Our 4D seismic tools have proven successful all over the world for many years and give us a clear competitive edge. They are the fruit of our state-of-the-art R&D, which has constantly invented new solutions to enhance the performance and expand the scope of application of geophysical monitoring of reservoirs.
Warping-4D: A success story
We owe our leadership in 4D seismic to the quality of seismic processing, the integration of our Geosciences teams and the quality of 4D attribute imaging obtained using our Warping-4D tool. Patented in 2006, Warping-4D optimizes the time-shift correction needed to compare repeat seismic surveys for reservoir monitoring purposes.
This tool is unmatched in the industry. The quality of its results is far superior to the correlations generally used by the industry to compare two 4D seismic vintages, known as the baseline or base seismic survey and the monitor seismic survey (i.e., the same survey repeated over time). This 4D post-stack* inversion* technique determines the best distribution of velocity changes along the seismic traces. While providing an explanation of the trace contractions and extensions linked to the increases and decreases in velocity, it also calculates the new wave amplitude, because the waveform shape is altered by the changes in wave velocity and rock density. In this way, we can visualize the changes that have occurred not only at the interface between geological layers, but within the layers as well.
*Post-stack: after several seismic traces have been summed into one to improve the signal-to-noise ratio.
*The inversion processing technique builds a subsurface model using data acquired from the surface.
Chipping Away at the Uncertainty Space
In addition to our warping tool, we have recently developed some quick and simple in-house tools for 4D impedance* inversion. While it can take contractors several months to pick the impedance variations within reservoir layers from the 4D data, we are now able to do so in a mere two-to-three weeks’ time. The impedance attribute is a complement to the warping attribute, and has proven highly valuable when interpreting non-unique 4D data, thereby contributing to better management of our uncertainties.
R&D efforts are ongoing to continually optimize our 4D attributes calculation tools. For example, we are working on two new versions, Warping-Multimonitor and 4D Inversion Sub-stack. Both these developments are aimed at reducing uncertainties as much as possible and yielding 4D images of ever-higher resolution. While the original Warping-4D allows inversion of the base and monitor surveys, the Multimonitor version is designed for inversion of the base seismic survey and all monitor surveys acquired on a given field, to suppress the biases inherent in the physical measurements recorded during each one. The 4D Inversion Sub-stack, on the other hand, is designed to give us more information about rock composition by providing a new type of data: variations in seismic wave reflectivity as a function of their angle of incidence at the layer interfaces. We can then observe the behavior of those generated shear waves.
FLEX 4D, the most recent addition to our toolbox, is currently being upscaled and offers an alternative to Warping- 4D. More versatile and up to ten times faster, FLEX 4D is designed for specific configurations where the correction of wave amplitudes by warping is not required.
*An elastic property of a rock equal to its density multiplied by the seismic wave velocity, which varies with changes in density caused by depletion, waterflooding, etc.
Toward Seismic 4D in Complex Geological Conditions
Warping-4D enables us to perform inversions on seismic images in poorly structured environments, but we must also have the capability to compare repeat seismic images in more complex ones such as salt diapirs. Ultimately, our goal is to be able to perform the seismic monitoring of production in structurally complex reservoirs in developments under way, particularly salt bodies in the deep offshore acreage of West Africa or Brazil.
This is where new tools currently being developed will have the greatest impact:
- WAVE 4D, like Warping-4D, is a post-stack inversion tool designed especially for environments for which the Warping-4D cannot be used, such as geological layers underneath the reservoir with significant dips (>10°).
- TOMO 4D, in contrast, is the first of our tools to make use of pre-stack (i.e., before summing) 4D seismic inversion. Pre-stack data contain much more information than post-stack data but represent a much greater volume. The pre-stack inversion will leverage the powerful capacity of our high-performance computer, Pangea III.
- LSI4D and FWI4D. A research program is currently ongoing, in order to adapt the most modern tools of seismic imaging to the specific case of 4D.