Page 156 - Handbook Of Multiphase Flow Assurance

152 5. Flow restrictions and blockages in operations

Liquid accumulation in horizontal and vertical wells
In vertical multiphase flow of gas and liquid the liquid phase may be transported as a film
on the tubing wall or as droplets carried with the gas. There are film models and droplet mod-
els which allow to predict the liquid accumulation in vertical flow. As soon as liquid starts
to accumulate in tubing, the flow from a well begins to have additional resistance to flow.
Eventually the well becomes filled with liquid or loaded up.
There are droplet models and film models for liquids transport with gas flow.
Droplet models include Turner et al. (1969) correlation and its modification by Coleman
et al. (1991). Film models include Barnea (1986) and Luo et al. (2014) models.
Both droplet and film models account for shear stress exerted by gas on the liquid surface.
Recent reviews of film models are in works (Shi et al., 2015; Chen et al., 2016).
Droplet models are among the most widely used in operations due to their simplicity.
Typical accuracy of empirical models fitted to data from wells producing some fluid in some
region may be as accurate as ±20%. Usually these correlations require operator adjustment
for a given field. Simpler models are more conducive to operational adjustment.
In addition to the empirical models there are also rigorous multiphase flow models which
are available commercially. The rigorous models are tuned to data sets for multiphase flow
of various mixtures of water, liquid hydrocarbons and gas. The empirical models may reach
accuracy of +/− 10% when all parameters are accurately determined.
Turner et al. (1969) correlation is the most widely known droplet correlation for liquid
loading onset. In its development both the film and droplets were considered, and droplet
mechanism fit the 106 field operating data better.
For vertical flow, the liquid accumulation starts when largest droplets are not carried up by
the gas flow. Terminal velocity of a particle falling in gas is determined by the balance of shear
force acting on the projection of the particle surface area, buoyancy of the particle in gas, and
the force making the particle fall.

F _drag = C _drag v densitygasArea particle 2
2
_
_
/
π
/
Forspheres ,mass m = πD 3 particle density_particle 6 andtheareaisprojectedarea = πD 2 p particle / 4
C drag))
v = 4 ( gD_ particle (densityparticledensity gas )/( 3density gas_ . 05
−
_
_
_
_
Turner assumed C_drag = 0.44. In field units, the Turner correlation for droplet unloading
minimum gas velocity is
. ( (
2
V gas _min [ ft s] = 192 ρ liquid − ρ gas)σ ρ gas) . 025
/
/
3
Surface tension σ [dyne/cm], density ρ [lb/ft ]. σ Water-Gas ~ 60 dyne/cm; σ Condensate-Gas ~ 20
dyne/cm.
Coleman et al. (1991) updated the Turner correlation to
. ×192 ρ
2
V gas _min [ ft s] = 08 . ( ( liquid − ρ gas)σ ρ gas) . 025
/
/
Belfroid et al. (2008) further updated the Turner correlation for use in horizontal wells
by using Fiedler shape function, based on laboratory tests with a 2 in. I.D. flow loop. The