Sedimentation and Riserless Drilling Risk
We were talking about riserless drilling in the Merlin office this week. Commonly done in sub-sea well top holes all over the world. The conversation turned to sedimentation (why not?), the process which creates the seabed over time. As drilling engineers, we’re all familiar with the concept of formations becoming “dewatered” as the depth of burial increases, leading to the rock increasing in density due to the increased overburden, squeezing out water from the pore throats between grains and crystals.
But what happens to this process as the rate of sediment accumulation increases or decreases? High rates of sediment deposition can, under the right set of circumstance lead to pockets of over-pressured fluid in porous and permeable rock, which can present a well control hazard. How can we tell? There are a number of ways, let’s stick to sedimentation for the moment.
Do we have any rules of thumb which might guide us as to the likelihood of risk? Well, for the Gulf of Mexico at least it turns out there are a couple of rules of thumb based on the rate of sediment deposition. Under 500ft per million years (how do you know? – ask your friendly neighbourhood geologist) the de-watering process is likely (key word) to have been effective, avoiding a build-up of trapped pressure.
What happens at higher rates depends on whether or not a seal has developed, creating a hydrocarbon trap. Shallow seismic is your friend here, allowing a geophysicist to map likely seal horizons. The safest course of action is of course to move location to avoid drilling into a potential trap in the first place however if that’s not an option, it’s prudent to assume that what lies below the reflector is waiting to bite you and to plan appropriately.
Riserless drilling and its risks are just one of the interesting and thought-provoking topics for discussion on Merlin’s 3 day Deepwater Drilling training course. Come along, engage with your peers and industry experts on this topic and many more.
See you there!