Non-Darcy Flow in Hydraulic Fractures: Does It Really Matter?

Abstract

Abstract In recent years, non-Darcy flow has seen a significant increase in interest in the petroleum industry, especially in flow in fractures—both artificial and natural. In hydraulic fracture stimulation, non-Darcy flow can have a major impact on the reduction of a propped half-length to a considerably shorter "effective" half-length, thus lowering the well's productive capability and overall reserve recovery. These non-Darcy flow effects in propped fractures have been typically associated with high flow rates in both oil and gas wells. This paper shows that non-Darcy flow effects have an impact on the performance of a hydraulically fractured well even at low flow rates. Although not as drastic as the effects on high flow rate wells, reductions in flow capacity of 5–30% can be realized in low rate wells. Such reductions are due solely to non-Darcy effects. When combined with other concerns, such as multiphase flow, the production reduction effects are even greater. Development of a simple spreadsheet is provided to aid engineers in assessing the impact non-Darcy flow may have in a given situation. The spreadsheet is not intended to replace more in-depth investigation of non-Darcy flow effects but instead provides a conduit to assess the sensitivity of certain parameters in a hydraulic fracture stimulation situation. For comparison purposes, results of the loss in long-term dynamic conductivity on well performance and cumulative gas recovery over time in low permeability reservoirs are also presented. These calculations were performed using dynamic conductivity loss calculations coupled with a transient gas reservoir simulator. These results showed that for the cases examined, non-Darcy effects could reduce cumulative gas production by up to 18.1% over a ten-year period. Introduction The existence of non-Darcy effects in the flow of fluids through porous media has been known for quite some time; however, characterizing and assessing the magnitude of these effects has proved difficult. Hundreds of papers have been published in the petroleum literature addressing everything from analyzing the effects of non-Darcy flow in pressure transient analysis to determining their role in gravel packs. Outside the realm of the petroleum industry, thousands of papers have been published on this subject as it affects a diverse range of subjects from irrigation to cigarette filter development. This paper demonstrates that non-Darcy flow effects can influence well productivity across the entire spectrum of flow rates, including low rates experienced in many producing wells. A simple spreadsheet model is developed which, although is not as rigorous as a fully 3D reservoir model, can provide engineers with the ability to estimate the effects of non-Darcy flow on production and take steps to minimize them if possible.