A rapid method for determining CO2/oil MMP and visual observations of CO2/oil interactions at reservoir conditions

摘要

A simple and rapid method to determine the minimum miscibility pressure (MMP) of crude oils with injected fluids such as CO2, natural gas hydrocarbons, and mixtures of different fluids was developed based on determining the pressure at which the interfacial tension between the injected fluid and crude oil phases decreases to zero. The height of a column of oil in capillary tubes suspended in a pool of crude oil is measured at different pressures of the injected fluid, and linear pressure versus height plots are extrapolated to zero height to determine the MMP value. Replicate MMP determinations show good reproducibility, with relative standard deviation (RSD) values less than 5%. The method has been used to investigate the effect of reservoir conditions on MMP, such as reservoir temperature (increasing the temperature greatly increases the MMP), the presence of natural gas components on CO2 MMP (e.g., MMP is raised with increasing amounts of methane), to comparing MMP values with different injected fluids such as pure CO2, and methane. Video recordings of the MMP determinations showed that significant oil mobilization into the injected fluid occurs even below MMP, and additional oil is mobilized as the pressure is increased above MMP. Similarly, significant oil is precipitated as the pressure is decreased from above MMP to the MMP and pressures below MMP. To confirm these visual observations, the oil suspended in the upper injected fluid (e.g., CO2) was collected at pressure, and analyzed to determine the mass of mobilized oil as well as the molecular weight distribution of the hydrocarbons in the injected fluid (upper) phase. These analyses demonstrated that increasing the pressure above the MMP (e.g., to 2300 psi for an oil with an MMP of 1450 psi) can double the amount of hydrocarbons mobilized into the injected phase. Similarly, when the pressure is dropped, significant precipitation of mobilized hydrocarbons occurs even though the pressure is still above MMP. Lower molecular weight hydrocarbons are preferentially mobilized at all pressures, but especially at lower pressures, indicating that higher molecular weight hydrocarbons could largely remain in the reservoir during EOR floods.