Page 69 - Handbook Of Multiphase Flow Assurance

64 3. PVT and rheology investigation

There are various correlations for the effect of slurry solids volume fraction on viscosity.
Nuland and Vilagines (2001, BHRG) proposed to use such correlation for hydrate slurries,
which is based on the correlation by Mills (1985) for apparent shear viscosity, which in turn
was based on the works of Einstein and Batchelor.

1 −Φ
µ =  
r
 1 − Φ 
 Φ max 
whereµ = µ slurry / µ fluid ,
r
Φ = solid volume fraction,
Φ max = maximumpaacking solidscolumefraction = 47 for spheres.
/

Other non-Newtonian behavior is also associated with wax crystals forming a gel. When slurry
becomes so concentrated that crystals overlap and form a network, a waxy “gel” forms. The term
gel is used in this context to signify that the whole fluid becomes nonflowing and non-Newtonian
in its rheology. A gel exhibits a yield stress, which means that some force needs to be applied to
disrupt the network before a gelled fluid starts to flow. In some cases waxy crudes with 3% or
greater wax content (measured by cold filtration) may exhibit gelling behavior.
Gel strength may vary depending on the cooling rate of the fluid. So, faster cooling (near a
pipe wall) results in smaller wax crystals and a weaker network. Conversely, core of the gell-
ing waxy oil cools at a relatively slower rate and grows larger crystals which form a stronger
network. This effect becomes most pronounced in larger pipelines. It is not uncommon to see
in a medium-diameter 2-in. pipe gel test that gel breaks near the pipe wall circumference, and
the gelled oil core is extruded from the test pipe. To overcome the discrepancy between gel
strength measured in a small laboratory tube and observed in a large diameter pipeline, gel
strength may also be measured in a temperature-controlled rheometer with a cone-and-plate
geometry to accurately reproduce the cooling rate history of a large diameter pipeline to ob-
tain a more accurate gel strength reading. Alternatively, larger size test tubes or field pilot test
sections may be used.

Emulsion characteristics

Emulsion stability is commonly measured and reported for new oil samples. The time it
takes to resolve an emulsion by gravity into oil and water layers is reported.
Emulsion stability is particularly important for offshore operations as residence time in a
separator is limited by size and weight of the separator to 5–10 min.
Water-in-oil emulsions are prepared by mixing for a prescribed time at several shear rates
and different water cuts and are allowed to resolve at different temperatures.
Foaming of the oil may also be reported if observed.
Formation of slop or solids-stabilized layer between oil and water layers may be reported
if observed.
Inversion point for an emulsion may be reported if observed at some water cut.