A new method for wellbore stability evaluation based on fractured carbonate reservoir rock breaking degree

Borehole wall collapse is rare in the drilling of carbonate rock formations. However, several wells in the Maantang–Leikoupo carbonate formation have collapsed and released blocks to varying degrees, which have caused sticking and hindered efficient development of oil and gas resources. Thus, through the study of macroscopic and microscopic characteristics of carbonate rocks in this area, it is concluded that the fractures in this area are developed. In order to facilitate the analysis of formation fracture development, the concept of formation integrity coefficient which is commonly used in rock engineering was introduced. Firstly, the feasibility of using the formation integrity coefficient to characterize the degree of rock fragmentation was verified by the laboratory acoustic experiment. Finally, the empirical formulas were proposed regarding the formation integrity coefficient and the cohesion of carbonate rocks in this area by combining the laboratory mechanical experiment and acoustic experiment, and a calculation model for the collapse density considering formation integrity was established. Results provide guidance for research on carbonate wellbore stability, especially for fractured carbonate strata.