Re-equilibrium of Asphaltenes by Repressurizing after Precipitation in Natural Depletion and CO2Enhanced Oil Recovery Schemes

In a series of isothermal depressurizing tests to measure asphaltene onset pressure (AOP), several depressurizing operations are usually conducted to multiple AOPs using the same lot of fluid subsamples. After one depressurizing operation is finished, the pressure/temperature condition is reset to the initial condition. Then, the next depressurizing run is performed. Before the next run, the fluid sample must equilibrate again by dissolving precipitated asphaltenes from the previous step. According to our many experiences with isothermal depressurizing tests for measuring AOPs, we know that redissolving asphaltenes depends highly upon the characteristics of a fluid sample. Some fluids redissolve asphaltenes quickly, while other samples need a lot of time. This study focused on fluid-dependent asphaltene re-equilibrium during a series of isothermal depressurizing tests. To investigate asphaltene re-equilibrium, the authors re-examined past experimental data for two types of crude oils. In the experiment, continuous power of light transmittance (PLT) was monitored for all stages in the series of AOP measurements. Two series of isothermal depressurizing tests were separately performed to evaluate CO2-induced asphaltene risks for two fluid samples. Three or four total steps of depressurizing were conducted in order of CO2addition ratios: 0, 20, 40, and 60 mol %. After a depressurizing process finished, pressure was increased above AOP to dissolve precipitated asphaltenes. PLT profiles in equilibrium steps between each depressurizing step were compared from a viewpoint of equilibrium time and/or PLT profile fluctuation. One of the fluids showed quick re-equilibrium of asphaltenes and smooth PLT profiles, whereas the other took much longer to achieve equilibrium and had fluctuating PLT profiles. This information suggested that easy-equilibrium-achievable fluids would be a preferred target for CO2enhanced oil recovery because of its potential use of pressure control for mitigating asphaltenes. In the case of easy-equilibrium-achievable fluid, when a production problem is caused by asphaltene precipitation, repressurization by temporary shut-in and/or production rate control can be an effective mitigation.