Page 654 - Air and Gas Drilling Manual

12-18 Air and Gas Drilling Manual
The equation for the pressure at the bottom of the horizontal segment, P ,
inside the drill string is
en [
P = P ex 2 + 2 a b T av ] 05 . (12-7) en
L
i
i
2
where P is the pressure at the exit to the horizontal segment (lb/ft abs).
ex
Minimum Volumetric Flow Rate
In the illustrative example below, the minimum volumetric flow rate of inert
atmospheric air will be discussed for the two directional drilling operational
situations, rotating the drill string and sliding the drill string.
Illustrative Example 12.2 Figure 12-8 gives a well profile for a directionally
drilled borehole that is to be initiated at a kickoff point (KOP) of approximately
7,000 ft. This well profile is similar to that given in Illustrative Example series 8.1,
8.2, and 8.3. However, in this example the well will be drilled directionally in the
3,000 ft of openhole drilling below the casing shoe at 7,000 ft. It is planned that
the directional segments of the well below the casing shore are to be drilled with the
PDM used in Illustrative Example 11.2. The curved segment of the openhole will
be a 318 ft medium radius borehole. The curved segment will be drilled to an angle
of 90˚ to the vertical. This will give an arc length of approximately 500 ft. At the
end of the curved segment, a 2,500 ft horizontal is to be drilled. The drill string in
the vertical cased section of the well is to be 4 1/2 inch, heavy-weight drill pipe (see
Table 3-3). The drill string in the openhole section of the well (curved and
horizontal segments) is to be API 4 1/2 inch, 16.60 lb/ft nominal, EU-S135, NC50
(IF) drill pipe. The MWD, bent sub, and PDM with a 7 7/8 inch drill bit will make
up the BHA. The anticipated rate of penetration for the directional drilling operation
is 30 ft/hr. The drilling operation surface location is at 4,000 ft above mean sea
level. The openhole section of the well is to be drilled with inert atmospheric air
(i.e., S g = 0.97). Determine the approximate minimum volumetric flow rate required
to clean the well when drilling at the end of the 2,500 ft horizontal (or 3,000 ft of
openhole drilling).
In Illustrative Example 12.3 below, the major and minor losses solution is
presented using the lumped geometry approximations introduced in the Illustrative
Example series 8.3a and 8.3b, and also used in Illustrative Example 11.2. This
solution can be used to determine the minimum volumetric flow rate of inert
atmospheric air while drilling at the end of the openhole section. This is
accomplished via a trial and error process that determines the volumetric flow rate
that will give an annulus minimum kinetic energy per unit volume of approximately
3
3.0 ft-lb/ft .
Rotating the Drill String
It is assumed that when the drill string is rotated in a directional well the flow
geometry in the annulus space can be adequately approximated as an annulus flow.
Therefore, assuming an annulus geometry of flow, it was found that the approximate
volumetric flow rate of inert atmospheric air that will give an annulus kinetic energy
3
per unit volume of 3.0 ft-lb/ft is 1,630 acfm. This critical annulus cross-section
where the minimum kinetic energy per unit volume occurs is just up-hole from the