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Asphaltene deposit accumulations, either in surface facilities or within the producing interval of the formation, can seriously decrease productivity up to the point where remedial work is required to remove the deposit and restore production. Focusing on the problem of asphaltene accumulation within the formation, the most common treatment is based on washing the reservoir in the near well-bore region with aromatic solvents either pure or doped with solvent-enhancing additives. These remedial treatments are often unsuccessful owing to incomplete removal of the deposit. In our opinion, underestimation of the specific asphaltene/formation rock interactions within the problem formation may be an important factor in determining the failure rate. This paper describes a new experimental approach for selecting or defining the best suited solvent or solvent/additive system for a given asphaltene/formation rock combination, so that more effective removal of the asphaltene deposits can be achieved.
Asphaltene deposits within the producing formation are constituted by two different types: the asphaltenes adsorbed onto the formation rock and bulk asphaltenes. Bulk asphaltenes are defined as asphaltenes aggregated from the oil medium and deposited onto the adsorbed asphaltenes. Because these two types of asphaltenes are characterized by interaction forces that are substantially different both in quality and intensity, they might be expected to present quite a different degree of removability with respect to a chosen solvent or chemical. In fact, considering those asphaltenes obtained by n-heptane precipitation from stock tank oil (i.e., the "softest" form of such material), toluene, as a typical solvent, shows a very high up-take (several tens of a %, w/w) when the asphaltenes are in the bulk state; on the contrary, the asphaltene up-take by toluene is very low (10–20 %, w/w) when the same material is adsorbed on a rock surface (clays, dolomia, quartz, etc.). In spite of this evidence, the most common remedial approach, based on solvent washing, refers only to bulk asphaltenes. Moreover, the activity of solvent enhancing additives, which range from aliphatic amines to alkyl benzene sulphonic acid (1,2), are still defined with respect to bulk asphaltenes. In our opinion, underestimation of the specific asphaltene/formation rock interactions within the problem formation may be an important factor in determining the failure rate. In fact, reports indicate only limited success with such solvent treatments, owing to incomplete removal of the deposits (3).