First Break - Volume 26, Issue 3, 2008

The nature of geological heterogeneity is well understood in proximity to a wellbore but is sparsely sampled laterally. Heterogeneity is both highly unpredictable and difficult to map with surface reflection seismics, but high productivity trends in tight gas reservoirs can be efficiently identified with precision crosswell seismic time-lapse travel-time data.

Low porosity (tight) gas sands contain important commercial reserves but, unfortunately, are difficult to image seismically with the quality needed to drive an effective drilling programme. These reservoirs typically have a small contrast in acoustic impedance relative to the encasing shales which results in a subtle seismic signature. This signature becomes even more difficult to identify in the presence of intra-bed multiples and other undesired seismic energy. Since this resource is predominantly exploited in the onshore arena, seismic imaging is also subject to the many cultural and near surface factors that affect land seismic data quality.

The Neuquina Basin is distinguished as being one of the most prolific producing areas in Argentina. Multiple types of reservoirs and trapping mechanisms characterize the petroleum systems of the basin. It is considered a mature hydrocarbon area if we consider both exploration and field development. Despite this, the level of activity continues to be high and still some technical challenges remain. New operational strategies include the development of deep and non-conventional reservoirs and the application of new technologies to enhance the recovery of existing producing fields.

This article represents a personal essay describing the fundamental differences between conventional gas accumulations and ‘tight gas’ accumulations. We have become accustomed to looking for gas in reservoirs that have good porosity and permeability to provide high flow rates. We demand excellent reservoirs to provide economic accumulations. We have avoided low permeability gas reservoirs.

Tight gas sandstone reservoirs are a vast resource. Due to their complexity and our poor understanding of these unconventional petroleum systems, new technologies are necessary to successfully exploit them. Some of the geological challenges present in tight gas sandstone reservoirs are: high reservoir heterogeneity, very low porosity and permeability, possible presence of natural fractures, uncertainty in gas/water contact, and the influence of over-pressures. This study shows the integration of core, log, and seismic data in tight gas sandstones and the use of Vp/Vs as an attribute can help to distinguish reservoir changes.

The Colorado School of Mines Reservoir Characterization Project (RCP) is currently researching multicomponent seismic applications to tight gas sandstone exploration and production at Rulison Field, Colorado. Results show that fractured sandstone bodies exhibiting enhanced permeability are detectable using high resolution shear wave anisotropy analysis. Multi-component seismic fracture detection has the potential to significantly impact future tight gas exploration and production.

Chris Anderson, Andrew Long, Anton Ziolkowski, Bruce Hobbs, and David Wright explain the principles of multi-transient EM technology and provide some recent survey results from the North Sea. The authors claim significant advantages for their method over the marine controlled source EM operations which currently dominate this emerging market.

Ivan Hrvoic, Mike Wilson, and Francisco Lopez* of Canadian company GEM Systems** review the available range of magnetometers used for various geophysical applications finding special benefits from the potassium version it has developed.

Chris Leech, Geomatrix Earth Science, and Ian Hill, Department of Geology, University of Leicester, describe the GEEP research project to develop a mobile equipment platform for shallow geophysical surveys.

Abstract At low frequencies the energy produced from a seismic vibrator is constrained by several mechanical and hydraulic limitations. These include reaction mass stroke, hydraulic pump flow, pump response time, servo valve stroke, accumulator size, engine horsepower, peak-decoupling force, harmonic distortion, and the vehicle chassis isolation. We have developed a model that can simulate seismic vibrator dynamics to evaluate the impact of these parameters on the vibrator fundamental force envelope at low frequencies.