This paper describes an investigation into a heat-exchanger fouling incident in an offshore installation that followed the flowback from a scale squeeze treatment. The phosphonate inhibitor used is well known to be incompatible with high concentrations of calcium at typical oilfield pH values, but the formation water in the field contained a very low calcium concentration and had been shown to be completely compatible even with high concentrations of inhibitor. Nevertheless, the solids recovered from the heat exchanger were found largely to consist of a calcium-phosphorus species, implying that incompatibility between dissolved calcium and the inhibitor was the cause; only traces were found of calcium carbonate, the scale mineral against which the squeeze treatment had been performed.
The paper presents a review of the timeline of events that led up to the incident and describes key observations that point to its cause, including the treatment design, well-performance, flowback practices, etc. Finding the most likely cause was primarily about determining how a stream had been created in the facilities containing elevated concentrations of calcium and the phosphonate inhibitor that would then be preferentially deposited in the heat exchanger due to its high skin temperature (the Ca·P complex is less soluble at high temperature). The plausibility of these mechanisms was then examined by a laboratory programme that attempted to reproduce conditions that could have occurred in the facilities.
The most likely origin of the high concentration of scale inhibitor was the squeeze flowback; that of the calcium-rich stream could not be conclusively assigned to a single cause, but was either (i) delayed flowback of spent acid-stimulation fluid from the intervention of a well 2 months before the fouling incident, or (ii) dissolution of previously deposited calcium carbonate scale (and/or conceivably, traces of this mineral in the formation) by the acid phosphonate inhibitor (most likely the well squeezed just before the incident), followed by re-precipitation of the Ca-P complex when the resulting solution was subject to the high skin temperatures of the heat exchanger.
Although it was not possible to distinguish conclusively between these possibilities, the understanding gained enabled preventative actions to be identified for future operations in this field.