A problem in mature oil fields is the large amount of water produced alongside the oil. At the moment that both phases are leaving the wellbore of an oil well, tight water-in-oil emulsions can be formed due to the turbulence in the choke-valve at the wellhead. This flow process was simulated in the laboratory, by pumping crude oil-brine mixtures through a model choke-valve represented by a calibrated orifice. The parameters varied were the water-cut, the production flow rate and the orifice dimensions. The water-in-oil emulsion droplets were encapsulated immediately after formation and the emulsion characteristics, i.e. droplet-size distributions and water fraction were determined by optical microscopy and DSC.

The results of the experiments revealed that water-in-oil emulsions could be formed in the choke-valve if a certain critical value of the average energy dissipation rate was exceeded. This critical value depends on the type of crude oil and on the oil-water ratio. The diameter of the emulsion droplets found in the experiments could be explained in the framework of inertial and viscous break-up theories.

In experiments with a crude oil with little asphaltenes the droplet diameters were in the order of the Kolmogorov length scale and the average diameter Dav of the emulsion droplets decreased with increasing energy dissipation rate ε following a power-law relation: Dav∝ ε n. The value of the exponent n was found to be −0.3 and −0.1 at a watercut of 30 vol.% and 50 vol.% respectively. In experiments with an asphaltenic crude oil, the final droplet diameter was determined by viscous forces, probably due to relaminarising flow conditions.

This paper gives new information with respect to the characteristics of brine-in-crude-oil emulsions being formed in a model choke-valve and can be used to obtain insight in the emulsion formation problematic at the wellhead of an oil well. Additionally, the paper provides data on the size of droplets formed in turbulent and laminar flow at high dispersed-phase fraction.

Keywords:
orifice, emulsion formation, orifice diameter, fluid compatibility, droplet diameter, production monitoring, chemical treatment, emulsion, breakup, brine
Subjects:
Drilling Fluids and Materials, Well & Reservoir Surveillance and Monitoring, Production Chemistry, Metallurgy and Biology, Fluid Characterization, Production logging, Downhole chemical treatments and fluid compatibility, Phase behavior and PVT measurements, Fluid modeling, equations of state, Completion Selection and Design, Completion equipment
Copyright 2001, Society of Petroleum Engineers
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