The Limits of Fluid Flow in Propped Fractures- the Disparity Between Effective Flowing and Created Fracture Lengths

Abstract

Abstract Post hydraulic fracture diagnostics often leads to an estimation of effective fracture lengths that are substantially shorter that the created length. (Ibrahim et al. 2017, Barree et al. 2003) This gives rise to questioning validity of the diagnostics or concerns regarding execution of the fracture stimulation itself. The objective of the paper is to show that short effective lengths are understood and predictable in the context of the physics of flow in propped fractures. Extensive laboratory testing of a broad range of proppants under a wide range of conditions has led to a databased and detailed understanding of pressure drop in a propped fracture as a function of fluid flow. The pressure drop in turn will dictate inflow from the reservoir at any point along the fracture. This interaction of pressure drop in the fracture and reservoir inflow will in turn dictate the flowing length of the fracture. Testing has demonstrated and it is well understood that the conductivity of a proppant under downhole conditions in a propped fracture is significantly less than that measured in the lab under ideal conditions of single phase Darcy flow at 2 lbm/ft2 loading. (Duenckel et al. 2017) This results from the combined effects of low proppant concentrations, frac fluid damage, multiphase flow and non-Darcy flow as well as other damage mechanisms. The paper will demonstrate that the pressure loss in the fracture resulting from the combined effects of these damage mechanisms quickly consumes the available energy to the point that the viscous forces are less than the capillary and gravity forces in the fracture, limiting further flowing length. In very low permeability reservoirs inflow from the reservoir quickly becomes insufficient to overcome gravity and capillary forces. The paper will also address the following question: if effective lengths are very short of what benefit is pumping large volumes of proppant? Further, how does one explain apparent field evidence for long fracture lengths (e.g. pressure communication) if indeed the effective length is much shorter? The paper will show how effective fracture lengths are not dependent on propped fracture properties alone. Given the limited effective conductivity resulting from a combination of damage mechanisms, the interaction of the propped fracture conductivity and reservoir inflow dictate effective fracture lengths. The resulting effective lengths are expected to be much shorter than the created lengths.