The theory of quantitative seismic interpretation (QSI) has been around longer than all currently practicing geophysicists have been alive.
Karl Zoeppritz outlined the set of equations defining energy partition of seismic waves at a rock interface in findings published after his death in 1919. In 1972, the Canadian Well Logging Society published what seems to be the first written post-stack seismic inversion paper, “Approximation of acoustic logs from seismic traces,” authored by Roy O. Lindseth. Clearly, this is a topic with great staying power and one that continues to be of importance to the E&P industry today.
So just what is QSI, and why is it useful?
The basic idea has not changed since Lindseth’s original implementation. QSI is about transforming seismic data, with units that are nebulous and vague at best, into physically meaningful units similar to a well log. Seismic data units are so unprecise that there is almost never a scale or legend on a seismic section that even attempts to describe the units. Imagine a thermometer that was not calibrated in degrees Celsius,Kelvin or Fahrenheit, but instead just read freezing, cold, tepid, warm or red hot. That’s what seismic data is like.
QSI methods use simplifications to the elastic wave equation by geophysicists Aki and Richard, Shuey, Ostrander, Bortfeld, or Zoeppritz to convert seismic data into properties which can be directly compared to those computed from well logs. That way, if an anomalous well log signature is present due to oil or gas, overpressure, or fractures, the same distinctive signature can be sought throughout an entire 3-D seismic volume.
To the non-geophysicist, QSI seems like a pretty dry topic, but it has major business implications for E&P.
- QSI can detect abnormal pressure or unsafe drilling conditions before the drillbit punches through them. This allows an operator to avoid a catastrophic industrial accident or environmental discharge;
- QSI can often distinguish between pay and non-pay, and increase an operator’s success ratio. Too few successes in today’s low price environment can be ruinous;
- Properties from QSI can be used to influence the population of reservoir models and improve flow simulation and history matching results and
- Fracking is a very expensive part of unconventional E&P. QSI can help detect where best to focus limited cash to complete and frack, based on brittleness, horizontal fractures and stress conditions in the subsurface.
Since QSI was defined theoretically in 1919 and in commercial use since 1972, what’s new in this space? Are there innovations that E&P operators should be paying attention to? The answer is yes.
Some of the main difficulties with legacy QSI software packages have been related to workflow and ergonomics. These products have always remained separate from the main flow of interpretation work, in terms of the data model, who does the work and their software user experience. The risks are sometimes described as “QSI hand grenades thrown over silo walls” or “QSI problems passed down the 1970s-style automobile assembly line (and accepted unknowingly by downstream customers).” The complexity of data movement and duplication, as well as the career management difficulty of training personnel for several different software packages, compounds the risks and inefficiencies of traditional QSI.
Since 2014, modern software has worked on two fronts to bring expanded capabilities to QSI interpretations. Those are:
- To bring prestack seismic and QSI functionality directly into the interpreter’s familiar main 3-D canvas and
- To implement amplitude versus azimuth (AVAZ) and velocity variations with azimuth (VVAZ) on the interpreter’s desktop, especially important for addressing the needs of unconventional E&P.
Paired with an integrated well and seismic database, the enhanced software allows interpreters to quickly and interactively test QSI ideas in minutes, rather than going through passing the data to a separate package, doing the work in a package with its own distinct ergonomics, then passing it back.
The time savings over sending QSI work out to external contractors is even greater. Time is money.
QSI quantifies the relevant parameters of the subsurface for each rock unit within the geometrical interpretation framework. (Source: Paradigm)
Thinking beyond the desktop QSI solution, it is also now possible through a cluster-based solution to create the best possible AVA/AVAZ gathers for QSI work by performing ray-tracing through complex VTI, TTI, and orthorhombic velocity models. There are many cases where simple source-receiver azimuthal angle sectoring fails, because seismic rays travel in the earth wherever the velocities dictate, regardless of the surface geometry.
Today, even more than in decades past, determining the impact of rock, fluid and fractures is vital to the success of both traditional and unconventional exploration and production efforts. By embedding QSI tools into the mainstream 3-D canvas, geoscientists can now ensure the cohesion of the interpretation scheme, reduce processing time and eliminate drilling uncertainty.
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