Self-Sealing Response in Kaolinite Mudrock Fractures: Implications for CO2 Storage and Unconventional Reservoirs

Fracture self-sealing in clay-rich mudrocks plays a critical role in subsurface energy systems, influencing both containment in storage applications and productivity in unconventional reservoirs. This study systematically investigates key factors governing self-sealing behavior of fractures in clay-rich mudrocks, focusing on the effects of saturation state, time, pore fluid polarity, and sealing integrity. Synthetic kaolinite mudrock specimens were prepared using high-pressure resedimentation, fractured by indirect tensile loading, and tested under controlled conditions. Self-sealing behavior was quantified through core flooding permeability experiments, supported by X-ray CT image processing to track fracture evolution and fluid distribution. Results showed that self-sealing initiated upon fluid contact, regardless of initial saturation, with higher saturation accelerating the sealing rate and spatial progression. Fracture permeability decreased by 4-5 orders of magnitude, from 10 -13 to 10 -17 -10 -18 m 2 , due to time-dependent self-sealing. Saturated specimens reached near-equilibrium sealing earlier than dry specimens, demonstrating that saturation primarily controls sealing kinetics rather than initiation. Brine-injected specimens exhibited rapid permeability reduction due to strong clay-water interactions, while oil-injected specimens showed limited sealing capacity. Sequential fluid injections demonstrated that brine could displace residual oil and promote sealing. CO 2 breakthrough tests showed that brine-sealed fractures withstood injection pressures up to 4 MPa before breakthrough, whereas oil-sealed fractures failed at a much lower pressure as 0.5 MPa, indicating a substantial reduction in sealing integrity under oil-sealed conditions. CT imaging showed fracture closure after fluid infiltration and self-sealing, with contrasts due to pore fluid polarity. These findings highlight the importance of understanding the factors affecting the fluid–clay interactions in self-sealing behavior, with implications for clay-rich tight rock performance in subsurface energy storage and unconventional reservoir systems.