Laboratory and field studies are described which demonstrate that diversion of well stimulation treatments can be improved dramatically by using buoyant ball sealers. These sealers have extended ball sealer applicability to treatments conducted at low perforation flow rates. The development of an improved ball sealer has resulted in the implementation of several successful field treatments. Introduction One major obstacle often limiting the success of well stimulation treatments has been our inability to divert adequately the treating fluids into the zones where they are needed. Numerous techniques have been used in an attempt to provide better fluid conformance. These include mechanical techniques (such as packers and bridge plugs) and various types of diverting agents (such as particulate materials, polymer gels, and ball sealers) added to or used in conjunction with injected fluids. These techniques vary widely in both cost and effectiveness, but none has proved entirely satisfactory for all types of treating conditions currently encountered in oilfield practice.The study reported here has concentrated on improving ball sealer diverting materials and techniques. Ball sealers have at least two inherent advantages over all other systems; they are inexpensive, and easy to apply. Originally introduced to the industry in 1956, ball sealers are small spheres intended for sealing perforations on the inside of the casing. They are added to the treating fluids during stimulation, carried to the perforations along with the fluids, and (when effective) seat on perforations accepting disproportionate quantities of fluid.Although ball sealers have been used widely, there is abundant evidence which shows that conventional ball sealer techniques often fail to provide satisfactory diversion. Attempts to overcome this poor reliability generally have involved increasing the number of balls or limiting ball sealer usage to treatments conducted at high flow rates. Some authors have gone so far as to propose that sealers have different shapes to enhance their reliability. Although these modifications may improve reliability under certain conditions, they have in no way provided a completely reliable diversion process.Recognizing the limitations and deficiencies of current ball sealer practices, this study was undertaken in an attempt to develop materials and techniques for improving the reliability and utility of ball sealer diverters. Laboratory and field studies have shown convincingly that ball sealers can be used with a very high degree of reliability, provided proper control is maintained on a few critical system variables. Laboratory Study The laboratory phase of this investigation consisted of a parametric study directed toward evaluating those variables of engineering significance that affect ball sealer seating efficiency. Tests were conducted in two vertical wellbore models: one 8.0 ft (2.4 m) long and the other 35 ft (11 m) long. Each had a 3.0-in. (7.6-cm) ID and contained drilled holes to simulate a perforated completion. The models were constructed of a transparent acrylic plastic, which enabled visual observation of the seating process and facilitated identification of the critical system variables controlling ball sealer seating efficiency. JPT P. 1903＾

The success of well stimulation treatments is often strongly dependent on selective placement of treating fluids into low permeability or damaged zones and away from high permeability or undamaged zones.A previous investigation conclusively demonstrated through laboratory and field tests that buoyant ball sealers can provide highly effective diversion. Buoyant balls have been shown to exhibit 100% seating efficiency provided they can be transported down the casing to the perforations.