Flowback of fracturing proppant during production is a common problem. The consequences of this can be extremely serious. Loss of fracture conductivity can occur due to reduced width, which can be further compounded by partial plugging of the pack by proppant fines. The latter are produced by non-uniform loading on the proppant with resultant failure and crushing. The produced solids can wreak havoc with both downhole and surface equipment, eroding chokes and nipples, plugging flow lines and filling separators. In the case of subsea wells, the problems are even greater and can compromise well security. Since the 1980's, the standard solution applied for prevention of proppant flow back has been the use of curable resin coated proppant. While this approach has met with some success, it is far from perfect. The use of RCP's can cause fluid compatibility problems and can interfere with well clean up. Such problems have prompted resin manufacturers to modify their products and have also opened the door for the introduction of new technologies. Newer developments have included the use of small fibres to try to bind the proppant pack together or heat-sensitive plastic film to partially encapsulate clusters of proppant. These materials have reportedly been used with some success but concerns have been raised with regard to their effect on fracture conductivity, amongst other things.
This paper describes the current methods in use and presents a new system that actually enhances fracture conductivity and minimises embedment and width-loss, while simultaneously helping prevent proppant back-production.
Proppant flowback has caused the oil industry a multitude of problems since it was first reported in the 1960's. Several solutions have been applied to the problem but none has met with complete success. However, without doubt, the most successful method applied to date has been the use of curable resin coated proppant (RCP's). RCP's were introduced in the early 1980's and their use has increased to the point that year 2000 estimates of consumption are around 500,000,000 pounds (ie. >215,000 tonnes).
Resin coating of proppant has several potential benefits apart from preventing flowback. Amongst these are increased crush resistance, reduced fines production, and a reduction in proppant embedment. There are two principle types of RCP -pre- cured (or tempered) and curable, but only the latter is useful in preventing proppant flow back.
The principal resins used in the manufacture of all types of RCP are phenolics. More accurately, they are products of the condensation between phenols and formaldehyde and are chemically related to plastics like Bakelite. Such materials are described as "thermosetting" as they can be moulded and cured at temperature only once. The curing process is irreversible and the resin cannot be softened or melted after curing is complete. These plastic resins are tough, mechanically quite strong and resistant to the effects of many chemicals, including acids and solvents, once they have been cured. They are, however, susceptible to rupture under confining stress, particularly if the thickness of the coating is too thin.