Cementing Subsea Wells
The cementing procedures for offshore wells drilled from jackup rigs or stationary platforms are similar to primary cementing operation on land rigs. Subsea wells drilled from a floating drilling vessel are cemented with a system somewhat similar to that used for cementing liners, except that because the larger wiper plugs cannot fit through the drill pipe, both the plugs and plug release system are integrated into an assembly within the casing below the casing hanger (Figure 1).
The cementing head on a floating drilling vessel is made up to the drill pipe which is used to land the casing string. The cementing head contains a launching ball and dart used to release the larger plugs in the casing. Offshore cementing heads are top-drive compatible, so the drill pipe and casing can be rotated during cementing.
After conditioning and circulating the mud and pumping washes or spacers – just before pumping the cement slurry – a bottom plug launching ball is released from the cementing head. It moves through the top plug and seats in the bottom plug held in the plug release assembly within the casing hanger. A 100 to 275 psi pressure increase behind the ball shears the connector pins and permits the bottom plug to travel down the casing until it bumps on the float collar at the bottom of the casing string, with the cement slurry behind it in the casing.
Extra pump and hydrostatic pressure extrudes the ball through its orifice seat and cement displacement continues around the casing shoe. Once the cement slurry has been pumped, the top plug launching dart is released at the surface. It seats in the top plug and the resulting increased pump pressure shears the holding pins, causing the second plug to move from the plug release assembly like the first one. When all of the cement slurry has been displaced the plug bumps on the float collar.
There are a number of challenges in cementing subsea wells, particularly those in deep water. These include:
- The added hydrostatic pressure from the additional distance above the subsea assembly and the smaller degree of separation between pore pressure and fracture pressure
- The low temperatures at the seafloor and within the shallower subsurface formations and the impact on cement slurry characteristics
- The presence of relatively shallow water-producing or gas-producing zones that must be cemented quickly and effectively
- The complications introduced by the extended reach or horizontal wellbores commonly employed to develop offshore reservoirs from single subsea well templates
A variety of specialized cementing tools and procedures have been developed to deal with these challenges.
