Solutions Finding reserves Seismic Inversion

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Inversion of the seismic data means extracting layer properties from reflectivity, which involves removing the effect of the seismic wavelet from the data, as well as calculating seismic layer properties (e.g. impedances, Vp/Vs ratio, density) from the interface reflections which depend on contrast.

The best output is often obtained using algorithms that take geology and rock physics into account, such as Ikon’s Ji-Fi (Joint Impedance and Facies Inversion).

Besides the reduction of interference effects due to the limited seismic bandwidth, the major advantage of seismic inversion is that it provides a common platform for communication because reservoir engineers and geologists work in a layered configurations as well.

Image shows a very good correlation between inversion predicted and well litho-facies. Strong gas-sand indication at recent Pyxis-1 location below K horizon. Highlighted by ellipse is another strong gas indication between Pluto-4 and Pyxis-1 at about the same depth as Urania-1 gas interval

Ji-Fi

Joint Impedance and Facies Inversion. Bayesian simultaneous inversion which jointly solves for impedances and facies.

Simultaneous inversion

Simultaneous AVO inversion that takes angle domain PP seismic data, low frequency background models and wavelets and inverts for absolute elastic properties Vp, Vs and Rhob.

PPPS inversion

Simultaneous AVO inversion that takes angle domain PP and PS seismic data, low frequency background models and wavelets and jointly inverts for absolute elastic properties Vp, Vs and Rhob.

Coloured inversion

Post-stack spectral shaping tool that takes seismic reflectivity data and ‘inverts’ to layer based relative acoustic/elastic properties by shaping the seismic reflectivity data to its corresponding earth impedance spectrum.

Model-based inversion

Post-stack model based inversion that takes seismic reflectivity data and inverts to its corresponding absolute impedance (AI, SI, EI etc) given a low frequency background model and corresponding.

Delivery inverter

Direct access to Delivery’s petro-physical inversion.

Stochastic inversion

Post stack and joint inversion performed on a stratigraphic grid at high resolution to generate multiple equi-probable realisations of impedance for reservoir property and uncertainty analysis.

Band-limited inversion

A simple inversion technique that takes seismic reflectivity data and inverts to corresponding absolute and relative impedance properties by de-convolving the wavelet directly.

AVO inversion

A variety of published reflectivity algorithms that allows the user to invert pre-stack seismic reflectivity data to AVO products such as P-wave, S-wave and Density reflectivity.

Ji-Fi

Ji-Fi is an innovative and unique Bayesian simultaneous inversion that takes partial angle stacks and solves jointly for Facies and Impedances. Ji-Fi combines the significant benefits of simultaneous seismic inversion with depth dependent Bayesian classification to leverage additional rock physics constraints that help to produce higher quality, more consistent and geologically reasonable results.

Some of the key advantages that Ji-Fi brings over other conventional model based inversion techniques include:

Some of the key advantages that Ji-Fi brings over other conventional model based inversion techniques include:

- No more complex low frequency models
- Tighter data integration
- Consistency of rock physics throughout inversion and interpretation
- Improved thick bed resolution
- Improved thin bed resolution
- Inclusion of regional/analogue information
- Improved noise reduction
- Efficiency

Simultaneous inversion

Simultaneous inversion is the industry standard model-based elastic impedance inversion. Angle stack seismic data can be inverted simultaneously for acoustic and elastic impedances, with the impedances generated dependent on the reflectivity model utilised in the inversion.

Multiple reflectivity models are available for use in the RokDoc simultaneous inversion, including Bortfeld (1961), Wiggins (1993) and Fatti (1994). The inversion uses a fast direct algorithm to minimise an objective function computed between the synthetics and input stack traces and between the inverted properties and the input base model. Above the usual parameters available for a simultaneous inversion, the RokDoc implementation includes the ability to set weightings on each angle stack, and model weight factors to determine how the model weight is applied to the individual inverted impedances. Both lateral and time variant wavelets can be used in the inversion to account for changes in seismic bandwidth throughout the survey.

