Deformation Mechanism of Asperities on Joints in Algal Clot Dolomite of the Dengying Formation, Sichuan Basin

The joints in algal clot dolomite reservoirs of the Dengying Formation in the Sichuan Basin are well developed. Under external pressure, the opening and closure of these joints significantly influence well productivity. The degree of joint closure is closely related to the deformation of asperities. However, quantitatively measuring this phenomenon at either the field or laboratory scale remains highly challenging. In this study, five sets of spherical-cap asperities with radii of 2, 4, 6, 8, and 10 mm were fabricated from algal clot dolomite using a CNC (Computer Numerical Control) system. These asperity samples were subjected to normal loading mechanical experiments, during which key parameters such as uniaxial compressive strength (UCS), peak strain, and normal stiffness were measured and analyzed. Due to the inherent presence of pores and microcracks in the rock, as well as the development of framework-like pores between clumps with multiple stages of dissolution, the pores are filled with granular and blocky sparry dolomite, resulting in strong heterogeneity. Consequently, the load-displacement curves exhibited pronounced nonlinear characteristics. The UCS and normal stiffness increased with asperity size, and larger asperities exhibited a longer plastic stage. Asperities of different sizes demonstrated two distinct constitutive behaviors: elastic-plastic and elastic-brittle-plastic. Post-compression observations of the asperity surfaces revealed a disc-shaped failure mode, where the asperity center was compacted. A primary crack was observed propagating outward from the center, accompanied by multiple secondary cracks. Furthermore, in situ CT loading tests validated that an initial primary crack formed inside the asperity under compression, with stress being released along the primary crack. As loading progressed, the primary crack widened and the peak of asperities flattened. The research findings provide new insights for validating fracture closure models and offer a theoretical foundation for studying issues such as lost circulation during the drilling process.