Preventing sour gas kicks during workover of natural gas wells from deep carbonate reservoirs with anti-hydrogen sulfide fuzzy-ball kill fluid

Workover of natural gas wells in deep carbonate gas reservoirs (DCRs) is facing multiple challenges such as fluid leakage, high temperature, and hydrogen sulfide (H2S) gas kick. Hence, well-killing fluids with satisfactory plugging ability, high-temperature tolerance, and anti-H2S ability are necessary. In this study, we developed a multifunctional anti-hydrogen sulfide fuzzy-ball kill fluid (AFKF) for preventing sour gas kicks in DCRs. The performance of the AFKF was optimized and analyzed via experiments and verified through a case study application. The results showed that the fuzzy-ball structures in the AFKFs demonstrated good stability under extreme H2S aging. The rate of change of certain critical rheological parameters such as apparent viscosity, dynamic plastic ratio, and density of AFKFs before and after hot rolling were no more than 5%. After plugging the fractures with the AFKFs at temperatures between 110 degrees C and 150 degrees C, the inlet driving pressure of the fractures increased from 20.73 degrees C to 21.07 MPa, and no fluid loss was observed at the core outlet after a secondary displacement of formation water. The permeability recovery rate was greater than 90%. Application of AFKF in well DW-2 showed that the shut-in pressure rose to 24.6 MPa with no trace of H2S gas at the wellhead after 4 days. The investigation of the H2S mitigation mechanisms revealed that the plugging of the seepage channels by AFKFs forms interconnected bonds that improve the mechanical properties of the reservoir. Additionally, AFKFs absorb H2S gases by protonation and dissociation reactions which convert the hazardous gas into an aqueous solution of sulfide ions (S2-). The proposed AFKF has proven to be effective means of mitigating H2S in DCRs, and minimize the negative impacts of environmental polution, health risks, and equipment corrosion.