Abstract

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.

Introduction

Aqueous-based scale inhibitor (SI) treatments have traditionally been the most effective way to control mineral scale formation in oilfield applications. Water-soluble inhibitors may be deployed by continuous injection in topside applications and in downhole "squeeze" treatments. However, such aqueous-based treatments are not suitable for all SI applications. In water sensitive formations, for example, squeezing with aqueous products can cause localized increases in water saturation and formation damage due to wettability alteration, both of which can lead to production decline[1]. Also, as increasingly complicated subsea satellite fields are developed, where access to individual wells after production has started is both difficult and expensive, the need for pre-emptive squeezing at zero or very low watercuts becomes a priority.

The application of non-aqueous scale inhibitors has been proposed to address some of these concerns[2–5]. Not only can these chemistries alleviate the problems described above, but they can also help with lifting the well on start-up due to reduced hydrostatic head. Enhanced treatment lifetime has also been claimed in some cases[6–10].

Non-aqueous systems are generally prepared by incorporating conventional aqueous scale inhibitors, such as phosphonates, polyacrylates or sulphonated co-polymers, into a non-aqueous medium. Many types have been reported ranging from invert emulsion-type technologies[4,5,11], microemulsions[7], encapsulated products[12], amphiphilic solvent systems[13], oil solubles[2,14,15], and water-free materials. Although their delivery systems may differ, the basic principle of operation is the same. The non-aqueous package is injected into the formation and, following a suitable shut-in period, back-produced as in conventional aqueous treatments. The inhibitor species then partitions into the aqueous phase on contact with the reservoir or injected brines, thus protecting the well and its associated equipment from scale formation.

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