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Abstract

Shale gas is natural gas from shale formations which acts as both the source and the reservoir for the natural gas. Each Shale gas reservoir has unique characteristics. Research work focus at bridging the gap between invariant characteristics at nano scale of sedimentary rocks and their macroscopic properties. 3D seismic is becoming successful because of the ability to identify fracutre and fault trends. Surface geochem cannot identify in the subsurface where the frac or fault systems will be intersected by the drill bit. This is why 3D is now being used aggressively and successfully. Unconventional reservoirs require some form of stimulation to obtain commercial production. Shale gas reservoirs require fracture stimulation to unlock gas from extremely lowpermeability 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, direction of fracture propagation, dimensions and type of fracture, and whether they will stay in zone. Increasingly, seismic is utilized to provide such information and guide drilling and completions. Three 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). 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). This means that the fracture system is comprised of vertical fractures which cause anisotropic effects on seismic waves as they pass through. These 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. 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. Ultrasonic Measurements of Anisotropy of Shales: Laboratory measurements of ultrasonic velocities have confirmed that compressional waves travel faster in the direction of applied stress. The reason may be that all rocks contain some distribution of microcracks. As stress is applied, cracks oriented normal to the direction of greatest stress will close, while cracks aligned with the stress direction will open . In most cases, waves travel fastest when their particle motion is aligned in the direction of the opening cracks.A noticeable feature of acoustic anisotropy is shear wave splitting, or polarization, typically caused by fractures.

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/content/papers/10.3997/2214-4609-pdb.377.39
2011-05-11
2021-12-04
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