European-US group CGGVeritas has developed seismic solutions to identify shale gas “sweet spots” which it is demonstrating at this year’s SEG Conference in Denver.
Shale gas is a hot topic today, with huge plays now being exploited and prospected in the US and the likelihood is that a similarly massive sector will develop in Europe.
The company says that 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, and reservoir engineers can use this information to optimise drilling and completion locations.
Unconventional reservoirs require some form of stimulation to obtain commercial production, however, and 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.
Three types of information extracted from seismic are useful in optimising drilling locations: fracture characterisation, 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). This means that the fracture system is comprised of vertical fractures that 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 multi-component 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.
The company says this information is of great value to production companies because it indicates the optimum horizontal drilling azimuth and offers the prospect of subsequent fracture stimulation as a solution to tap into existing natural fracture systems.
A clear understanding of the geomechanical properties and their distribution explains the reservoir heterogeneity and thus the variation in economic ultimate recovery (EUR) between wells.
CGGVeritas is able to derive a host of geomechanical properties from migrated CDP gathers, including what are known as “Young’s Modulus”, “Poisson’s Ratio”, and “shear modulus”, by first inverting the data for P and S-wave velocities and density. With this information, fracture dimensions can be predicted and wells drilled in the most brittle rock.
Linear Slip Theory for geomechanical properties is used to calculate stress values. Generally, the stress state is anisotropic leading to the estimation of both the minimum and maximum horizontal stress.
As the seismic data measure dynamic stress, results are then calibrated to the static stress that is effectively borne by the reservoirs at depth, making it possible to predict the hoop stress and the closure stress as key elements defining the type and motion of fractures.
At locations where the differential horizontal stress ratio (DHSR – the ratio of the difference between the maximum and minimum horizontal stresses to the maximum horizontal stress) is low, tensile fractures will form in any direction, creating a fracture swarm.
If the maximum horizontal stress is much greater than the minimum, then fractures will form parallel to the direction of maximum horizontal stress.