In scale inhibitor squeeze treatments, precipitation of the inhibitor within the formation can lead to extended squeeze lifetimes. However, such processes also have the potential to cause formation damage unless they are carefully designed and controlled. The formation of a partially soluble inhibitor/metal complex within a reservoir is the objective for almost all precipitation squeeze packages. However, historically there are numerous ways this is achieved almost all of which require a limited operational window to be deployed successfully. In this paper, we describe the development of a novel dual chelant system which provides a method for controlling both the "wanted" and "unwanted" precipitation of the scale inhibitor package within the formation. The highly tunable nature of the system allows for ease of pumping at more extreme conditions (higher and low temperatures, calcium levels etc.) than have previously been possible. By using the dual chelant mechanism described in this paper, a package can be tuned to precipitate within a certain time frame both at low and high temperatures in brines with varying degrees of salinity and hardness. The scale inhibitor (SI) itself is a chelant or ligand for divalent ions present (mainly Ca2+) and this is denoted L2 and the second chelant, L1, is added to the system at certain design concentrations, as explained in the paper.
In many situations, the high divalent metal ion content of a produced brine, or formation water can limit the successful pumping of a scale inhibitor due to high levels of calcium, for example. Under these conditions the dual chelant mechanism can also be deployed to prevent scale inhibitor phase separation.
This paper discloses the theory of how the dual chelant mechanism works using computer modeling and the subsequent confirmation of the simulations by laboratory testing. The importance of the pKa of the SI (L2) and the added chelant, L1, and the relative metal binding constant interactions between L1/ L2 and Ca2+ are explained and investigated. The comparison of the dual chelant mechanism versus conventional packages is demonstrated by core flood experiments. The dual chelant mechanism gives a clear improvement in squeeze lifetime and controllability and provides a platform for the development of many types of controlled solubility scale inhibitor treatment.