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 half-length 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. Introduction The integration of fracture design lengths with actual well performance can provide valuable insight into the effectiveness of the fracture stimulation. This process requires the effective integration of several analytical tools. The evaluation process starts with the pre-stimulation design and ends with an evaluation of the well's production performance. The actual rate and pressure response from the stimulation should be history matched to determine the placed fracture half-length. The resulting length should then be compared to both the pre-job estimates and to actual well production performance. Several analytical techniques are currently available to perform post production analysis including pressure transient testing and production analysis. Production analysis is a technique which incorporates the well's rate and flowing pressure into a type curve matching process to provide a consistent post stimulation analysis. The resulting well performance can then be evaluated by comparing the well's producing capacity with its actual performance. The evaluation of past performance has proven to be the best method of improving future performance. In many cases the resulting fracture half-length calculated from post production analysis is much shorter than planned. This discrepancy can be a source of contention between the team responsible for completing the well and the team responsible for the optimization of field performance. Frequently, these discrepancies can be quantified through a comprehensive, consistent analysis of the available information. Identifying the "problems" that result in short effective fracture lengths allows appropriate design changes to be made to improve future well performance. The example well case-histories described represent a subset of a more extensive field study. In general, the study identified the need for a change in the stimulation design in the field which has been successfully implemented and has resulted in significant production performance improvements.