Gas injection is a common method of enhancing oil recovery from a reservoir, but how does it affect the fluid properties and phase behavior of the oil and gas mixture? In this article, you will learn what is the miscibility of gas and oil in a reservoir, and why it is important for reservoir engineering and management.
Miscibility is the ability of two or more fluids to mix completely without forming separate phases. In a reservoir, the miscibility of gas and oil depends on the pressure, temperature, and composition of both fluids. When gas and oil are miscible, they form a single-phase fluid that has a lower viscosity and higher mobility than the original oil, making it easier to produce.
miscibility is the condition of pressure and temperature at which two fluids, when mixed in any proportion, form a single phase. The term miscibility refers to the ability of a liquid solute to dissolve in another liquid as a solvent. We can define miscible liquids as liquids that can mix to form a homogeneous solution.
There are two main types of miscibility that can occur between gas and oil in a reservoir: first-contact miscibility and multiple-contact miscibility. First-contact miscibility means that gas and oil are miscible at any ratio as soon as they come into contact, without any mass transfer or mixing. This is usually achieved by injecting a gas that has a similar or higher molecular weight than the oil, such as natural gas liquids (NGLs) or enriched gas. Multiple-contact miscibility means that gas and oil become miscible after several contacts and exchanges of components, resulting in a gradual change in the composition and density of both fluids. This is usually achieved by injecting a gas that has a lower molecular weight than the oil, such as lean gas or carbon dioxide (CO2).
first-contact miscibility (FCM) and multi-contact miscibility (MCM). At a given pressure and temperature, first-contact (FC) miscibility refers to the process in which two fluids mix in all proportions – at any concentration of either fluid – such that the resulting mixture remains a single phase. miscibility between a reservoir fluid and an injection gas develops through a dynamic process of mixing, with complex component exchange controlled by phase equilibria and local compositional variations along the path of displacement. In such a process, the injection gas and reservoir fluid are said to be multi-contact miscible (MCM).
The miscibility of gas and oil in a reservoir can be determined by performing laboratory experiments, such as slim-tube tests or rising-bubble tests, or by using numerical simulations, such as equation-of-state (EOS) models or compositional models. These methods can estimate the minimum pressure required to achieve miscibility, also known as the minimum miscibility pressure (MMP), and the effect of miscibility on the phase behavior and recovery factor of the oil and gas mixture.
The main benefit of miscibility is that it increases the oil recovery from a reservoir by reducing the residual oil saturation and improving the sweep efficiency of the gas injection. Miscible gas injection can also reduce the formation damage caused by water or gas coning, and prevent the precipitation of asphaltene or wax from the oil.
The main challenge of miscibility is that it requires high pressure and specific gas composition to achieve, which may not be feasible or economical in some reservoirs. Miscible gas injection can also pose some technical and operational difficulties, such as monitoring the gas-oil ratio (GOR), controlling the gas breakthrough, handling the produced gas, and mitigating the environmental impact of CO2 emissions.
Optimizing the performance and profitability of miscible gas injection requires following some best practices. Accurately and regularly characterizing the reservoir fluid properties and phase behavior is essential. Additionally, selecting the appropriate gas type and quality based on the reservoir conditions and objectives is important. Designing the injection scheme and pattern to maximize the contact area and the displacement efficiency of the gas and oil is also key. Monitoring the reservoir pressure, temperature, and composition, as well as adjusting the injection rate and pressure accordingly is necessary. Lastly, evaluating the technical and economic feasibility and risk of the miscible gas injection project should be done to ensure success.