Field experiments on multi-stage chemical diversion in low-permeability HPHT reservoirs

摘要

Efficiently producing gas from low-permeability reservoirs relies on a complex and dense fracture network. Since mechanical plugs or isolation tools become less reliable at high pressure and high temperature, chemical diversion is likely one of the most promising stimulation methods for the multi-stage hydraulic fracturing. To meet field operation needs, a series of diverters has been developed; they have different shapes and sizes, and can be applied in reservoirs with temperatures up to 200 degrees C and pressures up to 140 MPa. After the stimulation, diverters can degrade at the reservoir condition, leaving no residue that causes formation damage. Laboratory evaluation methods were introduced that could obtain the plugging process and plugging efficiency of diverters with different shapes and sizes, as well as the initiation of secondary fractures under tri-axial stress conditions. However, due to the limitation of sample dimensions, these methods still cannot capture the plug formation and fracture initiation in the middle section of previously generated fractures, and thus unable to evaluate the intrastage diversion that is more crucial to the generation of the complex fracture network. Therefore, different diversion treatments with different diverters were tested and compared in 43 vertical wells in the Tarim Basin with reservoir temperatures ranging from 120 degrees C to 181 degrees C. Results showed the successful operations of both intra-stage and inter-stage diversions during the multi-stage diversion treatment, where the former one seemed more effective on production enhancement. Moreover, the fiber-shaped diverters could enhance the formation of complete plugs in the previously generated fractures, and allow more secondary fractures to branch off from these fractures to achieve a better stimulation. Findings of this paper could provide crucial guidance on optimizing the diverters for stimulating the low-permeability reservoirs.