Detection of Scale Deposition Using Distributed Temperature Sensing

Abstract

Abstract The rapid deployment of distributed temperature sensor (DTS) systems in the oil and gas E&P industry provided the engineers with large amount of real-time, downhole data. Although the basic principles for DTS operations are simple, the interpretation of the downhole data presents a challenge for the production engineer. Inflow profiling has been promoted as the prime reason for the installation of DTS systems, though DTS data are currently used in all aspects of production engineering. The differences between the thermal properties of oil, gas and water allow the detection of unwanted fluids using DTS. Monitoring of the produced fluid temperature allow the engineer to prevent the formation of wax and hydrates, ensuring effective flow assurance. This paper examines a novel DTS application by analyzing the effect of scale deposition on the temperature profile of a conventional producing well. The low thermal conductivity of scale deposits increases the temperature of the producing fluid in the scaled region. A sensitivity study has examined the expected range of temperature increase caused by scale deposition to determine the conditions under which the flowing fluid temperature increase is a maximum. Low to moderate production rate environments with low water production yield the greatest increase in flowing fluid temperature when scale is present. The thermal insulation provided by the scale causes a unique temperature profile. Quantitative analysis allows the scale thickness to be determined. Introduction Solid deposition in the production tubing presents a big challenge for the production engineer; it reduces the well production, leads to equipment failure (e.g. overheating of ESPs, blocking of downhole valves), increases project operating costs (OPEX) due to the need for scale dissolver treatments and, in certain situations, creates health and safety issues. Examples of production solid deposition problems are:Organic DepositsWax DepositionHydrate FormationInorganic Scale DepositsCarbonate Scales (e.g. Calcium Carbonate CaCO3): precipitated by decreasing pressure (e.g. Venturi flow meters) and increasing temperature (ESPs)Sulfate Scales (e.g. Barium Sulfate BaSO4) forms when two different brine streams mix. Downhole monitoring for scale deposition could provide a valuable tool for the engineers overseeing wells in environments where logging is not practical, such as subsea developments. In North Sea Visund field [1], which is a subsea development, the impracticality of caliper logging did not allow the correct determination of the depth where CaCO3 scaling deposition started. Scale deposition can extend a significant distances along the tubing e.g. in the Brage field in the North Sea, CaCO3 scaling extended for 400m (˜1300 ft) [1]. One well in the Miller field in the North Sea represents an extreme scaling problem since scale treatments have to be carried out on weekly basis to avoid complete loss of the well [2]. The objective of this paper is to investigate the temperature behavior in wells where scale deposition is occurring and to quantify the temperature changes that occur within the scaled region. Understanding this behavior can aid in determining the possibility of detecting scale deposits in the production tubing using Distributed Temperature Sensor (DTS) downhole temperature data. Although, scale detection is never, and never will be, a major application of DTS; such a secondary application can form an extra incentive to justify installation of a DTS system.