To mitigate borehole wall instability in fractured carbonate formations in an oilfield, the main factors affecting borehole wall instability were determined by combining the characteristics of underground cores, logging data, and a series of laboratory mechanics experiments. The geometric morphology characteristics of a carbonate rock fracture surface were studied together with an artificial rock fracture surface formed by triaxial mechanics experiments. A relationship between the geometric morphology characteristics of a fracture surface and rock mechanical properties was established based on fractal geometry. Using the Mohr-Coulomb failure and weak plane failure criteria, a rock strength criterion based on the fractal characteristics of a rock fracture surface was established. Finally, the mechanical rock properties characterized by fractal geometry were imported into the established borehole stability evaluation model. The results show that a collapse formation is mainly limestone with relatively developed microfractures, and some fractures are filled with expansive clay. The anisotropy of mechanical properties of bedrocks and microfractured rocks is obvious, whereas drilling-fluid immersion has little effect on the mechanical properties of a rock. 3D scanning experiments of artificial fracture surfaces formed after a triaxial mechanical test of a bedrock and fracture surfaces of a rock with microfractures show that the geometric characteristics of fracture surfaces after bedrock failure were more complex than those of fractured rocks. The geometric characteristics of rock fracture surfaces were numerically expressed through astatistical analysis and fractal geometry. Function relationships among cohesion, an internal friction angle, and fractal dimensions of bedrocks and microfracture rocks were fitted. A numerical simulation of borehole stability based on the fractal model of a carbonate fracture surface shows that different fracture inclinations and borehole trajectories significantly influence the collapse pressure equivalent density of a borehole wall. On drilling a horizontal well along the inclination of a fracture, the collapse pressure equivalent density of the borehole wall is relatively low when the fracture inclination is along the direction of the minimum horizontal principal stress. Unlike that from a conventional borehole stability model, the collapse pressure equivalent density calculated from the fractal model will increase by 0.1-0.2 g/cm³. The study results provide a theoretical basis for safe and efficient drilling in fractured carbonate formations.

While drilling into the igneous rock formations of the Shunbei area, problems such as loss of well circulation and borehole collapse occur frequently, seriously hindering the efficient development of oil and gas resources. Aiming to solve this problem, the physicochemical and mechanical properties of igneous rocks are studied through a series of laboratory tests to determine the main factors influencing formation collapse and instability. In addition to the laboratory results, the weak plane effect of fracture mechanics, the seepage effect of drilling fluid, and the hydration effect of the drilling fluid on the borehole wall stability of igneous rock formations are evaluated and analyzed by establishing a mathematical model. The results show that the microfractures in the igneous rock are relatively developed and can be divided into unfilled fractures and calcite-filled fractures. The mechanical strength of the matrix igneous rock is higher than that of the rock samples with microfractures. The compressive strength of calcite-filled and unfilled fracture samples is 1/3–1/4 that of matrix igneous rocks, and immersion in the drilling fluid has little influence on the mechanical strength of igneous rocks. The fracture weak plane effect has the greatest influence on wellbore stability. With the increase of the number of fractures at different angles, the collapse pressure equivalent density of the formation subject to the mechanical weak plane effect increases by 21% compared with that of the homogeneous formation without fractures. The seepage effect of the drilling fluid on borehole stability is secondary, and the equivalent density of the formation collapse pressure increases by 11%. Because of the low content of clay minerals, the hydration effect of the drilling fluid on borehole stability was minimal and the equivalent density of collapse pressure increased by only 2%. During the drilling process, considering the weak plane effect of the microfractures and the seepage effect of the drilling fluid, the drilling fluid density should be controlled at about 1.82 g/cm3. The effective plugging ability and rheological properties of the drilling fluid should be improved for the formation with microfractures. The research results can provide a theoretical basis for the safe and efficient development of igneous reservoirs.

During the drilling process in the dolomite formation of the Leikoupo formation in western Sichuan, downhole obstruction frequently occurs, which seriously hinders the efficient development of oil and gas resources on site. In view of the problems of borehole stability existing in this type of formation, the geological characteristics, underground complexity, and fracture development of the research block were systematically identified and analyzed, and the physical parameters and mechanical properties of the rock were defined through physical and chemical tests and an understanding of the mechanical properties of the rock. These studies revealed the mechanism of borehole instability and the main factors influencing fractured dolomite formation. Finally, a model of borehole collapse pressure within fractured dolomite formation was established by selecting the appropriate strength criterion as the criterion of borehole instability. The results show that the dolomite fractures of the Leikoupo formation are relatively developed, the formation is dominated by low-angle fractures, and the core integrity is not high. The development of formation fractures has an obvious effect on the mechanical strength of rock, which decreases to different degrees with the decrease in the integrity coefficient of the rock mass, and the anisotropy of the rock's mechanical strength is obvious. The empirical parameters of the generalized Hoek-Brown (H-B) strength criterion were quantified by introducing rock mass acoustic data, and the accuracy of the improved generalized H-B strength criterion was evaluated using triaxial test data. Taking the M-C criterion, the generalized H-B criterion based on acoustic wave, the D-P criterion, and the weak surface strength criterion as wellbore instability criteria, combined with the distribution of wellbore stress, a prediction model of wellbore stability considering the degree of dolomite formation fragmentation was established. The results demonstrate that the generalized H-B criterion model based on acoustic waves can be used to evaluate the wellbore stability of dolomite fracture formation with more than one group of fractures. These research results have certain guiding significance for efficient and safe drilling in fractured dolomite formations.