One of the most intractable concerns when engineers try to reuse the produced water as frac fluid in the Bakken and some other shale plays is the scale formation caused by the incompatibility of produced water with additives in the frac fluids and with the formation. In order to obtain a more efficient scale treatment for a successful hydraulic fracing that handles the extraordinary amount of water with high supersaturation level, the better understanding of inhibitor retention and release in the production system is urgent.

To explore the mechanism of attachment/release of phosphonate to/from a mineral surface, calcite supersaturated feed solutions with different diethylenetriamine penta (DTPMP) concentrations were introduced into the steel tubing that was internally pre-coated with a thin layer of CaCO3. It is unveiled that DTPMP attachment was dominated by the precipitation of calcium phosphonate solid once the solution is supersaturated with Ca3H4DTPMP (pKsp=53.5), and the total amount of DTPMP attached on the calcite surface added up with the increasing supersaturation of Ca3H4DTPMP. The co-precipitation of CaCO3 and Ca3H4DTPMP has facilitated the attachment of the inhibitor with the increase of supersaturation of CaCO3. The retained phosphonate was released from the surface with a steady and low level inhibitor concentration over extended period of time. Combining with the kinetics of calcium carbonate precipitation in the presence of inhibitor, a 1500 gram of calcium phosphonate precipitation can protect the scaling for about 100 days (100 bbl/day) when the saturation index of calcium carbonate (SIcalcite) is as high as 1.3.

The results provide a better understanding of calcium-phosphonate-carbonate interaction, and show the phosphonate inhibitor can continuously accumulate on the carbonate and slowly dissolve. We anticipate this study can shed a light on how much inhibitor can be delivered to the unconventional reservoir as well as the theoretical limitation of inhibitor return in the flowback water.

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