The reactions between scale inhibitors and formation minerals determine the inhibitor retention and release after inhibitor squeeze treatment and hydraulic fracturing. Scale inhibitor is an important ingredient in fracturing fluids to prevent mineral scale depositions during fracturing, shut-in and flowback stages. The interaction of scale inhibitors between carbonate formation is well understood but the interaction of scale inhibitors between shale and sandstone formation has not been investigated thoroughly. The primary objective of this study is to develop mechanistic understanding of interactions between scale inhibitors and shale and sandstone minerals.

In this study, adsorption and precipitation of a phosphonate scale inhibitor – DTPMP on shale minerals were investigated at 70°C. Equilibrium adsorption isotherms were determined for various inhibitor concentrations at different pH values. At low phosphonate concentration, the interaction between inhibitors and shale can be characterized as surface adsorption; at high phosphonate concentrations, inhibitors precipitate with cations released from shale minerals. The interaction of scale inhibitor and sandstone formation was evaluated in coreflooding experiments by repeated squeeze and acid treatment. The inhibitor return of an identical second DTPMP squeeze showed almost the same return curve as the first squeeze. An acidizing pretreatment demonstrated improvements in squeeze performance. It is proposed that acid treatment dissolves soluble minerals such as calcite, and DTPMP inhibitors forms less soluble precipitate such as iron phosphonate in the formation, which results in the enhancement of squeeze. The dissolution rate constant of DTPMP precipitate was determined at three different temperatures and the established relationship can be used to predict rate constant under other temperature conditions.

The insights presented in this work will help to understand the fate of phosphonate scale inhibitor in shale and sandstone reservoirs, manage the use of scale inhibitor in fracturing fluids, and design optimum scale squeeze packages for scale control in oil field.

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