Reservoir simulation
Reservoir simulation is an area of reservoir engineering in which computer models are used to predict the flow of fluids (typically, oil, water, and gas) through porous media.
The creation of models of oil fields and the implementation of calculations of field development on their basis is one of the main areas of activity of engineers and oil researchers. On the basis of geological and physical information about the properties of an oil, gas or gas condensate field, consideration of the capabilities of the systems and technologies for its development create quantitative ideas about the development of the field as a whole. A system of interrelated quantitative ideas about the development of a field is a model of its development, which consists of a reservoir model and a model of a field development process. Layer models and processes for extracting oil and gas from them are always clothed in a mathematical form, i.e. characterized by certain mathematical relationships. The main task of the engineer engaged in the calculation of the development of an oil field is to draw up a calculation model based on individual concepts derived from a geological-geophysical study of the field, as well as hydrodynamic studies of wells. Generally speaking, any combination of reservoir models and development process can be used in an oil field development model, as long as this combination most accurately reflects reservoir properties and processes. At the same time, the choice of a particular reservoir model may entail taking into account any additional features of the process model and vice versa.
The reservoir model should be distinguished from its design scheme, which takes into account only the geometric shape of the reservoir. For example, a reservoir model may be a stratified heterogeneous reservoir. In the design scheme, the reservoir with the same model of it can be represented as a reservoir of a circular shape, a rectilinear reservoir, etc.
Traditional finite difference simulators dominate both theoretical and practical work in reservoir simulation. Conventional FD simulation is underpinned by three physical concepts: conservation of mass, isothermal fluid phase behavior, and the Darcy approximation of fluid flow through porous media. Thermal simulators (most commonly used for heavy crude oil applications) add conservation of energy to this list, allowing temperatures to change within the reservoir.
Numerical techniques and approaches that are common in modern simulators:
The simulation model computes the saturation change of three phases (oil, water and gas) and the pressure of each phase in each cell at each time step. As a result of declining pressure as in a reservoir depletion study, gas will be liberated from the oil. If pressures increase as a result of water or gas injection, the gas is re-dissolved into the oil phase.
A simulation project of a developed field usually requires "history matching" where historical field production and pressures are compared to calculated values.
It was realised at an early stage that this was essentially an optimisation process, corresponding to Maximum Likelihood. As such, it can be automated, and there are multiple commercial and software packages designed to accomplish just that. The model's parameters are adjusted until a reasonable match is achieved on a field basis and usually for all wells. Commonly, producing water cuts or water-oil ratios and gas-oil ratios are matched.
Other engineering approaches[edit]
Without FD models, recovery estimates and oil rates can also be calculated using numerous analytical techniques which include material balance equations (including Havlena–Odeh and Tarner method), fractional flow curve methods (such as the Buckley–Leverett one-dimensional displacement method, the Deitz method for inclined structures, or coning models), and sweep efficiency estimation techniques for water floods and decline curve analysis. These methods were developed and used prior to traditional or "conventional" simulations tools as computationally inexpensive models based on simple homogeneous reservoir description. Analytical methods generally cannot capture all the details of the given reservoir or process, but are typically numerically fast and at times, sufficiently reliable. In modern reservoir engineering, they are generally used as screening or preliminary evaluation tools. Analytical methods are especially suitable for potential assets evaluation when the data are limited and the time is critical, or for broad studies as a pre-screening tool if a large number of processes and / or technologies are to be evaluated. The analytical methods are often developed and promoted in the academia or in-house, however commercial packages also exist.
Many programs are available for reservoir simulation. The most well known (in alphabetical order) are:
Open source:
Commercial:
Application[edit]
Reservoir simulation is ultimately used for forecasting future oil production, decision making, and reservoir management. The state of the art framework for reservoir management is closed-loop field development (CLFD) optimization which utilizes reservoir simulation (together with geostatistics, data assimilation, and selection of representative models) for optimal reservoir operations.
Other references