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Abstract Flow assurance is a vital challenge that affects the viability of an asset in all oil producing environments. A proper understanding of asphaltene precipitation leading to deposition lends itself to reliable completions planning and timely remediation efforts. This ultimately dictates the production life of the reservoir. The Wireline Formation Tester (WFT) has traditionally aided the understanding of asphaltene composition in reservoir fluids through the collection of pressurized fluid samples. Moreover, the use of Downhole Fluid Analysis (DFA) during a fluid pumpout has augmented the understanding of soluble asphaltenes under in-situ flowing conditions. However, an accurate and representative measurement of Asphaltene Onset Pressure (AOP) has eluded the industry. Traditionally, this measurement has been determined post-acquisition through different laboratory techniques performed on a restored fluid sample. Although sound, there are inherent challenges that affect the quality of the results. These challenges primarily include the need to restore samples to reservoir conditions, maintaining samples at equilibrium composition, and the destruction of fluid samples through inadvertent asphaltene precipitation during transporting and handling. Hence, there is a need for WFT operations to deliver a source of reliable analysis, particularly in high-pressure/high-temperature (HP/HT) reservoirs, to avoid costly miscalculations. A premiere industry method to determine AOP under in-situ producible conditions is presented. Demonstrated in a Gulf of Mexico (GOM) reservoir, this novel technique mimics the gravimetric and light scattering methods, where a fluid sample is isothermally depressurized from initial reservoir pressure; simultaneously, DFA monitors asphaltene precipitation from solution and a high-precision pressure gauge records the onset of asphaltene precipitation. This measurement is provided continuously and in real time. An added advantage is that experiments are performed individually after obtaining a pressurized sample in distinct oil zones. Therefore, the execution of this downhole AOP experiment is independent of an already captured fluid sample and does not impact the quality of any later laboratory-based analysis. Once the measurements are obtained, these can be utilized in flow assurance modeling methods to describe asphaltene precipitation kinetics, and continuity of complex reservoirs. For the first time in literature, this study applies these modeling methods in combination with the AOP data acquired from a downhole WFT This approach has the potential to create a step change in reservoir analysis by providing AOP at the sand-face, along with insight that describe performance from asphaltene precipitation. The results of which have tremendous economic implications on production planning.
Dumont, Hadrien (Schlumberger) | Zuo, Julian Y. (Schlumberger) | Mullins, Oliver C. (Schlumberger) | Garcia, German (Schlumberger) | Mishra, Vinay K. (Schlumberger) | Harrison, Christopher (Schlumberger) | Fukagawa, Shunsuke (Schlumberger) | Sullivan, Matthew (Schlumberger) | Chen, Li (Schlumberger) | Montesinos, Jordi (Schlumberger) | Robert, Red (Schlumberger)
Deepwater oil reservoirs contain variable amounts of asphaltenes, defined as insoluble in n-C7 and soluble in toluene. These asphaltenes are in nanocolloidal suspension in the oil at reservoir conditions but as pressure and/or temperature change, part of the asphaltenes can precipitate out of solution. The characterization of this phase change (Liquid-Solid) in a pressure-temperature space is challenging due to the following factors: sample availability, sample integrity, laboratory methods, high pressure measurement apparatus, and oil near or at asphaltene onset at reservoir conditions. For example, many reservoirs are at pressures which exceed laboratory capabilities.
This paper describes an innovative method to measure Asphaltene Onset Pressure (AOP) using Downhole Fluid Analysis (DFA): the oil from the formation after clean-up is kept in the wireline tool and exposed to a pressure and temperature changes while the tool is pulled out of hole. During these changes in pressure and temperature, the measurements optical spectroscopy, density and viscosity are performed continuously on the oil. Spectroscopy analysis provide the following:
The advantage of this AOP method over laboratory AOP include measurement up to reservoir pressures, minimum sample handling, AOP measurement prior to sample cooling and real time ability to repeat measurements.