Engineering Criteria for Fracture Flowback Procedures

Abstract

Abstract Post treatment fracture flowback procedures during closure are often critical to the retention of fracture conductivity near the wellbore. Postfrac production performance largely depends on this conductivity. The importance of proper flowback procedure has been documented in the fracture industry, but definitive guidelines for flowback design have never been established. As a result, many misconceptions exist regarding the physics of proppant flowback and its effects on the final proppant distribution in the fracture. This paper presents a rigorous study of fracture flowback and proppant migration during closure using a fully three-dimensional fracture geometry simulator (GOHFER). The effects of rate of flowback, location of the perforation interval, final proppant concentration, and the fracture geometry prior to flowback on the retained post closure proppant concentration are discussed. Consideration is given to the fluid velocity field in the created fracture resulting from the flowback, and its effects on proppant movement and localized fracture closure. These studies illustrate the difference between "forced closure" and "reverse screenout" concepts in flowback design. Other effects such as crossflow between multiple perforated layers are also studied. Simulation studies indicate that selection of a desirable flowback rate is very sensitive to crossflow effects resulting from induced fractures in multiple stress layers. This crossflow can result in significant overflushing of proppant in the lower stress zones, if not countered by properly applied flowback procedures. Very high flowback rates, exceeding the total leakoff rate, may be needed to avoid such overflushing. The results of this study are assimilated into a set of recommendations for optimum flowback design leading to the maximization of the near-wellbore fracture conductivity and maximum attainable conductive length in communication with the perforations. Ideally, any properly applied controlled flowback procedure should induce a reverse screenout at the wellbore - forcing closure on the proppant by packing the near-wellbore area, not by depleting fluid pressure and "pinching" the fracture closed. Introduction Postfracture flowback procedure is known to be very critical to the production performance of a fractured well. It is particularly so in tight formations. Experience shows that improper flowback procedures often lead to poor retained conductivity near the wellbore due to proppant movement into the wellbore or proppant crushing at or near the wellbore. Robinson et al. discussed the merits of flowing back wells on a small choke to minimize the closure stress on the proppant resulting in crushing. These authors also recommended initiation of low rate early flowback of the fracture fluid, in case of excessive closure time common in low permeability formations. Longer closure time allows proppant to settle in the open fracture due to breaking of the cross-linked polymer gel or rheological deterioration of foamed fluids. This may severely reduce proppant pack conductivity at or near the wellbore. An early induced closure, suggested by Robinson et al., should lock the proppant pack between the fracture walls before much settling can occur Subsequent studies by Ely, et al. showed that such a forced closure technique, coupled with high proppant concentrations and appropriate fluid quality control, significantly improves the productivity of low permeability oil and gas wells. Ely, et al. also recommend a forced closure implementation procedure within thirty seconds of completing flush. They suggest less than 10–15 gallons per minute flowback rate up to 30 minutes after near wellbore fracture closure is detected from surface pressure measurements. P. 567