wireline and an interface to a gyro survey tool to provide the tool orientation when clamped.

The tool subs are interconnected with 'flex links' that are rigid in rotation to allow accurate sensor orientation but flexible in bending to allow each clamp arm to make independent coupling to the wellbore casing. The overall design is that of a logging tool, facilitating application via wellhead lubricators and allowing data to be collected during hydraulic fracture operations.

Pressure and temperature gauges measure the downhole environment which is otherwise calculated from surface measurements.

The tool is deployed in the stimulation well, giving it a ringside seat to catch induced seismicity immediately after the pumps stop.

We also encourage new ways of utilising the grandfather of the borehole seismic family, known in the industry as Vertical Seismic Profiling (VSP). A recent example of such efforts is shown through our co-development of pre and post fracturing time-lapse VSP analysis. Walkaround VSP geometries, or large surface source arcs around a vertical receiver array, have been used for over a decade to orientate fractures through analysis of shear wave birefringence.

Now, we are also acquiring data to characterise the fractures, pre and post fracturing, through analysis of diffraction patterns emitted as seismic energy stimulated by a vibroseis unit on the surface.

Our team of geophysicists has partnered with ConocoPhillips Subsurface Technology group in an effort to commercialise the technique to gain broader industry acceptance. Together we are carrying out surveys to assess how subsurface changes related to the fracturing operation manifest themselves in walkaround VSP data.

In addition to traditional difference sections we pay close attention to the diffractions that are at their strongest perpendicular to the fractures, revealing orientation and potentially telling us if the fracture is open or closed. VSP imaging, shear wave analysis, diffraction processing and other methods are fully incorporated with microseismic monitoring data.

In a recent project in December of 2008 ConocoPhillips and Weatherford acquired 58 VSP source positions in to a multilevel array tool for this very purpose.

The crew acquired the large VSP and monitored the microseismic activity during the fracturing operations. The walkaround VSP was then acquired again after the fracturing crew had left the site. So, by merging VSP and microseismic, we are now observing the reservoir pre-frac, syn-frac and post-frac for a better understanding of the hydraulic fracturing process.

Clarion, Weatherford's all-optical seismic sensing system, is installed during the completion phase and remains in place in the wellbore for a true life-of-field monitoring solution. This robust system enables production optimisation, increased recovery and enhanced reservoir management by making high resolution and on-demand reservoir imaging possible through the use of high performance optical instrumentation and miniature optical sensors. Clarion was specifically designed for hostile wellbore environments, and the sensors can be installed in production, injection and dedicated monitoring wells. The deployment system can even allow high quality imaging in producing wells.

The system enables reliable permanent, in-well seismic applications including both active and passive imaging and monitoring as well as repeatable 3D VSP. Clarion's sensors decrease uncertainty by providing consistent, unchanging reference points in the subsurface and eliminate repeatability errors that frequently result from redeployment of temporary sensors.

The optical seismic sensors achieve high sensitivity, high bandwidth and large dynamic range without the need for complex downhole signal conditioning, amplification and filtering typically required in most electronic sensor arrays. There are few,moving parts and no electronics downhole, reducing the need for well interventions. The system has been successfully deployed in onshore and offshore fields across the globe.

By working with and developing new tools and techniques Weatherford will continue to bring a new approach for tomorrow: borehole seismic 'in high definition'.

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Nicholas Brooks is Principal Geophysicist, Weatherford International Ltd, Global Borehole Seismic Services, Houston, Texas, USA.

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