A two-layer model for improved calculation of pressure drop during flow of a paraffin-laden crude oil in pipes and wells
P. Toma(*), J. Ibrahim(**), V. Rajan(***)
and G. Korpany(***)
(*) P R Toma Consulting Ltd, Canada
7328 Rowland Road, Edmonton, Alberta,
CANADA T6A 3W1
(**) PETRONAS, Malaysia
(***) Alberta Research Council, Edmonton, Canada
A two-layer model reported during the CJ OGNL conference in Banff, Canada, 2004 was first used to quantitatively describe the modifications (ageing - hardening) that occur during wax deposition in turbulent flow transport of a paraffin-laden crude oil in pipes/wells. The new model is based on observations made during the operation of a novel paraffin deposition apparatus which allows for visualization of the wax deposit within a very short time after the flow is interrupted and prior to the occurrence of congealing modifications of the deposit. The two-layer model further allows for a better description of frictional pressure drop during the transport of paraffin-laden oil.
Laboratory and field observed pressure drop values are generally considerably greater than those determined by conventional methods used for the calculation of pressure drop with stepwise modification of properties along the axial flow direction.
The paper uses the two-layer description of the flow for improving the conventional calculation of pressure drop. In this model, the “flow zone” is divided into two separate layers consisting of a non-Newtonian sub-layer of waxy gel and a Newtonian core of oil. This description leads to more realistic pressure drop results and avoids arbitrary manipulations of the roughness or/and the thickness of a stationary (composite) wax deposit used so far for matching measured and calculated pressure drop values. A radial distribution of temperature and a simplified wax appearance temperature (WAT)-Composition model as well as a two-coefficient description of paraffin oil viscosities versus temperature and shear rates are used to determine the thickness of each of the two layers, and the slip velocity.
The two-layer flow description and a deposit (solid wax) subjected to aging (and hardening) are considered in this improved calculation model. Observations and measurements performed with the aid of the novel, paraffin wax deposition loop are extensively used for model validation. Experimental procedures leading to visual observations and pressure drop data as well as the potential application of the model to field are described and discussed in this paper.