A review of diverting agents for reservoir stimulation

Compared with conventional fracturing and acidizing technology, temporary plugging and diverting stimulation technology (TPDST), which has been becoming more and more popular in many countries, can achieve diverting re-fracturing for old wells and diversion acidizing in long interval, and increase stimulated reservoir volume (SRV) in coal-bed methane (CBM) and shale reservoir. The diverting agent is used to plug temporarily the high permeability layers or treated areas during acidizing, which causes the acid fluid to divert to the low permeability layers or the untreated areas to achieve homogeneous acidizing. Besides, the technologies of hydraulic fracturing and acid fracturing rely on diverting agent to block the old fractures, natural fractures or the just-opened new fractures, forcing the fracturing fluid or acid to divert to create more new fractures, thereby achieving volumetric fracturing or network acid fracturing. Based on the differences of the temporary plugging mechanism, diverting agents can be categorized as chemical particle, fiber, gel, surfactant (including foam diversion acids, emulsified diversion acids, and self-diverting acids), composite and phase-transition diverting agents (including thermo-responsive diverting agent and liquid nitrogen). Researchers have summarized temporary plugging mechanisms of different types, commonly used materials, performances, and appropriate application conditions. After comparing and analyzing the advantages and disadvantages of various diverting agents, the results have shown that thermo-responsive diverting agent for fracturing and self-diverting acid for acidification have superior performance and accord with the likely future development trends in oil and gas resource exploitation. Thus, great effort should be made to improve and optimize the performance of thermo-responsive diverting agent and self-diverting acid.