(Wow, this has been an interesting week!)
The team that I'm working with are planning to core a Plate Boundary Fault, as part of the IODP Programme. Based on seismic interpretation, the fault lies at about 7000m TVDSS (2000m below seafloor). However there is currently a very large uncertainty of depth, which will be refined by a variety of methods. One of which will be Seismic-whilst-Drilling (SWD).
To add to the complexity, this Expedition will be a well re-entry after unstable borehole resulted in setting a liner significantly off-bottom and temporary suspension. It's hardly surprising that wellbore stability is an issue, given what we are trying to do.
The science party and drilling team will be working together to apply Real-time GeoMechanics to this programme, to better prevent anisotropic breakout.
An enabling technology will be continuous circulation. The supplier has been used before on previous expeditions to preserve the LWD, where BHST was > 300°C!
Now, for the challenge.
- Continuous Circulation means that we will continue circulating whilst making connections; this will help with hole-cleaning, minimise borehole pressure cycling, and possibly keeping us above the threshold for anisotropic break-out.
- Seismic whilst drilling acquires data during connections and - we are told - requires a 'quiet' environment i.e. no noise from pumping.
What we'd like to know is if any of you 'out there' (2600 members in 860 organisations) have encountered this issue. If so, have you managed to acquire SWD whilst circulating and, if so, what was the data quality like and how did you do it?
When replying, please can you keep 'on topic'. Feel free to forward this to others in your organisation.
I have never seen SwD used with Continuous Circulation systems, however whilst using Continuous Circulation systems we have seen the following issues whist performing MWD & LWD operations.
1. Mud pulse telemetry - Positive pulse seems to work better than negative pulse and no issues with any other telemetry formats. Ive never seen any pulser interference with LWD Sonic Logs but this would be one to look out for on the SwD if you are circulating then the pulsar will most likely be going ''off''.
2. Surveys across connections. Many MWD Tools use the power off sequence of stopping circulation to trigger a survey, which is then pumped up when the tool switches back on and starts pulsing again. There are tools which can be set up to give a survey on demand but remember the tool must be stationary to get a good survey back to surface. This may also be done by reducing the flow rate but not stopping it altogether.
3. Many LWD tools are run with a motor, if you're pumping then the motor will be turning and this again could induce some noise into the string and possibly effect the SwD measurements.
4. finally to top all of the above off the tools, motor and circulation system must all operate within a compatible circulation flow rate window. Which is doable but needs careful planning.