This paper describes the laboratory and field development of thermoplastic film materials used to reduce proppant flowback that can occur after fracturing treatments. The paper provides a summary of flowback mechanism theories and laboratory tests comparing flowback tendency for various types of treating procedures and materials. Some of these materials include angular proppant, proppant/fiber mixtures, and proppant with film strips tested over a wide range of temperature, closure stress, and flow-rate conditions. Field treatment procedures are discussed, and several case histories are presented.
All of the methods evaluated were effective in reducing proppant flowback under certain conditions. Heat-shrink film cut into thin slivers proved to provide flowback reduction over broad temperature and closure stress ranges and was found to cause little impairment to fracture conductivity with some dependency on use concentration, temperature, and closure stress. The film materials were more resistant to damage caused by blending and pumping than all other materials evaluated. In addition, proppant packs, including consolidated packs, were significantly more tolerant of large, repeated stress changes. Field results indicate that the use of the heat-shrink film material as a flowback control agent permits more aggressive bean-up procedures following conventional fracturing treatments. Conventional dry-additive metering systems were used to add the film material to the fracturing fluid proppant slurry.
Proppant flowback from propped hydraulic fractures causes tubing erosion, safety-valve erosion, disposal problems, and increased costs. Extra equipment and operators are needed for wells that produce proppant. In fact, the development of fields that for economic reasons require unmanned platforms or subsea completions has been inhibited because of the potential of proppant flowback. In most instances, proppant is produced during the cleanup after a fracturing treatment and equipment and operators are then available to handle the material that is produced to surface. Proppant back-production during the production life is generally the cause all of the above mentioned problems. Substantial work has been conducted in the industry to explain, predict, and reduce proppant back-production.
In fracturing operations, proppant is carried into fractures created when hydraulic pressure is applied to subterranean rock formations with such force that fractures are developed. Proppant suspended in a viscosified fracturing fluid is carried outwardly away from the wellbore within the fractures as they are created and extended with continued pumping. Upon release of pumping pressure, the proppant remains in the fractures holding the separated rock faces in an open position forming a conduit for flow of hydrocarbon, or formation fluids, back to the wellbore. Unfortunately, variations in the formation's in-situ stress and its mechanical properties can lead to nonuniform fracture closure. One possibility is the formation of open channels around proppant bridges. Fluid velocity in these channels may be very high and may increase the tendency of proppant detachment and flowback in this region. An incompletely closed fracture allows proppant to move freely (in suspension) if not trapped between the well and fracture wall. As a result, when the fluid flows back, this free-moving proppant can be brought back to the wellbore.
This paper gives a brief overview of possible causes of proppant flowback, along with the methods that have been applied to curtail the problem and their shortfalls. The mechanical properties and proppant flowback control characteristics of thermoplastic film strip (TFS) material are presented.