Risk assessment on the impact of Non-Darcy flow on unconventional well performance.

Abstract

Nano-pores in unconventional reservoirs can cause deviations from Darcy's law where permeability becomes a strong function of pore size and pore pressure. As pores become smaller, the interaction of gas molecules with the pore walls becomes increasingly dominant compared to interactions between gas molecules. This increases gas mobility leading permeabilities measured in the lab not to be accurate at reservoir conditions. Thus, corrections to Darcy flow may be needed when analyzing well production behavior. This paper demonstrates a novel workflow for modeling the impact of non-Darcy flow regimes (i.e., gas slippage, transition flow and Knudsen diffusion) from pore scale characterization to reservoir scale flow simulation. A stochastic approach is used for uncertainty assessment on non-Darcy corrections needed for methane mobility as pore pressure depletes based on reservoir properties, pore sizes, rock compressibility and compaction. Flow simulations are conducted for a range of scenarios to test the impact on the recovery of an unconventional well. Study is conducted using a commercial simulator since workflow modifies the input to the simulator and does not need a new code development. The workflow is demonstrated using field data and shows that non-Darcy flow regimes can have significant impact by increasing gas mobility at low pore pressures near hydraulic fractures. For the field under study, ignoring non-Darcy flow risks up to 20% lower estimations in cumulative gas production under certain scenarios.