Page 75 - Well Control for Completions and Interventions

66 Well Control for Completions and Interventions

Drilling the well normally starts with the placement of the surface
conductor. For land wells, a cellar is usually constructed. The cellar
accommodates the wellhead, and allows the tree to be positioned with
the base at ground level. The first casing string, conductor, is piled in to
the depth of refusal. Alternatively, a post hole may be drilled and the cas-
ing run and cemented in place. On offshore platforms, the surface casing
is normally run through guide slots that are part of the jacket structure
and piled in to the seabed. In subsea wells, the casing is normally drilled
and cemented into place. If the seabed is soft, it may be jetted into place.
With the conductor in place, the well is drilled to a depth where the
surface casing can be run. Whilst the surface casing hole is being drilled,
a diverter is in place on the surface to deal with shallow gas blowout. The
surface casing is normally the first pipe that will support the drilling BOP,
and the hole should be deep enough to ensure that formation fracture
pressure is high enough to withstand kick pressure as the next hole size is
being drilled. In most cases, the surface casing is cemented to the surface
(or seabed). Once the surface casing has been run, successfully cemented,
and tested, the drilling BOP will be installed.
Intermediate casing is used to deepen the wells where kick tolerance or
unstable formation mean that it is not possible, or desirable, to drill from
the surface casing to the reservoir in one continuous hole section. In very
deep or difficult-to-drill wells, more than one intermediate casing string
may be required. Cementing of intermediate strings will depend on a num-
ber of factors. For example, government regulation may require all or some
formations to be isolated, particularly before abandonment. This may mean
having to put the top of the cement above the shoe of the previous (surface)
casing string. In other cases, a deliberate decision might be made not to
cement above the shoe. Placing the top of the cement below a shoe might
allow the casing to be cut and pulled, should a side track be needed. It may
also allow annulus pressure to leak off into the formation at the exposed
shoe. This is deliberate, and can help reduce annulus pressure build-up
because of rising temperature as a well begins to produce (Fig. 2.1).
Some wells are drilled from the surface (or intermediate) casing shoe
all the way through the reservoir to total depth (TD). Production casing
is run across the producing formation and cemented in place. The casing
is then perforated to allow hydrocarbons to enter the wellbore, where
they will be produced through the completion.
In many wells, the production casing is set (and cemented) above the
producing interval. Once the casing has been tested, the shoe of the