Effect of interface defects on shear strength and fluid channeling at cement–interlayer interface

The fluid channeling during hydraulic fracturing has been seriously restricting the efficient development of low permeability oilfield and CBM Field. The occurrence of fluid channeling after hydraulic fracturing is hinged on shear strength at cement–interlayer interface (CII) and hydraulic fracturing pressure. Interface defects are key factors that influence the shear strength at CII. In view of this, the influence of interface defects on shear strength at CII and fluid channeling is discussed in this paper.The formation reasons of interface defects are analyzed firstly. Based on analysis of the essence of shear strength at CII, it is concluded that all interface defects amount to the missing amount of mud cake ring. Then a mathematical model between missing amount of mud cake ring and shear strength at CII is developed. In order to verify the accuracy of the model, a simulated experimental system is built. Based on the model and fluid channeling coefficient equation, a modified version of the fluid channeling coefficient is derived and the influence of interface defects on interlayer fluid channeling during hydraulic fracturing is evaluated quantitatively.The results revealed that the shear strength at CII decreases linearly with the increase of interface defects, and verification results show that the relative errors between calculated values by the model and experimental values are less than 10%. From the modified equation of the fluid channeling coefficient, this paper established a typical exponential relationship between missing amount of mud cake ring and fluid channeling coefficient, which indicates that interface defects drastically affect fluid channeling at CII.

► A mathematical model is developed for effect of interface defects on shear strength at CII.
► An experimental simulation system is built for verification of model.
► The verification results show the model in the experimental values matching.
► The interface defects have significant effect on fluid channeling at CII.