AE Pillay, B Ghosh, B Senthilmurugan, S Stephen and A Abd-Elhameed
Keywords: Strontium, barium, scale-formation, laser ablation, ICP-MS.
Abstract: The thrust of this work demonstrates the unique capability of laser depth-profiling to ‘peer’ within reservoir cores and reveal anomalies and irregularities associated with sulphate scale deposition. The intrinsic behavior of scale deposition within the pore space of near wellbore formation rock is relatively unexplored, and the essence of this paper concerns the application of an ablative laser technique to study the uniformity of strontium and barium distributions in suitable oil producing reservoir core sections. Information on the spatial and depth distributions of these metal components in sidewell core slivers could provide an insight into the location of relevant scale deposits, particularly barium and strontium sulphates. Nucleation and growth on the surface could indicate potentially favorable conditions for precipitation. On the other hand, sporadic ingrained deposits would signal abrupt changes in compositional or physical conditions within the specimen. The investigation was divided into two parts: (i) continuous ‘drilling’ by the laser at random points to uncover metal distribution at successive depths; and (ii) iterative surface scans at randomly selected areas to evaluate compositional consistency. The laser was attached to a high resolution ICP-MS instrument, and irradiations were conducted with a 213-nm beam of 30% total energy and 100 ?m diameter. Following iterative surface scanning, the laser ablated a total depth of 50?m at 10?m-intervals at each point. The study was largely semi-quantitative in the absence of standardization. Characteristic intensities originating from the metals of interest were measured. The experimental results showed distinct inhomogeneity in the distribution of Sr and Ba – declining sharply to negligible levels in some cases, and occurring sporadically at specific depths. Some data revealed that these deposits were markedly absent from some points in the core fragments, distinctly suggesting that specific conditions in the rock matrix are necessary to influence scale formation. The exact mechanism for this irregular behavior is not clear at this stage, and has considerable scope for extended research, including modeling studies. This work is of definite interest to geophysicists and petroleum scientists.
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