Larue, David K.¹ 
(1) ChevronTexaco Exploration and Production Technology Company, Bakersfield, CA

ABSTRACT: On Geological Uncertainty in Reservoir Modeling and Prediction

Geological uncertainty in reservoir modeling and prediction is a multi-disciplinary, multi-component, reservoir- and time-dependent problem with significant implications for capital expenditures in exploration, development and mature field management. Geologic uncertainty is multi-disciplinary because it involves all categories of earth scientists whose studies must all be integrated for geologic uncertainty to be appropriately defined. Geologic uncertainty is multi-component because it involves calculations of static reservoir volumes and reservoir characterization for dynamic behavior studies. Static reservoir volumes may represent the largest uncertainty throughout the history of evaluation and depletion of the reservoir. Derivative properties (those not directly modeled) such as connectivity, continuity and permeability heterogeneity mostly govern flow behavior. Geologic uncertainty is reservoir dependent, because each reservoir is characterized by its own bin of important uncertainties. Geologic uncertainty is time dependent, because as the field is developed, information becomes available that can reduce certain types of uncertainty, and increase others. 
There is a general perception that as a field is developed and more information about rock properties, fluid saturations and distributions are better understood, volumetric and production uncertainty is greatly reduced to values as low as 10%. Although aspects of this perception are correct, this is probably overly optimistic. Examples are shown in which volumes of a mature oil field are uncertain at >50%. There is a widespread belief that depositional environment strongly influences recovery efficiency. Based on database and reservoir modeling studies, this observation does not seem to be easily supportable, or is lost among other uncertainties.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.