4D seismics is hype. It’s the new stuff. But 4D is also somewhat obscure, so when I started telling people telling that I had landed a job in 4D seismics, the most common question was:
So, Jesper, what’s the fourth dimension.
The answer always causes some disappointment. “It’s time.” They were hoping for some obscure quantum dimension (not actually a thing, sorry). 4D seismics is interchangeable with time-lapse seismics. The principle behind it is seemingly simple. The execution not so much.
The Idea of time-lapse seismics
Take seismics for what it is, it’s a snapshot of the mere peel of the Earth, which has a history of 4 billion years. Rocks are known to be particularly lazy, they stay where they and move a couple centimeters at most per year. If they start moving faster, we usually have some type of catastrophe. So when looking at simple geological structures in seismic, it does not make a whole lot of sense to reacquire data over the same area unless there is something changing within the rock or the acquisition technique has improved. This is usually some type of fluid, such as hydrocarbons or water.
Although we call it timelapse, it’s important to remember that it is usually the difference between two datasets at different times that is investigated. Even several timesteps will have to be compared individually to a baseline.
The use of 4D seismic analysis
Monitoring the spatial extent of the steam front following in-situ combustion or steam injection used for thermal recovery
This is a particularly easy to grasp concept. Thermal change causes the oil to warm up and viscosity decreases. This change will also yield a change in seismic properties. In the special case of some heavy oils, it might even cause the shear modulus to disappear, so that it would initially appear on shear wave seismic images and vanish once the temperature treatment has begun.
Monitoring the spatial extent of the injected waterfront used for secondary recovery
Water flooding in improved or enhanced oil recovery, is very common nowadays. Complex reservoirs may cause the injected water to progress in unexpected patterns, sometimes even reaching the producer well, while directly bypassing hydrocarbons. That way, we directly produce water from the reservoir. Not the best situation.
Imaging bypassed oil
In 4D it’s important to not only look at the changes in reservoirs. You can also see which chambers are not connected to the tapped reservoir and develop strategies for recovery of these resources.
Determining flow properties of sealing or leaking faults
Seeing the time-lapse changes in leaking faults or even sealing may be important to assess the possible recovery rate and possibly intervene.
Detecting changes in oil-water contact.
When looking at the Oil-Water contact, it’s important to realize, not only the depth of the surface. Changes in dip may also give clues to hydrostatic changes.
Pushing this will be quite interesting. I am working with the Danish chalk fields. Those are considered a challenge according to some papers I read so far. Exciting times lie ahead!