This paper describes the development and field application of an acid-based treatment to remove downhole scale in several oil producing wells in a sandstone reservoir in Central Saudi Arabia. The sandstone reservoir is weakly consolidated and sanding problems were noted in several wells. To avoid sanding problems, these wells were gravel packed. In addition, the reservoir pressure and gas oil ratio are low and, as a result, electrical submersible pumps (ESP) are used to produce these wells. The sandstone reservoir contains 1-13.5 wt% clay minerals and the bottom hole temperature is 160°F. A thorough investigation was undertaken to identify the composition and location of the scale, to recommend a suitable acid formulae to remove the scale, and to assess the effectiveness of the treatment in the field.

Based on extensive lab studies, an acid treatment was designed and applied in more than 15 wells. These wells have a water cut that ranges from 3 to 80 vol%. Because of the low reservoir pressure, these treatments were conducted using coiled tubing and the spent acid was lifted using nitrogen gas. In addition, the formation is water-sensitive. Therefore, the pre and post flushes were designed to minimize formation damage due to fines migration and clay swelling. Chemical analysis of acid returns (spent acid) was used to determine the type and concentration of acid additives.

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