The Nature of CO2-Induced Organic Deposition.

Abstract

ABSTRACT This paper is an experimental investigation of reservoir parameters which influence CO2-induced organic deposition. A prototype high pressure/temperature apparatus is described which quantifies the onset and duration of heavy hydrocarbon flocculation, the extent of organic precipitation, and the alteration of wettability in porous media traps. CO2 is contrasted with pentane as a precipitating agent. Stainless steel filters were used to ascertain the role of miscibility, and to examine oil compositional effects. Native and treated Berea cores were utilized to probe the effects of initial rock wettability, brine, and clay activity. The results show that CO2-induced organic deposition resembles asphaltene precipitation by n-paraffins. Significant distinctions include that deposition using CO2 can be more extensive, is less abrupt, and is associated with liquid-liquid phase equilibria rather than the bubblepoint. The extent of CO2-induced organic deposition correlates with pressure and temperature as they relate to the development of miscibility. Deposition caused by CO2 was somewhat inhibited by rich gas. Sufficient levels of CO2-induced organic deposition typically made Berea cores less water wet and more oil wet regardless of the initial core wettability. Native cores could be altered to a mixed-wettability condition, and artificially neutral cores became strongly oil wet. The presence of brine reduced but did not eliminate CO2-induced organic deposition in strongly water-wet cores. Clays appear to be important deposition sites.