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.
Thermal simulators (most commonly used for heavy crude oil applications) add conservation of energy to this list, allowing temperatures to change within the reservoir.
As a result of declining pressure as in a reservoir depletion study, gas will be liberated from the oil.
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.
The analytical methods are often developed and promoted in the academia or in-house, however commercial packages also exist.