Iron Sulfide Scale Characterization: Insights from Spectral Induced Polarization Method

Abstract

This study aims to explore the use and effectiveness of the spectral induced polarization (SIP) method for identifying and characterizing iron sulfide scales in oil and gas wells. Laboratory experiments using scale-amended columns aimed to confirm the SIP method's capability to differentiate between iron sulfide scales and other naturally occurring minerals and better understand scale formation. The polarization of different iron sulfide scales was measured in response to an applied electrical field of 0.01-10,000 Hz frequency range, with polarization being dependent on the material's frequency-dependent response to the field. The scale types analyzed in this study mainly consist of iron sulfides such as pyrrhotite, troilite, and pyrite, as well as iron oxides such as goethite and lepidocrocite. To ensure consistency and eliminate any effects of grain heterogeneity, the samples were ground and sieved to a particle size between 425 and 600 μm. Each sample was tested in acrylic columns modified with Ag-AgCl current and potential electrodes. The findings of this study demonstrate that the SIP responses, such as phase shift, impedance, real and imaginary conductivity, and calculated chargeability, are highly sensitive in distinguishing various scale types, including those with minor composition differences. X-ray diffraction and X-ray fluorescence analysis validated the differentiation of the samples' mineral and elemental composition. Although phase and imaginary conductivity responses exhibit similar trends, phase clarified differentiation between the measured samples. The recorded signals present a clear trend dependent on the mineralogical composition and the iron content. Iron sulfide scales have a characteristic electrical response that differs from other natural iron-rich materials tested. This difference in electrical response is due to the unique electronic structure of iron sulfide minerals, which allows them to act as charge carriers. As a result, iron sulfide scales exhibit a high polarization response, which can be detected using the SIP method. SIP is a powerful geophysical technique that can provide valuable information about subsurface properties in micro-scale and fluid content. Unfortunately, due to poor exposure, SIP has not been widely adopted in the Oil & Gas industries. The SIP method offers several advantages over traditional methods of scale identification. With further research and development, the SIP method has the potential to become a standard tool for scale management in the oil and gas industry.