International Journal of Engineering Applied Sciences and Technology, 2020 Vol. 4, Issue 11, ISSN No. 2455-2143, Pages 537-541 Published Online March 2020 in IJEAST (http://www.ijeast.com) 537 INVESTIGATING THE IMPACT OF DIFFERENT MODELING ALGORITHMS AND THEIR ASSOCIATED UNCERTAINTIES ON VOLUME ESTIMATION (GULLFAKS FIELD, NORTH SEA) Emmanuel Umeagu Smart Osadebamwen Osarenogowu Centre of Excellence in Geosciences Centre of Excellence in Geosciences and Petroleum Engineering and Petroleum Engineering University of Benin, Edo, Nigeria. University of Benin, Edo, Nigeria Peter Onyedikachi Ibemesi Osayamen Julius Imarhiagbe Centre of Excellence in Geosciences Energy and Mineral Resources and Petroleum Engineering Geosciences Institute University of Benin, Edo, Nigeria. RWTH Aachen University, Germany Abstract— Reporting reliable results for hydrocarbon volume estimation is important for both economic analyses and making key decisions in reservoir management and development. Adequate facies and petrophysical modeling of static reservoir properties are key inputs for the derivation of a robust static reservoir model from which static volume is computed and inherent uncertainties are quantified. However, the choice of geostatistical algorithm for building the model depend on development and production maturity, degree of reservoir heterogeneity and the type, quality and amount of data. This study therefore aims at investigating the impact of the combination of stochastic and deterministic methods of property modeling on volume estimation and also perform uncertainty and sensitivity analyses to quantify uncertainties so as to aid exploration and production decision making process. Facies model were simulated/generated using both stochastic and deterministic algorithms. The resultant facies model formed an input for the petrophysical modeling process also using both stochastic and deterministic algorithms. For each combination, hydrocarbon pore volume was computed. Monte Carlo Simulation method was used to perform the uncertainty analysis where the low case (P10), mid case (P50) and high case (P90) was outputted. The results show that a combination of Sequential Indicator Simulation (facies) with Sequential Gaussian Simulation (petrophysical) captured a large range of hydrocarbon pore volume for the twenty equiprobable realizations simulated while the combination of Truncated Gaussian Simulation with trend and Gaussian Random Function Simulation gave a limited range. A combination of the deterministic algorithm gave a single estimated and more pessimistic volume. Uncertainty analysis indicated that the facies modeling process and the combination of SIS_SGS algorithm have a higher impact on volumetrics. Keywords— Reservoir, volume estimation, uncertainties, modeling. I. INTRODUCTION Reserve Estimation is a vital part of Exploration and Production Business decision making. Acting as a “spring- board” for E&P business decisions, it encapsulates the portfolio ranking and business viability of any E&P company's asset hence, the importance of correctly estimating volumes of reserves cannot be over-emphasized. Be that as it may, volume estimation remains a very uncertain task‟ due mainly to the heterogeneity of earth processes. To compute this volume, various algorithms, underpinned by geoscientific and engineering practices have been developed over time to reduce the possible errors associated with hydrocarbon volume estimation. These algorithms are geostatistical computations used to delineate the properties of an asset (property modeling). These algorithms are then subjected to uncertainty and sensitivity analyses to account for the dynamics of earth processes as well as inherent uncertainties of the algorithms. The Gullfaks giant oil field lies within the Norwegian license PL 050 in block 34/10 at 610N and 20E in the Norwegian sector of the North Sea. The Gullfaks Field was discovered in 1978 and was set on production in 1986. It is one of the largest oil producing fields in Norway. The Gullfaks field covers an area of 51km2 with water depths ranging from 135 to 220m. It is located in the central part of the East Shetland Basin on the northern North Sea Graben