Experimental Study on the Influence of Hydraulic Fracturing on Hydrogen Sulfide Generation in Longxi Block of Daqing Oilfield

Abstract Hydraulic fracturing has a significant effect on increasing production and injection of oil and water wells in low-permeability and tight reservoirs. Hydrogen sulfide (H2S) was generated after hydraulic fracturing of oil and water wells in Longxi Block of Daqing Oilfield. The maximum hydrogen sulfide content in casing gas reached 232 ppm, far exceeding the safety critical concentration value (20 ppm), which increased the risk and difficulty of production and construction operation. In order to reveal the influence mechanism of fracturing on H2S generation in Daqing oilfield, the experimental simulations of the thermal decomposition of sulfur (TDS), thermochemical sulfate reduction (TSR) and bioreduction reaction of sulfate-reducing bacteria (SRB) with different compositions of fracturing fluid at different temperatures were conducted in this paper. The results showed that the influence mechanism of the fracturing fluid compositions on the formation of H2S was different when the formation temperature was different. The critical temperature of TDS and TSR reaction in the formation was 100°C and 140°C, respectively. The higher the temperature, the higher the amount of H2S was generated. The potassium persulfate (K2S2O8) and sodium sulfite (Na2SO3) in the fracturing fluid could promote the TDS and TSR reaction to generate H2S, but did not change the critical reaction temperature of TDS and TSR reaction. When the formation temperature was not higher than 56°C, the guar gum and sulfur-containing materials in the fracturing fluid could provide nutrition for SRB in the formation, which promoted the rapid reproduction of SRB in a short period of time, and then generated a large amount of H2S. The mechanism of H2S generation for fractured wells in the Longxi block with temperature of 30∼56°C was the proliferation of SRB in the fracturing fluid. The mechanism of H2S generation in fractured wells with temperature of 100∼120°C was that sulfur-containing materials promoted TSR reaction. This study clarifies the reasons and conditions for the generation of H2S, and plays an important role in the subsequent research on H2S prevention and control measures.