A mineralogical study of the wellbore scale from Santos-operated Cooper Basin Wells

Abstract

Scale precipitation is a major issue in the petroleum industry limiting production and increasing maintenance costs. Scale composition and structure can vary considerably between different fields and over the life of a well. It is vitally important to understand the mechanisms of scale formation in order to prevent or treat them. In this study, three scale samples were selected for investigation from 2 wells, designated B2, DN12 and DN12B. The mineralogy of the samples was characterised using optical microscopy and scanning electron microscopy methods. Sample DN12 was dominantly barite and showed evidence of incompatible fluid mixing, which caused redox changes in the components. Possible dissolution structures also indicated the movement of saline fluids through parts of the sample. DN12B, a latter sample from the same well, was composed mainly of calcite and silica with isolated barite crystals. Multiple phases were also exhibited representing decreasing pH conditions resulting in reduced calcite and increased silica precipitation. The change in the dominant mineral indicated significant changes in production fluid composition which is normally considered stable. The washed sample B2 was predominantly silica and iron oxide with siderite, barite and pyrite and abundant halite before washing. The multiple observed phases indicated multiple redox changes caused by periodic fluid mixing. Stages of iron and silica precipitation may have also been caused by pH, Eh and compositional variations. The presence of pyrite, globular silica and iron oxide structures indicated the influence of bacteria in the mineral precipitation as a live bacterial slime accompanied the sample when collected.