Why acid fracturing is ineffective in carbonate-rich shale reservoirs: From laboratory evidence to field suggestions

Abstract

Carbonate-rich shale is a newly discovered, promising shale gas reservoir. But field stimulation practice showed that the effect of conventional acid fracturing was very limited. This is what we try to clarify through a comprehensive study. Using downhole cores from a carbonate-rich shale well, mineral quantitative evaluation, acid immersion, and acid corrosion experiments were conducted. Results showed that (1) The surface calcite of carbonate-rich shale was the first to be dissolved by acid; however, acid was blocked by quartz and unable to react with the internal carbonate minerals. (2) High strength was observed in carbonate-rich shale; however, acid immersion slightly weakened the peak deviatoric stress, Young's modulus, and Poisson's ratio by 2.22%, 13.39%, and 2.19%, respectively. (3) Only one acid-induced fracture was connected to the main shear crack under confining pressure, indicating a limited increase in the stimulated volume of this reservoir by acid fracturing. (4) In the carbonate-rich shale, where there was sufficient contact with the acid, the acid exerted a positive effect on fracture propagation; as the acid was consumed, the internal fracture changed insignificantly. The above evidence would explain the ineffectiveness of acid fracturing in carbonate-rich shale reservoirs. The stimulation suggestion discussed may bring about better stimulated volumes for carbonate-rich reservoirs.