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Acid stimulation of sandstone reservoirs can lead to substantial production improvements of hydrocarbons and can considerably enhance the economic value of a well. However, this technique is still considered risky because of secondary and tertiary reactions between the spent acid and the rock. Precipitates resulting from these reactions can deposit in the pores of the rock and eventually negate the positive impact of the primary reaction. Being able to assess the extent of the secondary and tertiary reactions under reservoir conditions is therefore critical for the acid treatment success. Traditional methods to evaluate overall treatment impact consist of injecting acid through a short core and measuring the change in permeability. However, these methods only capture accurately the impact of the primary dissolution reaction. The much slower secondary and tertiary reactions occur over time scales greater than the acid residence time in the core, and therefore cannot be fully taken into account by these experiments. This paper presents a new and economical technique to assess the extent of those reactions under reservoir conditions. It relies on one or two experiments performed on short cores of 2.5 to 15 cm to determine surface areas of reservoir minerals. This information is then incorporated in a geochemical simulator. The simulator includes an extensive database of reaction kinetics and thermodynamics properties of various acids and by-products with the minerals commonly encountered in sandstone reservoirs. Radial flow, reservoir heterogeneities, and damage mineralogy are rigorously accounted for and allow one to scale up to reservoir conditions. The type, amount, and location of precipitates can then be determined. This information is used to optimize the stimulation treatment. This methodology was successfully applied to three treatments. - The first case study considers a well in the Gulf of Thailand. The high temperature of the formation, which favors precipitation of silica, and the high carbonate content were challenges for the design of the treatment. - The second case study considers a well in the Norwegian Sea. A high degree of reservoir heterogeneity had to be accounted for in order to scale up the treatment to reservoir conditions. Without prior acidizing experience in this formation, a reliable means of designing the treatment was required. - The third case study considers a well in the North Sea. The new technique was applied to determine the damage due to secondary and tertiary reactions from an acidizing treatment on a well in the Galley field, after operational issues caused the acid to stay in the formation over a long period of time. For the first two cases, various fluid formulations were tested and sensitivity analyses were performed to improve damage removal and minimize the formation of precipitates. The results provided accurate guidelines for the design of the treatments.

A Method for Assessing the Impact of Secondary and Tertiary Reactions on Sandstone Acidizing Treatments