Summary 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 postfracture well performance. We also provide guidelines for the effective length of horizontal-well section in which ball sealers can be used for zonal isolation.

Conventional mud acid (a mixture of HF and HCl) is the most commonly used acid system for the stimulation of sandstone formations. However, well known disadvantages such as fast reaction rate, precipitation and matrix deconsolidation restrict the usefulness of this type of acid. A high-temperature deep-penetrating acid system (HTDP acid) is presented in this paper. Compared with regular mud acid, the HTDP acid penetrates deeper into the formation by generating HF at a slower rate in situ. HTDP acid is one type of retarded mud acid, and it is made up of composite phosphate and fluoride salt. A new corrosion inhibitor has been developed which makes HTDP acid suitable for formations with temperatures as high as 356°F. Very important, the HTDP acid has much higher quartz solubility compared with the mud acid and could reduce precipitation. The HTDP acid has been used in 23 wells in China offshore oilfields since 2006, and 22 of the stimulation jobs have been deemed effective. Among those wells, five are high temperature/low permeability wells (the highest temperature is 293 °F) and are located in China's Bohai Bay. Two of the wells are in a loose sandstone formation in the South China Sea. The production of these wells after stimulation was enhanced remarkably. Though developed and used in China, the acid together with the high temperature inhibitor could be extrapolated to other similar sandstone reservoirs world-wide.

Technologies for horizontal well fracturing in China include multi-staged 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 oilfield have been producing for a very long time and the perforations are all along the horizontal sections, it is practically impossible to use the technologies mentioned above. To prevent multiple fracture initiation in a given section, sealing-ball staged fracturing has been developed and has been used successfully in horizontal wells of the Huabei field. Three kinds of sealing balls, whose densities are <1, 1 and >1 g/cc, are pumped between stages of fracturing and they can seal perforations at 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. 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 where ball sealers can be used for zonal isolation.

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