Petrological analysis of mineral scales formed during gas production from Permianreservoirs in the Cooper Basin
Cotton, Timothy B.
Honours Degree, 2003
University of Adelaide
The precipitation of mineral scale occurs during gas production from wells across the Cooper Basin. Accumulations of scale can result in obstructions that can lead to reduced gas production or disruption to down-hole operations. In order to decide on the most effective method for removal and prevention of scale, it is necessary to determine its composition. Petrological analysis can also provide information on its origin and formation.
X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), optical microscopy and cathodoluminescence techniques were used in determining the mineralogy, and physical characteristics of scale samples removed from wells throughout the Cooper Basin.
The bulk mineralogy of 41 samples from 35 wells was determined using XRD. Scale-forming minerals occur as barite, anhydrite, gypsum, calcite, aragonite, magnesian calcite, ankerite, siderite, opaline silica and halite, either in isolation or as mixtures. An empirical method to determine barite/calcite ratios within samples using XRD peak areas was devised.
Selected samples analysed using SEM, EDX and optical microscopy were classed as 4 distinct types: (1) scale consisting predominantly of barite with a variety of crystal morphologies; (2) layered calcite-rich scale that displays variable crystal morphology between layers; (3) inter-layered barite and magnesian calcite scale, and; (4) inter-layered opaline silica, crystalline calcite and co-precipitated opaline silica/calcite.
The mixing of incompatible brines together with a decrease in brine pressure were identified as the likely causes of thermodynamic and kinetic changes resulting in scale formation. Variations in crystal morphology of both barite and calcite scale samples have resulted from differing rates of precipitation. EDX analysis of layered carbonate scale also identified variations in calcite morphology induced by incorporation of Mg2+ into the calcite structure. The localised occurrence of opaline silica has possibly occurred as a result of high silica saturation within formation brines and or favourable pH conditions for its formation.
Co-precipitation and layering of relatively insoluble barite and opaline silica with more soluble carbonate minerals was identified within analysed scale samples. There is potential for these scale types to reduce the effectiveness of remedial chemical treatments such as acidization. The variation in scale morphology seen within all samples will impact on the ease in which problematic scales can be physically removed.