SEISMIC SURVEYING 


Identifying shale gas ‘sweet spots’


 Seismic applications for shale gas reservoirs can now provide information to optimise drilling and completion locations.


 Seismische Anwendungen für Schiefergaslagerstätten können nun Informationen für optimale Bohr- und ergänzende Stellen liefern.


 Las aplicaciones sísmicas para depósitos de gas en arcillas ya pueden proporcionar información para optimizar los emplazamientos de perforación y terminación.


G


eophysical exploration company CGGVeritas has developed seismic solutions to identify shale gas ‘sweet


spots’. Advanced seismic processing and analysis of high-resolution wide-azimuth 3D surveys can define key reservoir properties such as brittleness, pore pressure and local stresses. Reservoir engineers can use this information to optimise drilling and completion locations. Unconventional reservoirs require some form of stimulation to obtain commercial production. Shale gas reservoirs require fracture stimulation to unlock gas from extremely low-permeability formations. As fracture stimulation is an important


aspect of well completions, production companies need to know basic information about fractures such as whether they will open (and stay open), direction of fracture propagation, dimensions and type of fracture, and whether they will stay in zone. Increasingly, seismic is utilised to provide such information and guide drilling and completions. Tree types of information extracted


from seismic are useful in optimizing drilling locations: fracture characterization, geomechanical properties, and principle stress measurements (vertical maximum and minimum horizontal stresses). CGGVeritas uses a series of methods


to derive this information, including appropriate data acquisition, careful AVAZ (Amplitude Versus Azimuth) processing, AVO, interpolation, and inversion. Some of these methods are mature


and have found new applications for the characterisation of unconventional reservoirs. Although information can be extracted from compression wave (P-wave) data alone, the inclusion of shear waves (S-waves) can be used as an additional source of observations to further constrain and narrow uncertainty in the results. Given the target depth of formations in shale gas basins that are being exploited today, the maximum principle stress is vertical, giving rise to HTI (horizontal transverse isotropy). Tis means that the fracture system is comprised of vertical fractures which cause anisotropic effects on seismic waves as they pass through. Tese anisotropic effects are observed on 3D seismic data as changes in amplitude and travel time with azimuth. In multicomponent data shear wave splitting can be observed. CGGVeritas uses the relationship between changes in P-wave amplitude with azimuth in anisotropic media to invert the observed seismic response and predict fracture orientation and intensity. Tis information is of great value to production companies


Fig. 1. In this figure colour represents estimates of Young’s Modulus, while the small vertical plates show DHSR. Large plates correspond to large values of DHSR. The prospect locations are where Young’s Modulus values are high (rock is most brittle) and DHSR plates are small (facture swarms will form).


IHSS 12


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