Evaluation of Reservoir Connectivity Between Gas and Oil Zones for Predicting Down Dip Oil

Rong Hwang¹ and Stan Teerman^2
1Energy Technology Co., Chevron, San Ramon, CA
²North America Exploration & Production Co., Chevron, Houston, TX

Assessment of reservoir connectivity is critical in field development and reservoir management and impacts the economics of development and production. The technique based on gas chromatographic (GC) differences between reservoired oils is widely used for this assessment. However evaluation of fluid communication between a gas cap and an oil leg is difficult because partitioning effects between gas and liquid phases that result in significant differences. The effects are difficult to quantify, reducing the correlation confidence. To overcome the difficulty a new approach was taken. The GC differences resulting from partitioning are minimized when comparison is confined to light hydrocarbons. As shown in the studies here these compounds with high fugacity show limited fractionation between gas and liquid phases.

In the Barrow Sub-basin, Australia, vertical wells completed in two different structures intercept a Mardie Greensand gas zone and an oil zone in the underlying Barrow sand. Fluid communication between the two sands was assessed for predicting if down dip oil occurs in the Mardie Greensand. In one structure, the fingerprint differences between the Mardie Greensand condensate and Barrow sand oil are less than the expected partitioning effects indicating fluid communication and the existence of a down dip Mardie Greensand oil leg. This was confirmed by a horizontal well drilled later. In the second structure, the differences in C7 hydrocarbon fingerprints were much larger than partitioning effects between the phases, indicating the presence of permeability barriers between the Barrow and Mardie Greensand reservoirs and thus the absence of down-dip oil in the Mardie Greensand. A horizontal well was drilled later in the Mardie Greensand confirming a gas accumulation in the sand. The drilling results were consistent with the geochemical predictions.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery