The narrow window between pore pressure and fracture gradient, especially prevalent in deepwater drilling has resulted in the need for more and more casing strings to be set in offshore wells. To fit this increased number of casing strings into the finite space available between the conductor pipe and a production string capable of flowing a well commercially, the available space between the strings has narrowed to the point where casing connections must be either flush or near-flush; there is just not the annular space available for casing collars. For equivalent circulating density (ECD) management and a good cement job the open hole beneath the previous casing shoe is usually under-reamed, meaning that to centralize the new string in open hole a magician has to pull something out of his hat. That special something has to be able to pass through an annular space only a few eighths of an inch wide yet open out to some inches greater than the casing outside diameter (OD) in the open hole. It must also still be robust enough to withstand the forces involved in the running process.
Bowspring centralizers can do all of the above and have been around for a hundred years but today's wells have some special demands which this paper will address and propose a number of different novel solutions depending on exactly how much annular space is available to pass through and how much stand-off is required in the open hole.
This paper will discuss the positive location on slick casing of various centralization devices using spray metal, a technology new to this particular aspect of the industry and a very robust process with many advantages over traditional methods. Centralizers can also be built entirely out of the spray metal itself.
Developments are ongoing and the unwelcome lull in deepwater GOM operations during 2009 ironically allowed some time for developing new products. Casing centralization is also commanding more attention in the post-Macondo world.
Typically oil and gas wells are drilled in sections, each having casing run into it to support that part of the well bore for the duration of its life. The casing also provides well control and, additionally, may be the conduit for well bore fluids to travel to the wellhead although this is normally through dedicated production tubing. To ensure formation isolation and prevent the possibility of well bore fluids traveling up the outside the casing it is invariably cemented in place, at least in part and especially across zones which require isolation. To achieve the best possible cement job the casing needs to be centralized, especially across potential producing zones, so that a uniform thickness cement seal is achieved in the annulus between the casing and the formation(s). An additional advantage of centralization is the avoidance of differential sticking, a hydrostatic pressure mechanism, which can sometimes irreversibly jam the casing when running into the hole.
Casing centralizers come in many forms to suit the needs of particular well characteristics. To be effective in all cases, they must be positively located in some way so that when the casing has been run to depth they are still where they were planned to be. Traditionally each joint of casing has a connecting collar forming the box end, the shoulders of which are sufficiently proud to act as very effective centralizer stops. As wells get more complex and the demands on the casing more stringent, the casing connections have become an integral part of the casing itself, initially with threads cut into a slight upset at either end, and now more and more often, particularly in deepwater drilling, perfectly flush pipe with the threads cut within the wall thickness of the casing itself.
