Asphaltene Studies in On-Shore Abu Dhabi Fields, Part IV: Development of a Surface Sensor

Abstract

The selection of optimal chemical solutions to an asphaltene challenge has been an integral part of the flow assurance strategy for a large on-shore field in Abu Dhabi. Previous studies in the field have demonstrated good performance by mixing heavy aromatic naphtha with some dispersant chemicals and then bull-heading that mix to allow it to soak and then flow back. Laboratory studies using dispersant tests were performed to better understand the effectiveness of carrier solvent and dispersant mixtures; the economics of different fluid delivery methods (jet blasting, bull-heading, etc.) were analyzed for cost-effectiveness; and significant field-testing was performed to validate the integrated approach. But despite all of this activity, there was still no direct measurement. Inferences of asphaltene removal or redeposition needed to be made from indirect sources such as surface pressure gauges and flow meters or via intervention, such as running an accessibility check using gauge cutters. There was no hardware available in the industry for direct measurement of the asphaltene. This led the operating company to help sponsor development of a real-time sensor. A ruggedized version of that sensor has now completed its first field-test in Abu Dhabi. The physics behind the sensor relies on the use of a known quantum property of asphaltene, namely that asphaltene free-radicals can be resonated by an external magnetic field with a particular ratio of frequency to magnetic field strength, a phenomenon known as Electron Paramagnetic Resonance (EPR). Contributions from metal ions such as nickel, manganese, iron and vanadium can also be resonated. Spectrometers using the EPR effect have been used, for example, in the geochemical industry for concentration analysis of organic free matter, but only inside dedicated laboratories. To take the asphaltene study to the next level, real-time data would be needed directly from the wellhead. By focusing primarily on the asphaltene response, rather than a broad range of chemicals, it proved possible to miniature and ruggedize the device for oilfield application. Fluid can enter and leave the device via side-streams from the main flowline. The spectral output gives a direct measurement of spin concentration and hence the percentage of asphaltene flowing past. The goal of the first field test was to validate the device resolution in a field application. It is known from previous laboratory and field data that the total asphaltene ratio would be less than 1%, so the EPR signal might be anticipated to be small. Results exceeded expectations and repeatability was better than 0.1%. One initial surprise was that the asphaltene level in each well changed over time, even during steady production. Some wells showed significant variation from one day to the next with a standard deviation near 5%. Other wells showed barely 1% variation. The wells with the higher standard deviation seemed to correlate against those wells which had historically seen more problems. While the system was in country, the operator took the opportunity to investigate cleanup response. It was determined that after flow back was showing 100% crude then the surface asphaltene level was low and stayed low for about 24hrs before returning back to the baseline level. The data is indicative of deposition in the wellbore and the initial hypothesis is that the asphaltene was at least partially redepositing onto sections of tubular that had been cleaned by the solvent. This is the first time in industry that asphaltene data has been available at the wellhead on a continuous basis. The field test also demonstrated the utility of more frequent sampling, which led to the launch of a device upgrade that can be permanently plumbed into the flowline and which will give surface data every 5 minutes. It is anticipated that such an industrial Internet of Things (IoT) device will make possible the optimization of chemical program and asphaltene remediation by incorporating the surface data into an integrated flow assurance management system.