RE:RE:RE:risk now comes from testing results of chinook and the deep.A simiplistic approach explanation. Typically all formation pressure scales are related to the water gradient...so ~10 kPa/m or 10 kg/m3 density. If encounter a zone at 1000m one would expect ~10,000 kPa formation pressure (slightly higher depending on brine salinity maybe 10.5MPa). If pressure encountered higher than this gradient it's classed as over pressured and if less under pressured. Normal pressured gas tells us we're in a normal hydrocarbon migration system. Overpressured typically tells us the system is closed...pressure hasn't leaked off by breaking the seal through the fracturing process. The source rock is generating the hydrocarbon creating more volume which then starts to migrate. Depending on the scale of the source rock and the volume generated and the trapping container size. If the size of container is constrained and you keep adding volume to it the pressure rises hence the source rock is typically close or in the container. If ones dealing with over pressured the video explains why it's improtant to ensure a good mud system with keeping the wellbore pressure greater than the formation pressure. There are limits to the mud system densities one can drill with. The cheapest option is a fresh water based polymer system and you typically can weight up with addititives like CaCO3 to I think ~12.5kg/m3 - easily controlled as you go higher need different weight materials. If ones really overpressured one needs to go up to barite from 15-19/20 kg/m3 I think is the upper limit. A driller should be able to confirm. Drilling with high mud density slows down your penetration rate and barite impacts the logs so most geologists would prefer it not be used. The gas kicks are the dangerous ones due to the compressibility and the dramatic volume expansion as they come to surface. The risks with mixing up drilling muds too much overweight is the zones ie drilled earlier will have higher mud invasion and damage which can impair future productivity. Won't bother getting into the mud filtercake. Your protection when drilling (well control) is your drilling mud and the last set of casing that has been cemented in place If you have a deep set casing you can afford to take a high volume kick (loose all your drilling mud) and be exposed to a full column of gas and the full formation pressure and you can do a full complete shutin without risk...If not you need to make sure you have enough drilling mud on location, sufficient rig pumps and horse power and weight material to get to the kill weight you need to.. So I'm not sure how much over pressure and where they set the intermediate casing depth on Cascadura 1 but they likely built a plan to set it deeper on Casc Deep and Chinook. Sounds like they designed for the pressure gradient observed in Cas1 vs higher overpressure gradients as they got deeper....The extra sets of casings mean you need to start at larger diameter, slower penetration rate and result higher cost wells but safer..
Hope this helps.