Shale Gas

Production of gas from shales is not new, although development on a large scale is relatively recent, going back 20 years and highlighted by the technical understanding gained, and success achieved, in various shale fields in the USA. Shale gas is the fastest growing energy sector in onshore USA, and interest is rapidly spreading around the rest of the world.

What is Shale Gas?

In a shale gas reservoir the organic-rich shale is both the source and the reservoir rock. Unlike conventional gas production, shale gas potential is not confined to limited traps or structures, and may exist across large geographic areas.

Colorado shale gas play

In this example, from a shale gas play in North America, we used a unique CGG technique to seismically derive a horizontal stress ratio (seen in the variance of color) that enables better well placements for fracking and production. The Colorado Shale becomes clearly visible.

Gas is held in the shale not only in tiny pores, but also in a solid solution bound onto the rock grains. The key to producing these shales is connecting the pores through the introduction of an artificial fracture system, and lowering the pressure in the rock (through production) to allow the gas in solid solution to become gaseous and flow.

The extent of these shale reservoirs changes the criteria for establishing production away from one of location dependency (finding the traps where the gas is present) towards one of optimizing drilling, stimulation and completion techniques.

Seismic Solutions

CGG draws on its advanced toolkit to provide complete shale solutions that can be tailored for any reservoir. We use advanced processing and reservoir analysis to estimate the natural fracture intensity and orientation, the porosity, the density and the stress regime. 3D seismic data can be used to:

Increase Productivity by identifying 'sweet spots' to drill and frac'
- Various seismic attributes that are used to characterize shale gas can be extracted, such as:
  - Young's Modulus
  - Poisson's Ratio
  - Rigidity
  - P-impedance
- Vertical, maximum and minimum horizontal stresses can be estimated from anisotropic analysis of long-offset wide-azimuth data, or from multicomponent seismic.
- Closure stress - how much force is required to open fractures in the rock - can be estimated from Azimuthal AVO in combination with seismic attributes.
- Existing fracture densities and orientation, can be estimated from fracture analysis on wide azimuth data, or from multicomponent data.
- Optimal areas for hydraulic fracturing can be identified from stress estimation.
- The reservoir fractures can be monitored using microseismic or SeisMovie
Reduce Risk:
- By prioritizing wells on 'sweet spots'

$Map of best frac zones$

Map showing probable zones of better hydraulic fractures, estimated from seismic.
Green is where fracture swarms will form, red is where rocks are more ductile and hydraulic fracturing won't work, and yellow is where aligned fractures will occur. Note that only about a quarter of this reservoir is optimal for hydraulic fracturing.

Increased Drilling Success

CGG uses seismic azimuthal anisotropy to identify fracture density and orientation with a success rate of upwards of 80% in unconventional gas plays. The best areas for inducing hydraulic fractures can also now be identified by calculating the stress state.

Each shale reservoir is unique and must be understood in order to maximize production. Shale properties vary spatially so the reservoirs are heterogeneous. To maximize the potential of these shale gas reservoirs it is necessary to integrate all the disciplines - engineering, geology and geophysics. CGG has the global expertise, experience and technology partnerships to create an integrated solution to understanding each unique shale gas reservoir.