In recent years, a number of non-aqueous delivery systems for scale inhibitors (SI) have been developed which are designed to be applied as low damage, low watercut or pre-emptive squeeze treatments, e.g. in critical or expensive subsea wells. The mechanisms through which non-aqueous scale inhibitor systems operate is an important technical issue. Only when a good understanding of the transport and retention mechanisms is developed, can this be built into a model for designing such squeeze treatments (such as the SQUEEZE VI model). The experimental work in this paper focused on a specific "oil soluble" version of a standard penta phosphonate inhibitor (DETPMP), which has been described previously in the literature. Several floods have been carried out comparing corresponding non-aqueous and aqueous applications of DETPMP in order to better determine the main features of the transport and retention mechanism of the system. Novel core flood experiments with tracers in both the aqueous and oleic phases have been performed and are reported in this paper. Unique information is generated by designing very detailed flooding cycles and carrying out tracer floods at each stage. Results point to the formation of an immobile "emulsion like" third phase in this system. In order to confirm our proposed mechanism, non-aqueous inhibitor has also been applied at zero residual water saturation (100% oil) to investigate whether or not the tailing effect (or third layer deposition) could be generated without prior oil/water partitioning of the scale inhibitor. For this case, tailing of the brine tracer only was observed. Mass balance showed that there was still considerable retention of scale inhibitor that was ultimately produced during the aqueous post-flush.
The corresponding aqueous inhibitor coreflood showed similar returns to that of the previous non-aqueous experiment and no retardation of either the tracer species or the metal ions in the brine. No formation damage occurred for either phase after the inhibitor was injected. A further control flood (no SI treatment) proved that the brine tracer tailing arose as a direct result of the treatment.
The use of the tracer species in each (water and oil) phase is a genuine innovation, which provides a powerful additional technique for demonstrating the effect of chemical treatments on the flow and the retention of all fluids in the core.