Closing the Gap: Fracture Half Length from Design, Buildup, and Production Analysis

Abstract

Abstract It is commonly observed that hydraulically fractured wells perform as though the effective fracture half-length were much less than the designed half-length. This observation has been explained by various "models" including poor fracture height containment, poor proppant transport, proppant falling out of zone (convection), ineffective proppant pack cleanup, capillary phase trapping, multi-phase flow, gravitational phase segregation, and non-Darcy flow, with combinations of any of these mechanisms. With recent improvements in diagnostic measurements of fracture geometry, some of these explanations have lost credibility, but the problem of low effective fracture length persists. This paper presents detailed evaluation of hydraulically fractured well behavior using continuous production analysis, pressure transient (buildup) analysis and fracture treatment evaluation using actual field data from a tight-gas reservoir in the Rocky Mountain Region. The various analyses explain the observed producing behavior of the well and lead to a consistent determination of the actual "effective" fracture halflength compared to the physically created or propped length. Problems relating to semantics and inconsistent fracture and reservoir description, especially the physical processes encompassed by various analytical techniques, will be addressed. Methods will be outlined for predicting the useful effective length from available proppant conductivity data. The process outlined helps to close the gap between designed frac length and producing length and points out the causes for remaining system bottlenecks that limit post-frac well productivity. Finally, the understanding of these mechanisms provides a means to arrive at an economic optimum fracture treatment design for a reservoir, once key parameters are known.