Technologies for horizontal-well fracturing in China include multistaged fracturing and hydraulic jetting. As in many other parts of the world, this type of well stimulation employs mechanical zonal isolation or sand and polymer plugs. All of these technologies are used by us in a number of reservoirs, but they depend on the presence of newly targeted and intentionally perforated sections of horizontal wells that are not already perforated. However, because most horizontal wells in the mature Huabei oil field have been producing for a very long time and the perforations are all along the horizontal sections, it is practically impossible to use these technologies.
To prevent multiple-fracture initiation in a given section, sealing-ball staged fracturing has been developed and used successfully in horizontal wells of the Huabei field. Three kinds of sealing balls--with densities of < 1, 1, and > 1 g/cc, respectively--are pumped between stages of fracturing. They can seal perforations 360° around the horizontal-well section. Higher-permeability zones are sealed off preferentially. Because staged fracturing can be carried out effectively without stopping the pumps while running ball sealers downhole, the process can be accomplished in a significantly shorter period of time than other techniques. And, because these sealing balls are made of wax, they can be dissolved and removed later by the oil produced from the formations.
In this paper we present field case studies of staged hydraulic fracturing using ball sealers, and we demonstrate the success of this form of zonal isolation in both execution and especially post-fracture well performance. We also provide guidelines for the effective length of horizontal-well section in which ball sealers can be used for zonal isolation.
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