f Ultra-Deep Offshore Well Casing Design Process

Drilling an ultra-deep water well is one of the most challenging tasks that an Exploration and Production Company can undertake. Not only are there technical, logistical and environmental issues, but the high operating costs makes every aspect of the well planning and execution process critical. As in any other well delivery process, the casing design is one of the most important deliverables as this will affect the lead time for procuring equipment, the most likely time estimate for drilling the well and finally the cost. The first step of casing design involves coming up with a robust conductor design as this is the main support structure for the well. Detailed bending studies should be carried out because it will carry hundreds of thousands of pounds. Secondly any possible shallow flow (shallow water and gas flow) should be considered which needs to be isolated by an additional casing string. Once initial studies are done for first two sections, rest of the well design will be mainly limited by pressure gradients (PP and FG). As rule of thumb in a hole section the pore pressure at section TD should not exceed last casing shoe’s fracture gradient. If this happens mud losses will occur which could become a costly event and lead to an influx or kick. To eliminate this risk a detailed PPFG study should be carried out by geological and geophysical experts, because this study will be the guide for planning engineers to make casing design. As nature of offshore environment, the higher the water column the slower the overburden gradient increases as depth increases. This will directly affect the fracture gradient and make it increase slowly also. In case of having any pressure ramp at pore pressure the gap between PP and FG will decrease significantly which will cause more casing or liner strings to be run. In Black Sea region the most complicated casing design up to now had 8 (eight) casing strings starting with 36” conductor and ending with a 7” production liner. The number of strings to be run is affected by geological properties of lithologies, pressure profile of the well and technical limitations. These factors should be examined in detail to make the casing design effective and suitable for well conditions.