Page 164 - Handbook Of Multiphase Flow Assurance

160 5. Flow restrictions and blockages in operations

TABLE 5.5 Data for paraffin deposits collected from various depths of the same well
Composition (wt%)
Melt
Paraffin sample Resins, via Solid temperature Residue (oils)
depth (m) Asphaltenes silica gel hydrocarbons (°C) (wt%) Loss (wt%)
36 2.2 8.9 64.8 72 21.8 2.3
88.5 1.7 6.2 65.7 72 22.0 4.4
133.5 1.6 7.9 64.0 74 26.3 0.2
193.5 2.3 6.4 59.1 75 28.7 3.5
328.5 2.1 5.9 53.8 75 33.9 4.3
448.5 2.7 7.8 49.1 77 35.0 5.4

Structure

Wax crystal habit depends on the cooling rate. Fast cooling leads to many small crystals.
Slow cooling leads to fewer large crystals. Wax usually crystallizes as 2-dimensional platelets.

Wellbore and reservoir plugging

Deposit thickness was reported for wellbores and surface gathering lines for Devonian
paraffinic oil production in Tatarstan, Russia (Mazepa, 1965) as shown in Fig. 5.32. The author
also reported the microphotographic research on adhesion on a steel plate of precipitated
paraffin particles flowing in oil. The study indicated that particles sized between 0.002 and
0.2 mm adhered to the steel plate.
Production operating parameters for the above wells are summarized in Table 5.6 below.
The wells 4136 and 4138 were producing undersaturated oil, at pressure above the bubble
point. The author (Mazepa, 1965) produced the first proof that paraffin deposition may occur
from undersaturated flowing oil. Additional results for 40 wells were also presented in this
work.
The author indicates that the relatively low length of sections with wax deposition for
low-rate wells was caused by the stratified multiphase flow of oil and gas. This flow regime
ended at a relatively short distance from wellheads. The stratified flow in the gathering lines
was confirmed in the field using radioactive density meter, sample valves, and transparent
spools installed on the pipelines. The critical flow rate at which the stratified flow may still
occur was determined from the critical Froude parameter (which accounts for the effect of
gravity on flow).
)
2
2
5
Q = Fr ( 1− Gasfraction π 2 g D / 16
crit
ν
2
Fr crit = ω / g ( )
c
Q = oil flow rate, D = pipe diameter, ω c = mixture velocity, g = gravity acceleration, and
ν = hydraulic depth.