Abstract

Over a period of 4 to 6 months one of the Kristin wells had experienced a rapid decline in production. The drop was unexpected and at the time there was no detectable formation water breakthrough in the well. The decline in productivity was interrupted by a number of erratic, but temporary, surges in production that fell off again soon after. By comparing the well performance with other wells having the same downhole completion type, attention was soon focused on the likelihood of in-situ CaCO3 scale deposition. The subsequent discovery of CaCO3 deposition in the near wellbore area was partly anticipated. However, the discovery of halide scale (by precipitation) and salt (by evaporation) deposition came as a major surprise given that the Kristin formation water contains less than 10% of Total Dissolved Solids (TDS). This paper will describe the early diagnostic approach to screen and finally identify the most likely cause of the productivity loss. It will present the multiple well treatments that followed, the logics, the results and their interpretations. In addition to the results from these multiple treatments the paper will provide a mechanistic explanation of such deposition processes. Most interestingly, a novel observation will be put forward in this paper to underline the erratic and temporary surges of production during the decline period and that can be attributed to a ‘self-healing’ mechanism.

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