Evaluation of the relationship between fracture conductivity, fracture fluid production, and effective fracture length

摘要

Low-permeability gas wells often produce less than predicted after a fracture treatment. One of the reasonsfor this is that fracture lengths calculated after stimulation are often less than designed lengths. Whileactual fracture lengths may be shorter due to fracture growth out of zone, improper proppant settling, orproppant flowback, short calculated fracture lengths can also result from incorrect analysis techniques. It isknown that fracturing fluid that remains in the fracture and formation after a hydraulic fracture treatmentcan decrease the productivity of a gas well by reducing the relative permeability to gas in the regioninvaded by this fluid. However, the relationships between fracture fluid cleanup, effective fracture length,and well productivity are not fully understood.In this work I used reservoir simulation to determine the relationship between fracture conductivity,fracture fluid production, effective fracture length, and well productivity. I simulated water saturation andpressure profiles around a propped fracture, tracked gas production along the length of the proppedfracture, and quantified the effective fracture length (i.e., the fracture length under single-phase flowconditions that gives similar performance as for multiphase flow conditions), the "cleanup" fracture length(i.e., the fracture length corresponding to 90% cumulative gas flow rate into the fracture), and the"apparent" fracture length (i.e., the fracture length where the ratio of multiphase to single-phase gas entryrate profiles is unity).This study shows that the proppant pack is generally cleaned up and the cleanup lengths are close todesigned lengths in relatively short times. Although gas is entering along entire fracture, fracturing fluidremains in the formation near the fracture. The water saturation distribution affects the gas entry rateprofile, which determines the effective fracture length. Subtle changes in the gas rate entry profile canresult in significant changes in effective fracture length. The results I derived from this work are consistentwith prior work, namely that greater fracture conductivity results in more effective well cleanup and longereffective fracture lengths versus time. This study provides better explanation of mechanisms that affectfracturing fluid cleanup, effective fracture length, and well productivity than previous work.