Compositional Grading in the Ghawar Khuff Reservoirs

Abstract

Abstract The paper presents the results of a study to investigate the variation of gas composition and key PVT properties with depth in the Khuff carbonate reservoirs of the Ghawar field in Saudi Arabia. The study was performed as part of a larger effort to characterize the reservoir fluids for compositional simulation and development planning. Laboratory PVT reports on over twenty appraisal wells were used as a basis for the study. It was found that both condensate content and hydrocarbon compositions decreased with depth and/or temperature while acid gas composition increase with depth. The composition of hydrogen sulfide varied from zero, at a threshold depth and temperature, to over five mole percent at greater depths. The observed variations of composition with depth were contrary to what would be expected from gravity-chemical equilibrium considerations. We propose that the dominant process for compositional grading in the Khuff is the generation of hydrogen sulfide by thermo-chemical reduction of sulfate. This process is believed to occur at temperatures above a threshold value (about 270 0F for the Ghawar Khuff), and utilizes hydrocarbons as one group of reactants. Thus, as is shown in the paper, hydrogen sulfide composition increases with depth at the expense of hydrocarbons in a way that upsets the effect of gravity- chemical equilibrium. P. 685