Study on the Mechanical Properties and Failure Law of Rocks with Interbedded Sand and Mud

Abstract

In view of the borehole instability during the drilling process of the thin sand and mud interbedded sections in the Shahejie Formation, the physicochemical and mechanical properties of sand and mud interbed rock were studied through a series of laboratory tests to determine the main factors influencing the formation instability. The effects of fracture development of interbed sand and mud, mechanical weak plane, borehole trajectory, and seepage effect on borehole stability were evaluated and analyzed through the established model. The results indicated that the microcracks are developed on the lithologic interface due to the change of lithology of the sand–mud interbed. The anisotropy of the mechanical properties of the rocks with interbedded sand and mud is obvious, which leads to the great mechanical weak plane effect on the wellbore stability. The bottom-hole pressure difference leads to the seepage effect, which reduces the effective supporting force of the drilling fluid on the borehole wall and promotes rock sliding and failure along the lithologic interface developed by the sand–mud thin interbedded layer. Considering the influence of the borehole trajectory, mechanical weak plane, and seepage effect, the recommended drilling fluid density in the deviated section is 1.69 g/cm3. The wellbore stability is the best when drilling along the direction of the minimum horizontal principal stress in the horizontal section. Further strengthening the plugging performance of the drilling fluid, drilling through the microfractures vertically on the interbedded lithologic interface, and reducing the lateral vibration of the drilling tools as far as possible are necessary. The research results provide a theoretical basis for the safe drilling of thin sand and mud interbedded strata.

Rock Mechanical Properties and Wellbore Stability of Fractured Dolomite Formations

“…In the process of drilling fractured carbonate formations, wellbore instability occurs frequently, and it is more common in areas with concentrated geological structural stress, which is also a common technical problem faced by the oil drilling industry. , The main factors affecting wellbore instability include geological structure characteristics, the mechanical properties of the surrounding rock, the occurrence of formation cracks, the magnitude and direction of ground stress, and field construction. The main cause of wellbore instability is the redistribution of the original stress equilibrium state caused by the formation of new boreholes, and the peripheral stress formed by the redistributed structural stress and drilling fluid column pressure exceeds the bearing capacity of rocks, resulting in borehole wall instability. , If fractures are relatively developed in the formation, drilling fluid will flow along microfractures and bedding, resulting in pressure penetration between the formation and the wellbore, weakening the effective supporting effect of the fluid column pressure on the wellbore, and further aggravating wellbore instability, especially when the formation is relatively broken, resulting in more complex downhole problems. – …”