Modeling Bitumen Presence and Its Impact on Reservoir Performance in the Arab D Reservoir, Dukhan Field, Qatar

A recent integrated geologic modeling and reservoir simulation history-matching effort in the Arab D Reservoir, Dukhan Field, Qatar, has found that the presence of bitumen and the extent to which it saturates porosity and impairs permeability are the key factors in matching water production history in wells located near the oil-water contact (OWC). By using production data, the ability to predict the location and continuity of permeability-reducing bitumen was greatly increased. In this study, many past attempts at visual and petrophysical identification of Dukhan Arab D bitumen were quantitatively compared, and visual macroscopic core descriptions were found to be the most accurate. By integrating core and petrophysical data with the structural history and the timing of hydrocarbon migration and bitumen generation, a predictive model was constructed to identify the part of the reservoir most likely to contain bitumen, the “bitumen prone interval” (BPI). Within this interval, the presence of bitumen was determined stochastically away from well control using facies modeling techniques. In each bituminous cell, bitumen saturation was calculated through statistical analysis of core plug porosity data from inside and outside the BPI. The resulting reservoir model provided both a conceptual match to the working knowledge of bitumen distribution at Dukhan and a directional match to the overall field production history. However, this approach was unable to match individual well performance or the variability in areal sector behavior with a high degree of accuracy. While direct observation of bitumen is limited to cored wells, producing wells above the BPI provide indirect evidence of bitumen presence for model calibration. Through iteration with the dynamic reservoir simulation model, bitumen distribution, saturation, and permeability were modified systematically within the predicted BPI to better match the behavior of both individual wells and areal sectors of the field.