First Break - Volume 29, Issue 7, 2011

The peaking of Europe’s oil and gas production is undisputed but the current EU 2020 energy conservation policy does not address this decline. Ruud Weijermars highlights the emerging fossil fuel supply gap which demands formulation of a new EU energy strategy.

Tom Bradley, Douglas Patterson and Xiao-Ming Tang introduce a method developed to image geological structures in a cased borehole by using dipole array acoustic waveform data. The resulting fracture connectivity model can reveal migration pathways for the gas within a salt body.

Graham Gaston and Tad Bostick of Weatherford International report on experience with optical sensors used for downhole monitoring and suggest the way ahead with the potential applications of distributed acoustic sensing technology.

Bob Heath casts a critical eye over current passive seismic acquisition equipment and operations and suggests that experience with past new technology evolution in land seismic may offer some valuable lessons and pitfalls to avoid.

Scott A. Wessels, Alejandro De La Peña, Michael Kratz, Sherilyn Williams-Stroud and Terry Jbeili of Microseismic describe how in microseismic monitoring of low permeability reservoirs the use of source mechanism inversion, b values, and energy release rates enables identification and differentiation between fracture stimulation and fault activation, critical issues for effective hydraulic treatment.

A new methodology for building salt velocity models is proposed that allows effective testing of different salt interpretation scenarios. In this methodology, we combine the strength of efficient interactive imaging with the accuracy of layer-stripping RTM to derive a more accurate salt geometry. A set of interactive imaging tools enables a large number of salt interpretation scenarios to be quickly tested and narrowed down to a small number of likely salt interpretation cases. This is followed by a reduced number of layer-stripping RTM runs to single out the final salt velocity model. Redatuming the wavefield from the surface to a user-defined subsurface datum plays a pivotal role in this methodology; it enables the improvement of the quality of beam migration and the efficiency of RTM.

It has been recognized that extending the bandwidth of seismic measurements below 10 Hz can bring many benefits for geophysical exploration. Due to physical limitations in vibrator mechanical and hydraulic systems, the ability of seismic vibrators to produce significant output power at low frequencies is limited. This paper focuses on the key factors that limit vibrator performance at low frequencies and demonstrates them empirically. Our main purpose is to help geophysicists understand seismic vibrator performance at low frequencies so that they can design an optimal low frequency sweep.

We show how to apply the statistical factor analysis method to effective stress prediction from seismic attributes. Two examples are discussed, one based on a laboratory dataset and relevant to the unloading mechanisms of abnormal pore pressure generation, and the other based on well data and a seismic section, relevant to the compaction disequilibrium mechanism. Arguably, this method of seismic interpretation is superior to the existing deterministic and statistical approaches as it reduces the multitude of available seismic attributes to a small number of factors most relevant to the effective stress and,in addition, provides a mathematically systematic way of arriving at most probable effective stress values as well as the confidence range.