Identifying shale gas 'sweet spots'

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

Geophysical 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.

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).

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 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.

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. 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 derives a host of geomechanical properties from migrated CDP gathers, including 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.

Broadband marine solution

Meanwhile, BroadSeis, the CGGVeritas broadband marine solution, continues to provide exceptional broadband images in different geological settings.

Since its launch at the Barcelona EAGE convention, BroadSeis has generated considerable interest. Eight projects have now been recorded in various locations around the world, from Australia to the Arctic. These include both 2D and 3D geometries in a variety of geological settings, water depths and target depths. These surveys are both proprietary and non-proprietary and clients include a number of major oil companies.

Deepwater data examples from West Africa and the Gulf of Mexico were shown at the recent SEG convention, providing exceptional bandwidth of 2.5 - 155Hz, almost 6 octaves of data. These results included true amplitude deghosted gathers, suitable for AVO analysis.

BroadSeis acquires both low and high frequencies simultaneously using Sercel Sentinel solid streamers with a variable depth profile (patent pending). The cable ghost notch varies along the streamer with the cable depth and our new proprietary deghosting and imaging techniques (patents pending) use this notch diversity to produce a very sharp wavelet, with a superior signal-to-noise ratio and the maximum bandwidth.

The low-noise characteristics and precise low-frequency response of Sentinel streamers have proved central to the success of BroadSeis. Sentinel is quieter and can be towed deeper than other streamers (up to 60m). Since Sentinel has an instrument low-cut of 2Hz, BroadSeis provides significantly better low frequencies, recording high-quality signal down to at least 2.5Hz.

A key feature of BroadSeis is that the cable depth profile can be tuned for different water depths and targets so that the notch diversity and output spectra are optimised for each survey. Operational experience has shown that the control of solid streamers to variable depths is robust and stable for both 2D and 3D configurations, even down to 60m, especially when using Sercel Nautilus streamer control devices. This has been proven even in areas with variable currents such as the Gulf of Mexico and variable salinity conditions such as West Africa. Sentinel has retained its exceptional low-noise characteristics at all depths, with all streamer profiles and under all conditions.

BroadSeis includes a new, true-amplitude, fully-3D deghosting algorithm that makes no 2D assumptions. This gives it unrivalled suitability for 3D and all wide-azimuth acquisition configurations, as well as 2D data. Whilst the proprietary deghosting is a part of the BroadSeis technique, the raw field data is supplied to clients providing them with flexibility to apply any processing techniques desired.

From shallow objectives to deep targets beneath complex overburdens, BroadSeis is delivering data with exceptional bandwidths and excellent penetration. For clearer imaging of deep targets, as well as greater stability in seismic inversion, providing reduced exploration risk, BroadSeis is proving to be the Safer, Broader, Better choice for marine acquisition and processing.

CGGVeritas' headquarters is in Paris, France.

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