As described in U.S. Pat. Nos. 2,254,246; 4,244,425; 4,410,387; and 4,505,334; U.S. Pat. Appln. Publ. 2009/0255674, and Intl. Appln. No. WO 2009/050681, it is common practice in completing oil and gas wells to set a string of pipe, also known as a casing, in the wellbore. The outside of the casing typically is surrounded by concrete sheath. Perforations are provided through the sheath and casing to allow fluid to flow between the interior of the casing and the geological formations surrounding the sheath. This structure of the well permits the flow of fluid between the casing and the formations to be limited to selected zones or strata. The well operator determines from which zone or zones hydrocarbons are to be collected from during recovery operations, or into which fluids are to be injected during a stimulation operation such as fracturing or “fracking,” and then perforates the sheath and casing in the area of those zones.
Ball sealers are spheres which are sized to be slightly larger than the openings of the perforations inside the casing. They are employed to seal the openings of certain of the perforations to thereby mechanically divert fluid flow from those perforations to other perforations in the wellbore.
Ball sealers and are incorporated in the working fluid used in the fracturing or other treatment operation, and are injected into the wellbore with the fluid. The balls are carried to the perforations by the fluid flow and seat in the openings of the perforation wherein they are held in place by differential pressure. The effectiveness of the seal depends on factors such as the differential pressure across the perforation, the geometry of the perforation, and physical characteristics of the ball sealer.
As additionally described in U.S. Pat. Appln. Publ. No. 2007/0169935 and U.S. Pat. Nos. 2,754,910; 4,102,401; 4,421,167; and 4,488,599, ball sealers, which may be either soluble or non-soluble, are made in a variety of diameters, densities, and compositions to accommodate different wellbore conditions and perforation sizes. Non-soluble ball sealers of the type herein involved generally consist of a rigid solid or hollow core covered by a rubber or other coating.
Presently there is an interest in advancing fracking technology to improve the extraction rates of existing wells. The goal is to be able to use higher pressures and to increase the number of well zones that can be isolated. The key to this advancement is through the development of ball sealers having a higher specific strength, that is, an increase in strength without an increase in weight.
Ball sealers are used in hostile environments and need to be robustly constructed. Ball sealers also need to be buoyant within the working fluid, and therefore are required to be of a density which is within a specific range. This range is relatively low and limits the materials and constructions that can be employed. Those in the field have long struggled to create a ball sealer that is light and strong enough to be buoyant in the working fluid, but strong enough to endure high pressures without distorting or shattering.
Ball sealers initially were used with lower pressures and typically were constructed of high strength plastics. As working pressures increased, ball sealers needed to be constructed from higher strength materials such as a resin-infused syntactic foam core covered with an elastomeric, plastic, or other material. Alternate constructions employed a light-weight, hollow core covered in a skin made of machined aluminum or made by wrapping a high strength filament impregnated with a thermosetting resin. As wellbore working pressures continue to increase, it is to be expected that continued improvements in ball sealers would be well-received by the oil and gas industry.