The RokDoc simultaneous inversion implementation has multiple quality control tools including:

- Synthetics generated from the final impedance results using the selected wavelets
- Residuals between the seismic and synthetic trace
- Relative impedance quantities generated by subtracting the low frequency background model
- Spectral plots for seismic, synthetic and residual quantities
- Well-based analysis including comparison of well-dervied and inverted impedance profiles, XCC and PEP values and cross-plot analysis

The RokDoc simultaneous inversion module is fast and allows thorough QC of the inverted impedances. The latest version of RokDoc includes a fast direct inversion (rather than iterative) and also allows model weighting factors to be specified for the second and third impedance terms. In fact, the new algorithm is 25 times faster than the original method!

Licensing: Add-on module to RokDoc 3D. Can be purchased standalone, or as part of the "RokDoc Inversion Essential" bundle.

PPPS inversion

To fully describe an elastic, isotropic earth, three parameters are required. These are normally P-wave velocity, S-wave velocity and bulk density (RhoB). Density estimates in traditional P to P wave reflection seismic (or PP) inversions are made based on the high incidence angle P-wave reflection behaviour close to critical angle. With limited good quality high angle PP data normally available, the three parameters (two velocities and density) cannot easily be determined. To overcome this, generally two attributes are inverted for from PP seismic (e.g. Fatti et al., 1994 - AI and SI). The amplitude of reflected S-waves (P to S or PS) depends on only two parameters for isotropic elastic media, allowing better estimates of S-wave velocity and density from PS data.

The RokDoc PPPS simultaneous inversion tool simultaneously inverts PP and PS seismic data to acoustic impedance (AI), shear impedance (SI) and bulk density (RhoB). The use of both PP and PS data allows more robust estimates of AI, SI and RhoB. Each inversion (PP and PS) can be parameterised separately (e.g. using appropriate wavelets for the PP and PS angle stack data) and a weighting function allows user control over the influence of the PS data in the final impedance estimates.

Where PPPS seismic data has been recorded, the RokDoc PPPS simultaneous inversion allows the geoscientist to get the most out of it via the generation of more robust AI, SI and density estimates.

Licensing: Add-on module to RokDoc 3D. Can be purchased standalone, or as part of the "RokDoc Inversion Advanced" bundle.

Coloured inversion

Coloured inversion is a simple and robust technique for generating relative impedances from zero-phase seismic data, developed by Lancaster and Whitcombe (2000). The method is a convolution process: an operator is designed that performs a -90 degree phase rotation and spectral shaping to the seismic trace in order to generate relative impedance profiles. The spectral shaping matches the amplitude spectrum of the seismic to the earth impedance spectrum over the bandwidth of the inversion. The method does not require the generation of a background model, so is particularly appropriate in exploration settings. Depending on the attribute that has been inverted, the use of layer properties versus reflectivity can greatly assist the interpreter. Coloured inversion is a quick and robust way of generating layer properties from seismic.

The RokDoc implementation of coloured inversion allows the user full control over the seismic amplitude spectrum and earth impedance spectrum used to derive the operator. The low frequencies can be added to generate absolute impedances via the use of a spectral merge function. In addition the module also allows quick and easy QC of the results of coloured inversion at the well locations.

The RokDoc implementation of coloured inversion allows the user full control over the seismic amplitude spectrum and earth impedance spectrum used to derive the operator. The low frequencies can be added to generate absolute impedances via the use of a spectral merge function. In addition the module also allows quick and easy QC of the results of coloured inversion at the well locations.

Licensing: Add-on module to RokDoc 3D. Can be purchased standalone, or as part of the "RokDoc Inversion Essential" bundle

Model-based inversion

Model-based inversion is the industry standard approach to single stack seismic inversion for a single absolute impedance. The method uses a starting background impedance model and equivalent seismic trace (e.g. AI and zero-offset reflectivity) to generate an impedance estimate via the minimisation of an objective function. The background model is perturbed until the difference between the resulting synthetic and seismic trace is minimised.

