Page 175 - Air and gas Drilling Field Guide 3rd Edition

166 CHAPTER 7 Reverse Circulation Models

P e
P in (All)

P
P bca

d P
H
P bdpa P bdpi

P bdci
P ai
P bdca =P bh

FIGURE 7-1. Schematic of reverse circulation. P in is the injection pressure into the top of
the annulus, P bca is the pressure at the bottom of the casing in the annulus, P bdpa is the
pressure at the bottom of drill pipe in the annulus, P bdca is the pressure at the bottom of drill
collars in the annulus, P bh is the bottom hole pressure in the annulus, P ai is the pressure
above the drill bit inside the drill string, P bdci is the pressure at the bottom of drill collars inside
the drill string, P bdpi is the pressure at the bottom of the drill pipe inside the drill string, and P e
is the pressure at the exit at the top of the inside of the drill string.

It is assumed that compressible gases can be approximated by the perfect gas
law. Further, it is assumed that the mixture of compressed gas and incompressible
fluid will be uniform and homogeneous. When solid rock cuttings are added to
the mixture of compressible gas and incompressible fluid, the solid rock particles
are assumed to be uniform in size and density and will be distributed uniformly in
the mixture of gas and fluid [1].
The assumption of uniformity of the two or three phases in the mixtures is an
important issue in light of the technology developed for gas lift-assisted oil pro-
duction [2, 3]. The aeration (or gasification) of oil (or other formation produced
fluids) from the bottom of a well with the flow of gas from the surface (down
the annulus between the casing and the production tubing) is basically the same
as the aeration of drilling fluid and rock cuttings from the bottom of a well to the
surface through the inside of the drill string using a flow of gas and fluid from
the surface (down the annulus). However, in most oil production situations the
two-phase flow takes place inside of the tubing [4].