At some point during a 4D project you will encounter stress – not just the human kind of course, but the kind that changes the rocks in your reservoir and its surroundings. Unfortunately, all reservoirs experience this during production lifetime, although admittedly to a lesser, or greater extent depending on the degree of connectivity, pressure support, depletion rates and development strategy.
Don’t stress about stress before there is even stress to stress about. It is ok for a reservoir to have a lot of stress, not you.
Stress changes appear in your data as variations in seismic impedance, velocity, and density – key properties of relevance to feasibility studies, 4D interpretation and modelling. Obviously, these are essential to understand in order to clearly detect pressure and saturation variations over time. Unfortunately, the effect of stress is notoriously non-linear (see figure below) and difficult to calibrate in situ. So how do we know how big the variations are likely to be? What can help guide us? There are four solutions currently available:
- Measure the in situ response using time-lapse logging – these data are rare and usually expensive to acquire.
- Squeeze core samples in the laboratory – this is our preferred option, and if we get the stress paths right with a decent selection of subsurface core samples, we can do a pretty good job of replicating reality.
- Use an existing database combined with a semi-empirical trend equation – a lot of data already exist from past studies. Finding an analog for your field is possible, and a handy trend equation helps ….
- By reverse engineering, it is possible to back out the desired stress sensitivity from the 4D seismic data itself using a few select wells in your field.
Useful added information – whilst stress can be viewed as making life difficult during interpretation, it can also help in many cases! Indeed, stress must lead naturally to strain or deformation of the subsurface. In turn, this creates reservoir compaction and also overburden extension, leading to fault reactivations, a loss of production at wells, or creation of zones dangerous to future drilling. This is especially the case in the highly compacting chalk reservoirs, HPHT fields, or unconsolidated sands. Thankfully we can monitor these strains using 4D seismic, backing out the geomechanical effects by detecting minute time-shifts in the data induced by the velocity changes. We can then decipher these changes to give a good indication of the overall distribution of stress and strain in and around the reservoir.
The non-linear response associated with a stressed subsurface rock.
In the next article of this series, we look at another problematic issue for reservoirs – the presence of gas, and whether it is a friend or foe?
Author: Prof. Colin MacBeth, University Associate of iRes-Geo. Professor in 3D/4D reservoir geophysics, Heriot-Watt University. Project Leader of Edinburgh Time-Lapse Project
Please feel free to contact us should you have any questions about 4D stress.