Risk Assessment of Concurrent Hydrate and Wax Formation for Deepwater Production Restart Modeling and Experimental Results

Abstract

This paper presents the successful implementation of a flow assurance strategy for a new deepwater production system offshore Africa derived from results of experimental assessments of associated paraffinic and hydrate formation. The rheological behaviour of production fluids under field conditions at various pressures and temperatures are compared with results from conventional PVT and flow modelling including latest models specifically designed to review pipeline clearance times and pressure wave propagation during restart. These assessments differentiated between the individual contributions of hydrate formation and paraffin gelling and included analysis of the interaction between hydrate crystals and the wax-gel network. Additionally, the performances of Low Dosage Hydrate Inhibitor (LDHI) and Pour Point Depressant (PPD) were assessed for various production conditions in the presence of other production chemicals in the temperatures range of 65°C to 4.5°C. The results showed excellent performance of both LDHI and PPD and additionally an incremental performance from the synergy between the two applied chemistries. The predicted flow behaviour of the multiphase system based on the modelling was aligned significantly with the findings of the experimental assessment creating confidence for planned optimisation of the shutdown/restart procedure. Simultaneous formation of gas hydrates and paraffinic gels during multiphase transportation poses significant potential risks for interruption of production in deepwater operations. An accurate assessment of these risks is critical in implementing a cost-effective flow assurance strategy which will minimises downtime during shutdown and restart procedures. This investigation realises the potential development of a best practise solution to gelling problems associated with complex micro-crystalline paraffin wax. Crude oils which contain large fractions of branched and cyclic alkanes typically show poor performance with conventional PPD chemistries, but may show improved effective treatments with new chemistries specifically designed to lower the yield stress. Extension of this new treatment technology on complex waxy crude oils can encourage development of new reserves in regions such as Southeast Asia, west coast Africa, and South America where traditional chemistries are ineffective.

Viscosity of Water-in-Oil Emulsions from Different American Petroleum Institute Gravity Brazilian Crude Oils

“…In addition, the simultaneous formation of gas hydrates , and paraffinic gels during the multiphase flow can lead to important changes in the rheological behavior of the produced fluids. It will thus increase the potential risks of production interruption, especially in deep-water scenarios, where a cold water environment prevail …”

“…It will thus increase the potential risks of production interruption, especially in deep-water scenarios, where a cold water environment prevail. 9 Rheological studies show that the temperature, shear rate, and water volume fraction have important impacts on the viscosity of W/O emulsion. 10 Rheological analyses are an important tool to predict and curtail different flow-assurancerelated problems.…”