Application Prospect of Oxidative Stimulation of Organic-Rich Shale Gas Reservoir: A Case Study of Longmaxi Formation in Sichuan Basin

Oxidative stimulation has the ability to increase the permeability and produce fractures in a shale matrix. However, oxidation will inevitably change the mechanical properties of the shale gas reservoir. Moreover, its ability to generate and support fractures through expansion stress under high effective stress in formation conditions has not been verified. Taking the shale from Longmaxi Formation in Sichuan Basin as an example, this paper aims at evaluating the long-term fracture conductivity after oxidative stimulation by testing its influence on the permeability and rock strength of the shale matrix. The results show that the damage from oxidation on the uniaxial compressive strength of the shale reservoir was up to 80%. The oxidation of shale showed an uneven etching on the fracture surface, which is similar to the influence of dilute acid on carbonate-rich shale, forming or increasing the degree of surface concaveness and convexness, relieving the reduction in conductivity led by closure of the fracture. The experimental results also verify that oxidative stimulation can effectively improve the seepage capacity of organic-rich shale under high effective stress. By analyzing the variation trend of the shale fracture conductivity over time and the Young's modulus of formation and combining with the variation characteristics of the Young's modulus of shale over time under the oxidation, a model to predict the permeability's retention rate under high stress was established. After fracturing, the conductivity of the shale fractures decreased to 52.9%, 42.3%, and 39.2% after 3, 5, and 7 days, respectively. After 7 days of acid fracturing, the conductivity of the shale fractures decreased to 33.04%, indicating that the fractures formed by oxidation had a similar long-term fracture conductivity to those formed by fracturing or acid fracturing. Therefore, oxidative stimulation had a positive impact on increasing the matrix permeability, protecting and improving the conductivity of unsupported fractures under pressure loads, delaying the rapid decline of gas well production, and improving the shale gas recovery.