Coupled adsorption/precipitation experiments: 2. Non-equilibrium sand pack treatments

Scale inhibitors (SI) are retained in the reservoir formation by both adsorption (G) and precipitation (?) mechanisms, and these are responsible for the duration of the squeeze lifetime. However, processes such as the adsorption of SI and dissolution of precipitated SI complexes are not instantaneous. During a scale inhibitor treatment in a rock core, the fluid/mineral contact time may be quite short compared to the equilibrium time for the SI/mineral system for many practical experimental flow rates. This is also the case in the near-wellbore region where flow rates are very high and hence dynamic (non-equilibrium) effects may be significant. The SI retention process occurring in the sand packs, and indeed within the reservoir formation, are therefore likely to be non-equilibrium processes for at least some period of the squeeze process. If this hypothesis is correct, non-equilibrium behavior can be observed from the change in observed SI effluent concentrations as the flow rate changes in the sand pack flood. Therefore, non-equilibrium effects must be taken into account in the derivation of the parameters governing the squeeze such as the adsorption isotherms and/or the precipitation/dissolution model.

In this paper, we present a range of novel experimental results from a series of variable rate adsorption and precipitation sand pack floods for OMTHP ( hexaphosphonate) scale inhibitor. The sand pack experiments were conducted using silica sand as the adsorbing mineral and all the floods were conducted using identical procedures. The unique feature of this series of floods is that the bulk coupled adsorption/precipitation behaviour of this system (OMTHP/sand) has been fully characterized in previous work (Paper 1, Ibrahim et al., 2012). Therefore, we know precisely when the system is in the “adsorption only (G)” or in the “coupled adsorption/precipitation (G/?) regime. The sand pack effluent results show non-equilibrium behavior as the flow rate changes. These results indicate the importance of flow rate on the derivation of dynamic isotherms (G) and precipitation models (?) prior to field application modeling. We believe that the results presented in this work probably yield the most complete dataset of well characterised adsorption/precipitation SI pack floods assembled to date.