Summary
Calcium carbonate (CaCO3) scale is a common problem in the oil industry,
leading to reduced well performance and obstruction of the safety valves.
Recent field experiences indicate that scaling may be a problem at lower
saturation ratio (SR) than earlier anticipated. In a high-pressure,
high-temperature scale cell, crystal growth in synthetic formation water has
been studied. Induction times and growth rates have been determined by use of a
microscope cell. The results have been interpreted by use of nucleation theory.
The findings indicate that wells may be safely operated at SR below 2.5 at
85°C. At higher temperature, crystal growth is less dependent on SR and the
temperature dominates the system. Care should therefore be taken when operating
wells at temperatures greater than 100°C.
Introduction
Precipitation and deposition of CaCO3 is a common problem in the oil
industry. The scaling problems occur during pressure decrease of the produced
formation water. The consequence may be reduced performance of the well and in
some cases influence on the operation of downhole safety valves
(DHSVs). The carbonates precipitate from the produced water as release of
CO2 gas gives increased pH and supersaturation of CaCO3 . The kinetics of the
reaction is a function of temperature (i.e., slow kinetics at low
temperature).
The rule of thumb for prediction of CaCO3 scaling problems has until lately
been based upon the work by Oddo and Tomson. Their conclusion from field
experiences in the Hitchcock field in Texas, and Farida offshore Indonesia, was
that produced water from vertical wells may be supersaturated up to saturation
ratio SR=2.51 before any scaling problems occur. The bottomhole temperature was
moderate (88 to 104 °C). In some relatively recent field developments, the
design of the completion and the need for chemical placement in new wells
(e.g., scale inhibitor injection lines) have been partly based on these
findings.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
30 June 2004
- Revised manuscript received:
20 July 2005
- Manuscript approved:
20 July 2005
- Version of record:
15 December 2005
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