Mechanism of Borehole Stability in Fractured Formation With Discrete Fracture Network

ABSTRACT Due to the high degree of rock block dispersion and the extensive development of natural fractures in the fractured formation, the drilling fluid loss and wellbore instability accidents occur frequently during the drilling process. At present, in the numerical simulation research on wellbore stability in fractured formations, the randomness of actual fracture dip angle and dip direction, as well as the influence of the coupling effect of fracture seepage and matrix seepage on wellbore stability have not been considered. In this paper, the discrete element ‐ finite element coupling modeling is adopted, and the DFN (discrete fracture network) is used to simulate the actual random multi‐fracture development of the fractured formation. The dual medium seepage effect of fractures and matrix pores is comprehensively considered, and the seepage‐stress coupling model is established and numerically solved. The research shows that the actual random fracture distribution is significantly different from the uniform fracture distribution commonly used in current numerical simulation in terms of fracture opening, drilling fluid loss and wellbore surrounding rock damage. The collapse pressure under the influence of random fracture distribution is higher than that under uniform fracture distribution. Under the random fracture distribution, the fractures do not show extensive opening as in the uniform fracture distribution, and only some fractures show a relatively high opening degree near the wellbore. In terms of drilling fluid loss, due to the mutual interference of fracture seepage, the range of drilling fluid loss along the fractures under random fracture distribution is relatively smaller than that under uniform fracture distribution. In terms of the degree of fracturing, the higher the degree of fragmentation (the larger the number of fractures), the greater the opening degree of fractures around the wellbore and the more severe the wellbore instability.