(49c) Investigating Asphaltene Deposition in Pipe Flow at Elevated Temperatures

Asphaltene deposition is a longstanding flow assurance issue. It has been studied extensively at near ambient temperatures where asphaltenes are commonly considered to precipitate as glassy particles [1-7]. Asphaltene deposition at these conditions involves the adhesion and accumulation of asphaltene molecules, particles, and aggregates at the surface of a pipe or vessel. Several models have been developed to capture asphaltene deposition mechanisms accounting for asphaltene aggregate size and concentration, shear conditions, fluid properties, and surface properties [8-9]. However, asphaltene deposition at higher temperatures encountered in deep, offshore production has not been rigorously investigated. Precipitated asphaltenes at higher temperatures are liquids rather than glassy particles and may deposit differently. The goals of this project are to: 1) design and commission a deposition test apparatus for temperatures up to 130°C; 2) measure the effects of asphaltene solubility, asphaltene-rich phase properties, time, and adhesion on deposition rates; develop a model or correlation relating deposition rates to changes in temperature, pressure, flow rates, and solvent composition. In this contribution, the following are presented: design of the proposed deposition apparatus, validation with deposition measurements at 50°C, and preliminary results at higher temperatures. To date, deposition experiments have been performed in a horizontal flow configuration with a Western Canadian bitumen mixed with n-heptane.

The new apparatus was designed to measure asphaltene deposition from once-through flow of mixtures of oil and solvent at temperatures from 50 to 130°C and pressures up to 10 MPa. Solvent induced precipitation was selected as a design basis over pressure drop induced precipitation to obtain measurable and easily controlled quantities of precipitate. While the apparatus does not directly match offshore wellbore conditions, it permits an examination of deposition mechanisms at elevated temperatures. Deposition is detected via a change in the pressure drop through a 30 cm long, 1.4 mm I.D. tube (the test section). The mass and location of the deposit can be determined by removing the test section. The diameter of the tube was small enough to observe a measurable incremental pressure drop with deposition and large enough to obtain a quantifiable mass measurement.

The apparatus consists of two transfer vessels, a custom built co-axial nozzle-like static mixer, and the test section. A Quizix SP-52000 pump system and a Teledyne ISCO Demi 2510S pump are used to displace pre-diluted bitumen and n-heptane, respectively, from the transfer vessels to the test section. The differential pressure between the inlet and outlet of the capillary tube is measured with a Rosemount 1151 pressure transducer. The apparatus is housed inside an air bath.

In each deposition experiment, the bitumen was pre-diluted with n-heptane to a point below the onset of precipitation to reduce its viscosity and promote rapid mixing. Then, the pre-diluted bitumen and additional solvent was fed through the static mixer and displaced through the capillary tube. The pressure drop across the tube was monitored during the flow period. At the end of the flow period, the free liquid in the tube was displaced with cyclohexane, depressurized and disconnected, and the mass of deposit determined from the change in mass of the tube. In some experiments, the tube was cut open to examine the location of the deposit. The controlled variables are the type of solvent, solvent/oil ratio, temperature (operating range of 20 to 130°C), pressure (up to 10 MPa), and flow rate (2 to 5 cm³/min).

A critical factor in these experiments was the mixing of the bitumen and solvent. The effectiveness of the static mixer was assessed by comparing asphaltene yields (mass of precipitated divided by mass of bitumen in feed) at the exit of the mixer with asphaltene yields measured in test tubes all at ambient conditions. Poor mixing in the static mixer would give lower yields than observed in the well mixed test tube tests. The yields at the exit of the mixer matched the test tube yields confirming effective mixing above the onset of precipitation.

However, an issue with mixing at solvent/bitumen ratios below the onset of precipitation was identified. As expected, no precipitation occurred at these conditions in the test tube tests. Similarly, any pressure drop observed in the apparatus for a feed of premixed bitumen/solvent below the onset ratio was less than the error of the measurement. However, pressure drops indicative of deposition were observed in the deposition apparatus when bitumen and solvent were mixed in the static mixer even when the final composition was below the onset condition. A potential explanation is that there are local high solvent content zones within the mixer that cause local precipitation and subsequent deposition. Therefore, any data collected below the onset of precipitation are suspect.

The data collected above the onset of precipitation at 50°C were consistent with the literature. The pressure drop measurements indicated cycles of deposition and erosion during the flow period. Asphaltene deposits were found to be concentrated near the inlet of the capillary tube. Preliminary data collected at higher temperatures will be presented and the effect of higher temperatures on the deposition rates will be discussed.

References

1. Broseta, D., Robin, M., Savvidis, T., Féjean, C., Durandeau, M., Zhou, H. Detection of Asphaltene Deposition by Capillary Flow Measurements. SPE paper 59294, SPE/DOE Improved Oil Recovery Symposium, Tulsa, Oklahoma, 3–5 April, 2000.

2. Wang, J., Buckley, J.S., Creek, J.L. Asphaltene Deposition on Metallic Surfaces. Dispersion Sci. Techno., 25:3, 2004, 287-298.

3. Nabzar, L., Aguilera, M.E. The Colloidal Approach. A Promising Route for Asphaltene Deposition Modelling. Oil & Gas Science and Technology – Rev. IFP, 63, 2008, 21-35.

4. Seifried, C.M., Al Lawati, S., Crawshaw, J.P., Boek, E.S. Asphaltene Deposition in Capillary Flow. SPE paper 166289, SPE Annual Technical Conference and Exhibition, New Orleans, Louisiana, USA, 30 September – 2 October, 2013.

5. Hoepfner, M.P., Limsakoune, V., Chuenmeechao, V., Maqbool, T., Fogler, H.S. A Fundamental Study of Asphaltene Deposition. Energy Fuels, 27, 2013, 725-735.

6. Bôas Fávero, C.V., Hanpan, A. Phichphimok, P., Binabdullah, K., Fogler, H.S. Mechanistic Investigation of Asphaltene Deposition. Energy Fuels, 30, 2016, 8915-8921.

7. Ghahfarokhi, A.K., Kor, P., Kharrat, R., Soulgani, B.S. Characterization of asphaltene deposition process in flow loop apparatus; An experimental investigation and modeling approach. Petr. Sci. Eng. 151, 2017, 330-340.

8. Vargas, F.M., Creek, J.L., Chapman, W.G. On the Development of an Asphaltene Deposition Simulator. Energy Fuels, 24, 2010, 2294-2299.

9. Eskin, D., Ratulowski, J., Akbarzadeh, K., Pan, S. Modelling Asphaltene Deposition in Turbulent Pipeline Flows. J. Chem. Eng., 89, 2011, 421-441.