Asphaltene Deposition in Production Facilities

Abstract

Summary Asphaltene deposition in the tubing and production facilities caused operational problems in the Ula reservoir. Laboratory and field studies were carried out to determine the point of asphaltene precipitation. Laboratory observations were made on the regimes of precipitation. Laboratory observations were made on the regimes of deposition as a function of decreasing pressure down to and below the bubblepoint of "live" crude oil samples. The tubing areas affected by deposition in the field corresponded to those identified in the laboratory and by subsequent pressure and temperature modeling of the wells. Large asphaltene deposits were also found in the production separators. Introduction Ula Reservoir. The Ula field, operated by BP Petroleum Development (Norway), is situated in the Norwegian sector of the North Sea, 280 km southwest of Stavanger. A dip-closed salt-dome strucmm in the Upper Jurassic sandstone, the Ula reservoir is located 3345 m below the seabed. Oil produced has a gravity of 0.83 g/cm 3 with a GOR of 97.263 std m/m. The asphaltene content of the reservoir old is 0.57 wt%. Reservoir temperature is 143 deg. C and initial pressure was 49 054 kPa. History of Operational Problems on Ula. Downhole Safety Valves. In Nov. 1986, the downhole safety valves (DHSV's) in the two producing wells, Wells A and B, became increasingly difficult to open. The valves were removed and examined (Fig. 1). A black material later identified as asphaltenes had deposited in the valves and interfered with the opening mechanism. The deposited asphaltenes increased the friction between the flow tube and the valve housing and prevented the valves from opening with normal surface hydraulic pressures. The valves were modified to increase the opening and closing forces to overcome the extra friction. Wireline Operations. The asphaltene deposition in the well tubing caused a number of problems during wireline runs. Access to wireline plugs was hampered by asphaltene settling on top of the plugs. Wireline tools were damaged by asphaltenes, interfering with plugs. Wireline tools were damaged by asphaltenes, interfering with their operations. These problems led to extensive delays in wireline operations, and changes to running procedures were required. Equipment modifications were required to overcome these problems. problems. Production Facilities. In Aug. 1987, the two main separators on Ula were opened for inspection and 25 Mg of asphaltenes was found inside. The asphaltenes had built up to the weir levels within the separator over a period of 9 months. As a result of asphaltene compaction, the asphaltenes could be removed only by being dug out manually (Fig. 2). Production Restrictions. Asphaltenes were found to deposit on the well tubing wall below (in terms of depth) the crude's bubblepoint. This has also been observed in the Hassi-Messahoud and Ventura fields. However, the downhole flowing pressures are large enough in Ula for the restriction caused by asphaltene deposition in the production tubing to have no significant effect on the oil pro-duction. Multiphase modeling indicated only a small increase i pro-duction. Multiphase modeling indicated only a small increase i frictional pressure drop along the tubing attributable to asphaltenes. if the downhole flowing pressures drop significantly, then the potential for future production restrictions still exists. potential for future production restrictions still exists. Background of Asphaltenes Asphaltenes are a constitutent of the heavier and polar fraction of crude oils. The remaining polar species are resins and aromatics. Structural studies indicate that the asphaltene and resin molecules have a similar molecular weight range of 500 to 1,000. Asphaltenes, however, can form aggregates with molecular-weight distribution of 103 to 105. These aggregates are stabilized in solution by the resins and aromatics, which act as peptizing agents. When this protective shield is removed by preferential dissolution of the resins into the fluid phase (e.g., on addition of an alkaline), the asphaltene molecules start flocculating and aggregating into particles large enough to result in deposition. particles large enough to result in deposition. Asphaltenes are broadly classified as n-heptane insolubles or n-pentane insolubles. Resins can be defined as the fraction of oil not soluble in ethylacetate but soluble in n-heptane, toluene, and benzene at room temperature.