Superior Algorithms
EXBOS is not of the same lineage as most models for petroleum reservoir simulation. It uses several novel algorithms which correct persistent problems in the field of petroleum reservoir simulation thereby bringing the technology to a new level. The core of the model consists of a flashing algorithm which converts mass densities to saturations and pressure (and saturation pressure in the undersaturated case). Models without a flashing algorithm may conserve mass but compute incorrect pressures as is easily demonstrated in a single-block test case where the pressure can be computed explicitly. The pressure errors are even more pronounced when a reversible process is simulated by reversing the rates from production to injection. Quite simply, the flashing approach is the proper method although it is somewhat harder to develop.
The model is implicit [without the usual restrictions] including the implicitly coupled multiple-perforation wells. The blocks of a well may be perforated and shut at will in the course of a simulation. By default, a three-point temporal scheme is used for added stability.
The code is written in the C language and uses dynamic memory allocation throughout. The linear algebra for the solution of the sparse linear systems is implemented in terms of block three-vectors and matrices for superior efficiency. This makes the model available on a variety of platforms usually without requiring any porting changes.
Other features include a theoretically correct rock compressibility model and a novel three-phase relative permeability blending model. This replaces Stone's method II which is still supported for compatibility.

Features
The algorithms used in the EXBOS model correct several persistent problems in the field of petroleum reservoir simulation and bring the technology to a new level. The following features set the model apart from the competition:

• Implicit mass-conservative model using three-point temporal scheme for added stability.
• Implicit multiple perforation well model for stability.
• Novel flashing algorithm for correct results.
• Theoretical rock compressibility model based on porosity.
• Linear three-phase relative permeability model.
• 20 different well types (including LIT wells) all of which allow arbitrary well connectivity (e.g. for horizontal wells). Automatic and reliable switching between rate and pressure conditions.
• Pressure maintenance option with fresh water and disposal water injection.
• Restart file for use in forecasting situations.
• Radial grid option for reliable and stable simulation of well tests.
• Implicit van Everdingen-Hurst aquifer and other aquifer models.

User Interface
Exbos does not have a sophisticated graphical user interface for input data preparation. Data entry uses mnemonic keywords and is otherwise quite flexible. Included with the model is the Tracer interpolation and contouring program which is used for data preparation from original well data. A simple viewer is also included to examine areal data computed by the model and The model is well documented with an up-to-date 75-page user's manual and a library of annotated examples.

Compatibility
EXBOS has in use since 1993 and has been installed and tested on PC systems running DOS and Windows as well as Linux and also on UNIX systems including the IBM RS/6000, DEC ALPHA and Sun Sparc systems.

More Information
For more information, contact the author: Jentje Goslinga

Paper
This paper discusses a new blending method for three-phase relative permeability. This is the most obvious method for blending the permeability data for the water-oil system and the data of the oil-gas system into three-phase data.