Detection and Identification of Scales Using Dual Energy / Venturi Subsea or Topside Multiphase Flow meters
Authors: Bertrand Bertrand , Ségéral Gerard , Moksnes Paul Ove 
Published: October 2008

Scale deposition poses serious challenges to maintaining production and minimizing operating costs and it ranks high amongst the concerns of operators presenting a major threat to flow assurance, not least for remote subsea and satellite unmanned developments. A new methodology enabling the early detection, type identification, estimation of the thickness of the scale deposit and its rate of accumulation over time is presented using dual-energy spectral gamma ray / venturi multiphase flow meters.

Several field examples of scale built-up are presented with detailed analysis showing the sometimes-surprising speed of deposition even at very low water cuts. The data from dualenergy spectral gamma ray / venturi multiphase flow meters was used to define the type of acid treatment required and to evaluate its efficiency. The impact of scale deposition on the metrological performance of the meter is illustrated and the procedures for quantification and compensation for maintaining the measurement quality over time is presented.

Introduction

Deposition of scale represents a major threat to flow assurance, with particularly serious consequences on subsea developments, and it poses serious challenges to maintaining production and processes optimisation. It can represent a large loss of production and diminish a field's net present value. Tjomsland et al1 quantified the value of scale control in the Veslefrikk field in Norway at U.S. dollars 1.1 billion. Quantification of scale deposition is of primary importance for the determination of remedial / corrective action. In some areas such as Canada and the North Sea, where entire regions are prone to scale, scale is recognized as one of the top production challenges.

Many mechanisms can lead to the deposition of scale in the reservoir, in the completion, in surface lines and production process equipment. For example, scale can be generated by the incompatibility between injected seawater and reservoir water. Van Khoi Vu et al2 report that more than 320 g of barium sulphate could potentially be deposited per cubic meter of coproduced water in the Girassol field in Angola.

Scale can develop in the formation pores near the wellbore - reducing formation porosity and permeability. It can block flow by clogging perforations or forming a thick lining in production tubing. It can also coat and damage production completion equipment such as safety valves, gas lift mandrels and nipples. Scale also accumulates in surface production equipment, such as well heads, master and wing valves, manifolds, flow lines, water knockouts, and test and production separators. It may clog up control lines and wet legs of instruments, affect the proper operation of automatic control and emergency shut down valves all of which are detrimental to production and increase field maintenance costs.

Scale deposition may be somewhat controlled in a number of ways such as: injection of inhibitors, chemical or mechanical removal, by changing the operating conditions - pressure, temperature and the composition of injected water. Some mineral scales can be dissolved and removed by acids, whilst most others cannot. Mitigating scale therefore is a difficult task often involving a combination of the above techniques.