Genesis and Phase Characterization of Oil-field Calcium Sulfate Scales

Abstract

Abstract The scale sample used in this work was obtained from a test well (used for testing the compatibility of seawater and formation water). The sample was analyzed at 7 regions as well as at the pipe scale interface and the scale water interface to determine the genesis of the scale-formation. Chemical analysis of the bulk scale sample indicated that it was mainly made of Ca, S and Sr elements (based upon XRF and ICP-AES analysis). Small to trace amounts of Na (0.4%, Fe (0.1%) and Ba (0.03%) were also present. XRD analysis, performed with a Debye Scherer Camera, of a small portion from the top layer (pipe scale interface) of the scale sample showed the presence of strontium sulfate (35%) and calcium sulfate anhydrous (65%) phases. Perhaps, the concentration of iron sulfide and barium sulfate phase in the sample was too small to be detected by XRD. The XRD analysis of the scale/water layer, in addition to hemihydrate, also indicated the presence of gypsum at 20 percent. Powder X-ray diffraction analysis of the bulk sample determined only calcium sulfate hemihydrates (CaSO4.0.5H2O) phase. EDS analysis of the pipe/scale interface showed the presence of sulfur and iron with the traces of calcium, strontium, and barium elements. The presence of iron and sulfur elements in the pipe/scale interface suggested the formation of an iron sulfide phase such as pyrrhotite (Fe0.98 S) or pyrite (FeS) at the commencement of the scale sample. The formation of these iron sulfide phases at the well pipe can be explained on the basis of the reaction of aqueous H2S (normally persist in an oil pipe) with the pipe surface at elevated temperatures (80 to 100 °C).