Navtej Kohli – Pros & Cons of 2D and 3D Seismic Surveys ( Navtej Kohli – President, Granox Ltd)
I am trying to elaborate on the advantages and disadvantages of using the geophysical technique in the oil exploration industry. A geophysicist would be able to give a more “expert” answer, but I will offer what opinions and experience I have.
The problem with most of the subsurface data we have to work with in oil exploration is, that they are all indirect evidence. For example, we cannot directly measure the porosity or fluid content of rocks in the wellbore, so we run electric logs in the well and bombard the rock formations with gamma rays, measure the resistivity, etc. From these measurements of indirect properties, we try to calculate the porosity of the reservoir and what kind of fluid may be in it, but we aren’t measuring these things directly, we are only extrapolating an answer based on a bunch of related but indirect data. Over time, many of these kinds of data have proven to be worthwhile and sometimes accurate, but it is wise to remember what these data really are. The only “real” subsurface data (in my opinion) are cores taken of the rock formation and drill cuttings of the rock formations, and they are limited in amount and the scope of what they can tell.
The seismic method suffers from the same kinds of problems. You cannot directly record or measure subsurface structure, porosity, the presence of hydrocarbons, or anything else really useful – all you are measuring with seismic is the time it takes for energy to be reflected off of some subsurface feature and return to the surface. You can also extract some more information from the nature and shape of the waveform data that come back, but again these are very indirect measurements.
With that general limitation in mind, the practical pros and cons of seismic (from my point of view) are these:
1. It is a way to gather subsurface data over a large area in a short amount of time. In rank basins where there are few drilled wells, it may the only way to obtain any subsurface information.
2. Seismic (especially 3D) when used appropriately can reduce the risk of drilling wells.
3. The data are acquired digitally and can be worked off site by even small companies and individuals.
4. Recent technological advances have vastly improved the accuracy and usefulness of seismic data. New processing and interpreting techniques like “coherency cube” may be able to extrapolate even more useful information and correlations out of the seismic data set.
5. Seismic is fairly well understood and appreciated by the industry in general. It can add credibility to a drilling prospect.
1. It is inexact, and data quality is subject to a host of uncertainties due to environmental conditions and etc.
2. It is very expensive. The seismic business is very dependent on both high tech computer equipment and skilled people.
3. The data are extremely subject to interpretation; you must have a skilled interpreter with specific knowledge of the area he is working.
4. It requires rocks with a velocity contrast to work, otherwise you will not get any useful reflections. In plain terms, it will not work everywhere, in every basin, play, or series of rock formations.
5. Some people put way too much faith in it. It is just another tool, not the be-all and end-all of exploration. I much prefer to use subsurface (well) data whenever possible, but I am biased since I am a geologist and not a geophysicist.