The beauty of modern well-tie software is that it is easy to pull in a few curves, to add some pin points and to move the synthetic to match the seismic, stretch a bit of the top, perhaps squeeze a bit of the base. So let’s think about what we are actually doing when we apply these processes.
Often the entire synthetic appears to be too shallow or too deep. There is likely a discrepancy in the datum or the replacement velocity used to anchor the uppermost point of the synthetic. In this case it is valid to apply a bulk shift but always check the datum and review the interval velocities in the near surface to make sure that they are reasonable.
Stretch and squeeze
In my experience this is the most contentious part of the well-tie process because it is easy to abuse. This is not a procedure by which you simply select peaks and troughs and match them to the seismic. You can make anything tie that way.
The idea is that a distinctive reflector on the seismic section is also observed on the synthetic (and is thus observable on the borehole logs) and there is a certainty that it represents the same event. If the event is identified in the borehole as a specific unconformity that has been correlated from other wells on the seismic, so much the better.
There is well-documented dispersion between sonic and seismic velocities. Dispersion is the phenomenon of frequency dependence of acoustic velocity in the rock. We usually need to make a correction for this by reducing the sonic velocities by a small amount. This is most easily undertaken by stretching the synthetic seismogram so that the depth extent occurs over more time. All geophysicists have their own thoughts on the procedure, but I like to first slightly stretch the entire synthetic so that, in effect, a single drift correction is applied and the major reflectors are correlated.
That said, there might be a significant change of lithology (e. g. clastic to carbonate) so a single drift correction may not apply. In this case, you might need to insert some intermediate knee points.
At this point, it is good to remember that the end game is to correctly place the depth data onto the seismic section so that we can extrapolate away from the borehole or drill a prospect, for example. It is important not just to know where we are in the borehole but to be honest about how certain we are about where we are in the borehole.
Think about what you are doing
With each modification to the synthetic we should think about why we are applying a certain process, what the effect is, and whether it makes sense. Anderson and Newrick (2008) highlighted what can and does go wrong with synthetic seismograms and I add to that here:
- Quality check the logs with the help of a petrophysicist. If adequate velocity and density curves are not available, then substitute computed curves when necessary, but be clear about what you did. There are many ways to model missing data (e. g. Gardner, Faust, Smith) so ensure that the one you choose is appropriate for the geological setting. Present a series of synthetics to illustrate the substitution (i. e. the raw curves with gaps; more complete pseudo-curves) indicating where the real data are and where the computed data are.
- Acquire VSPs for a detailed time–depth relationship. This will provide both a corridor stack to match to the seismic and synthetic, and a time–depth relationship. To extract checkshots, select some key impedance boundaries and use those time–depth pairs.
- Check that the interval velocities are reasonable for the expected strata in the borehole, and if they are not, find out why. It could equally be a data problem or interesting geological information.
- Always make a few synthetics with a variety of edited curves, different wavelets, and even different seismic volumes, such as full, near, mid, and far stacks.
- Remember that a poor synthetic tie is not always caused by poor synthetic inputs — the seismic data may not be optimally processed or correctly positioned due to many reasons, including strong velocity variations, anisotropy, or complex structure.
- When the tie just doesn’t seem to work consider the amplitudes and zones of interest, i. e. the dim zone generally ties to the low amplitude zone on the synthetic, but in this case be honest about how good the tie is.