Abstract: Numerical Basin Modelling for Fluid Flow, Phase and Pressure Prediction

Vear, Alwyn; Stephan Düppenbecker and Mark Osborne - BP

When robust 2- and 3-D basin simulation programs first began to displace the established technology of 1-D modeling tools in the early 1990s, they were regarded by many as specialist, or geochemical, tools whose results were to be treated with caution. Early 2-D models, for example, simply reproduced known distributions of oil and gas (Fig. 1), and were only tentatively used to modify exploration risks. As modellers gained experience in using the tools, more time was spent in sensitivity analysis, and more confidence was then attached to model- influenced risk assessments. In some companies, these simple basin models often became focal points for the subsurface teams. By providing spectacular images and allowing easy visualisation of the basin?s geological history, they facilitated discussion between different technical specialists, and allowed easy communication of often complex ideas to senior management. In this paper, several examples of 2-D basin models from around the world will be shown, and we will highlight the impact they had on exploration decisions within BP. From the UK?s Atlantic Frontiers, models will be shown which helped predict the presence of oil in stratigraphic Tertiary plays. In Indonesia, 2-D modeling re-focused the exploration effort to concentrate on a previously under-stated risk element of the traps.

Simple 2-D models can still be valuable for visualisation alone. However, we would like to concentrate on some of the more challenging, business-focused applications of basin modeling - for studying petroleum phase behaviour during migration, and for predicting subsurface conditions (pressure, temperature, etc.) ahead of the drill bit (Fig. 2). Examples will be shown from a variety of basins, comparing pre-drill predictions with post well reality. Those from the UK and Norwegian sectors of the North Sea appear to be particularly robust. Petroleum phase predictions from the Gulf of Mexico will also be discussed. The results show why numerical models are gaining the respect of wider user communities - not only basin modellers, but also exploration geologists, geophysicists, and drilling engineers - and why usage is passing from the hands of specialists in corporate research centres to generalists in integrated, multidisciplinary teams.

The main challenge for software developers now appears to be making the tools faster so that multiple simulations, capturing the true range of uncertainties, can be performed in sensible time frames. Speed becomes even more critical as we move towards 3-D basin models. Here, the tools must accurately represent geological reality, while quantitatively predicting volumes of different petroleum compounds reaching a trap, so that the economic value (phase, quality) of an accumulation can be estimated. Complex 3-D basin modeling software is now being developed both internally in the larger oil companies, and externally by research institutes. Some recent examples from the Gulf of Mexico will be shown using one of these prototype tools, more to highlight the problems which still need to be resolved rather than to discuss details of a case study. Developers should continue to work these issues. Because once the industry has modeling tools which can accurately predict the volumes and value of yet-to-be-discovered petroleum accumulations, whilst keeping exploration teams ?honest? about the risks involved, then the science of Basin Modeling will truly have arrived.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil