Risk Assessment of Petroleum Fields—Use of Numerical Simulation and Proxy Models

Abstract

Abstract The development of petroleum fields is a complex task due to the high influence of uncertainties on E&P projects. During the appraisal and development phases, uncertainties related to geologic and fluid models play an important role, especially in offshore heavy oil fields due to the low economic return, limited flexibility, and importance of reservoir modeling. The flexibility is limited because of the necessity to design the production facilities based on a low amount of information. The reservoir modeling process is important because risk of field development projects is normally caused by a high uncertainty on the recovery factor. Due to the necessity of a more robust evaluation of recovery factor, risk assessment methodologies normally are integrated with reservoir simulation, which is the best available tool to predict reservoir performance. However, higher precision on prediction of reservoir behavior is normally associated with fine simulation grid and high computation effort. In this article, some alternatives are presented to improve the efficiency of risk assessment, considering precision and computation effort. Among these alternatives are (1) use of coarse models, (2) use of coarse models modified to reproduce fine grid results, (3) simplifications on the risk assessment procedure, and (4) use of proxy models based on statistical (experimental) design and response surface methodology. A general discussion, including each alternative, use of upscaling techniques, reduction of grid size, number of attributes, use of parallel computing, and use of proxy models are made based on previous publications and results of a case study. The methodology applied to quantify risk involves a sensitivity analysis in order to reduce the number of critical attributes and simulation of reservoir models obtained through the combination of these attributes. Afterward, a statistic treatment is used to evaluate the risk involved in the process. Based on a case study, it is shown that the use of faster simulation models and proxies can speed up risk assessment, but a few steps must be performed to guarantee the quality of the results. Keywords: analytical modelsforecastingMonte Carloupscaling ACKNOWLEDGMENTS The authors wish to thank UNISIM, CNPq, FINEP (CTPETRO), PETROBRAS, CEPETRO, FAPESP, and the Department of Petroleum Engineering for the support to realize this work. Notes a The relative permeability model was simplified to perform this analysis; Corey model was used with exponent equal to 3 both for water and oil (kr0). For kr1, the exponent for water and oil were 2 and 3, respectively; for kr2 the exponent for water and oil were 3 and 2. The relative permeability at maximum saturation was 1.