Fault Rock Property Prediction in Deepwater Reservoirs

Stephen J. Naruk and U.O. Onyeagoro
Shell International Exploration and Production Inc, Houston, TX

Recent production from DW (deep water) fields shows that the impact of subtle faults on reservoir performance has been systematically and significantly underestimated. This bodes ill for future DW developments, most of which are much more complexly faulted than the relatively simple developments impacted to date. The culprit faults are typically those at the limits of seismic resolution, with displacements << the reservoir thickness, leaving the reservoir in fault contact with itself. These faults typically exhibit no pre-production pressure differences across them, and they are commonly not even included in predictive reservoir simulations. However, syntheses of published and proprietary data show that for typical DW reservoirs, the faults' permeabilities are 1/100th to 1/10000th of the original reservoir permeability, yielding fault transmissibilities (i.e., the ratio of flow with the fault divided by the flow without the fault) on the order of 0.01 to 0.00001), even at very small throws. Because of the high DW production rates and pressure drawdowns, significant flow and pressure buildups can still occur at these low values, depending on other fluid and reservoir variables, particularly the stratigraphic architecture. The subtle faults thus have an enormous and complex impact on DW production rates, reservoir pressures, and well drainage areas. Research and field studies show that these impacts are predictable prior to initial production by careful integration of seismic, structural geologic and stratigraphic data into reservoir simulations. Such integration is essential to optimizing well counts, placements and production rates in future DW developments.