After discussing, in the last two chapters, how geologists (chapter 3) and petrophysicists (chapter 4) can get involved in a geomodeling project, we now look at the role of geophysicists. This chapter is the last one on the relationships between geoscientists and geomodelers. The next three chapters will talk about the role of engineers.
Geophysicists in oil and gas companies worked mostly on 2D seismic data before the technology to acquire 3D seismic data was developed and became popular. Companies are now also using seismic data to monitor the development of their fields thanks to the acquisition of 4D seismic data. Nowadays, geomodelers still have to integrate some 2D seismic lines into their models and some are starting to work on the integration of 4D time-lapse data. But for most of us, when we think about geophysical data, we have in mind the integration of 3D seismic data into our geomodels. This topic is the focus of this chapter.
First, one has to ask: Why do we need a geomodel if we have acquired 3D seismic data? After all, seismic cubes give us an image of our reservoirs between the wells. Don’t they solve all of our problems? Using a technology as a stand-alone product has its use. When integrated with other technologies, such as geomodeling, that is when it shows its potential.
A seismic cube does give us a 3D image of the reservoir; however, the resolution is usual too low to capture the level of detail engineers need to truly understand the reservoir. Wells data provides the level of detail we are looking for, but this data can be cumbersome to interpolate in 3D. The solution is to integrate well data and seismic data.
We use trends extracted from seismic to guide the interpolation of wells data in 3D. Some uncertainty about the reservoir will remain, coming both from the seismic interpretation process, from the work on the wells and from the integration of the well data with the seismic. This uncertainty needs to be taken into account as well. Integrating different type of data together and understanding the impact of uncertainties in a model: these are two tasks that geomodels are designed for.
As such, it’s advisable to build a geomodel whenever seismic data and seismic interpretation are available. In the meantime, building a geomodel only based on well data while seismic is readily available would be a shame. Good seismic information will always tell us more about the reservoir than the results of pure mathematical interpolation techniques. If a geomodel is about to be built, a geomodeler should always ask if seismic data are available.
Geomodeling can be used for time-to-depth conversion. This will be covered in the first section. Secondly, seismic interpretation is extremely useful to guide how the geomodel 3D-grids shall be built. The second section focuses on the integration of stratigraphic interpretation, while the third section looks at the integration of structural interpretation in geomodeling. Lastly, considerable information about the rock characteristics can be extracted from the seismic cube, such as seismic attributes or fracture density for example. This data can be used to guide geostatistical algorithms. It will be the topic of the last section. Each of these sections will also cover the associated uncertainties.
To close the introduction, a second question is in order: who should really be accomplishing all these tasks that we will cover in this chapter? The geomodeler with his geomodeling software or the geophysicist with his 3D geophysical package? Over the last few years, software companies specializing in geophysical packages have added more and more tools of grid construction, of geostatistics and of 3D-grid analysis (volume computation for example). In a similar way, geomodeling packages are now able to accomplish large parts of many geophysical workflows. So who should integrate seismic and well data and study the remaining reservoir uncertainties? The geomodeler or the geophysicist? We believe that it doesn’t matter who does the job. Our goal is to highlight aspects of a reservoir study in which we believe geophysics and geomodeling shall be combined. Each team will decide who has the time, the tools and the experience to do the agreed-upon workflow. Ultimately, it’s all a team effort. It doesn’t matter who is pushing the buttons on a computer, as long as the job gets done.