RokDoc allows the use of laterally and time variant wavelets to capture the variation in seismic frequency content across the survey. As with the RokDoc simultaneous inversion module the RokDoc model-based inversion has a variatey of quality control tools available to the user:

- Synthetics generated from the final impedance results using the selected wavelets
- Residuals between the seismic and synthetic trace
- Relative impedance quantities generated by subtracting the low frequency background model
- Spectral plots for seismic, synthetic and residual quantities
- Well-based analysis including comparison of well-derived and inverted impedance profiles, XCC and PEP values and cross-plot analysis

Licensing: An add-on module to RokDoc 3D. Can be purchased standalone or as part of the "RokDoc Inversion Essential" bundle

Delivery inverter

Delivery is a Bayesian based petro-elastic inversion program designed for inverting near and far stacked seismic data to yield distributions of properties sub as net-to-gross, fluid saturations, porosities, etc., in individual bodies. The Delivery software was written at CSIRO and is freely available for download and use. To simplify the program’s use, and to harness data that has already been created or loaded to a RokDoc projects, an interface to the Delivery-Inverter and Delivery-Analyser is available. Data, such as wavelets, seismic, trends and bodies that has been worked up in a project, can be pushed into a Delivery session directly from RokDoc. Conversely, the inverted data can then be pulled back from Delivery’s output and displayed in a RokDoc-3D session. This gives an easier and safer access to Delivery since you are spared the need to set up and edit its input parameter files.

Combining Delivery and RokDoc into one tool, simplifies the creation of a probabilistic seismic inversion for a layer based reservoir model. With an inversion being inherently non-unique, Delivery will give a distribution of possible solutions directly in terms of petro-physical properties that can be used as part of a RokDoc workflow.

Combining Delivery and RokDoc into one tool, simplifies the creation of a probabilistic seismic inversion for a layer based reservoir model. With an inversion being inherently non-unique, Delivery will give a distribution of possible solutions directly in terms of petro-physical properties that can be used as part of a RokDoc workflow.

Licensing: Add-on module to RokDoc 2D.

Stochastic inversion

Stochastic inversion allows uncertainty in reservoir properties to be taken into account, producing multiple possible geological realisations. These can then be used to create probabilistic forecasts of oil-in-place, NTG and other model properties.

Licensing: Add-on to RokDoc ChronoSeis.

Band-limited inversion

To approximate the impedance of the subsurface using seismic data, it is necessary to account for the band-limited nature of seismic data, especially at low-frequencies. Waters (1978) described an inversion scheme which he dubbed ‘SAIL’ (Seismic Approximate Impedance Log). This method is simple approach to deriving impedance values from seismic data. An impedance estimate, from a well-log (or stacking velocities), is first combined in the frequency domain with integrated seismic data. The result is inverse Fourier transformed to provide an impedance trace. Detailed impedance values are thus provided by the integrated seismic data, and the low frequency trend is provided by the well-log.

Licensing: Band-limited inversion module

AVO inversion

We compute the reflectivity (or contrast) terms that are defined in terms of petrophysical quantities when solving equations used by Aki and Richards (1980), Smith and Gidlow (1987), etc. and which have the common form of:

**Rpp(θ) = a(θ) R1 + b(θ) R2 + c(θ) R3**

θ is the angle of incidence - commonly the stack angle,

a(θ), b(θ) & c(θ) are constants depending on θ and K=(Vs^2/Vp^2)

and R1, R2 & R3 are the reflectivity (or contrasts) in petrophysical or impedance quantities.

These equations have a similar form for each of the petrophysical models, with the outputs representing different quantities depending on the model chosen (Kemper 2010).The following models are supported:

The main required inputs to the function are a set of partial angle stacks, the selected model and a value for K (Vs^2/Vp^2). The function then computes the reflectivity terms (R1, R2, R3) from the angle stacks according to the chosen model. These outputs can be rotated, inverted or used directly to infer rock types and fluid fill from seismic data.

Licensing: This module is activated with any inversion license.

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