Wax flow assurance, an extensive experimental study on risk evaluation and deposition monitoring

Abstract

Flow assurance involves ensuring fluid flow in well and pipelines. In a deep cold environment wax deposition can form and easily reduce the flow. When the temperature drops below wax appearance temperature, wax particles precipitate out of solution, crystallize and form a gel. Deposition of these gels makes flow through pipelines difficult and challenging which leads to increases in operational and remedial costs. Pipeline failures in this condition is a potential threat in some cases such as restart of the production. Reliable experimental approaches mimicking pipeline conditions for wax studies are critical to reduce the cost of production and transportation of crude oil. This thesis presents a rigorous investigation using a high-pressure rheometer aimed at identifying the optimum starting temperature for wax studies. The technique can also be used for measuring the proper temperature in terms of wax inhibitor injection. The next step in laboratory wax studies is cooling down the sample from initial temperature to a test temperature. Since pipeline passes through different environments with different temperatures from well head to the production unit which fluid poses to different cooling/heating rate when crosses these areas. The impact of cooling rate on the various wax parameters including wax appearance temperature (WAT), wax disappearance temperature (WDT) and viscosity was investigated using a rheometer. A coaxial shearing cold finger was used to study the effect of test time and shear rate on deposition with a blank oil sample. In addition, the impact of subcooling on the performance of a number of wax inhibitors was studied by a coaxial apparatus. The main work of this thesis was aimed at investigating the reason for discrepancies between results from different conventional experimental techniques in terms of wax inhibitor screening. The result in some devices contradict with other devices which make it difficult to decide which one is more realistic and applicable in the field. The equipment included Quartz Crystal Microbalance (QCM) technique, rheometer, coaxial shearing cold finger, Near InfraRed (NIR) spectroscopy and an in-house built flowloop. A wide variety of different approaches were used to obtain reliable data with different apparatus such as employing different ageing times, flow/shear rates, subcooling, presence of impurities, test geometries, water cuts, the impact of circulation in the loop, the influence of thermal cycle, conditioning time, etc. In addition, the dependency of wax deposition on subcooling in the presence of a watercut, various thermodynamic hydrate inhibitors and two different commercial low dosage anti-agglomeration (AA), were investigated using a flowloop. The adhesion tendency of wax particles and the rheology of fluids were also studied in the presence of AA’s using both QCM and rheometer. All the tested parameters will be experienced during the well/field life cycle. Downhole samples which are the more likely source of crude for testing pre-development will in many instances have some degree of mud contamination. The possibility of using several wax related parameters including WAT, WDT, adhesion tendency and viscosity obtained by rheometer and QCM, to determine levels of oil based mud contamination in downhole samples was investigated.