Tracking the Reaction Front in Siliceous-carbonate Mudstones During the Post-Fracturing Shut-In Period

Surface phenomenon in reactive flows can potentially alter the capillary forces, yet there is limited understanding of its impact on the pore-scale fluid transport. In this work we explore the coupling of reactivity and spontaneous imbibition in an organic-rich siliceous carbonate mudstone by conducting a series of spontaneous imbibition experiments designed to mimic the post-fracturing shut-in period by using a reactive fluid and observing the chemical evolution of the rock and fluid phases. Seven cuboidal samples of size 10 x 10 x 20 mm are extracted from a siliceous carbonate mudstone block and coated with an epoxy on all sides but the square cross-section. Separately, an equilibrated brine is synthesized by reacting a rock chip with DI water for 21 days and determining its composition using GC-MS. The samples are placed in an acidized equilibrated brine, generated by titrating HCl with the equilibrated brine until pH 2 is achieved, and aged for 2, 4, 7, 14, 28, 56, and 100 days. The aged samples are then removed and cut horizontally to expose a fresh surface. μXRF scans are conducted to track the imbibition front and SEM-EDX is used to determine the physical and chemical changes on the exposed rock surface. Separately, the reacted fluid time series is determined using a GC-MS. XRD analysis on the shale samples shows that it consists of 29.2% calcite, 4.4% dolomite, 58.5% quartz, and 7.9% apatite. The elemental maps generated from μXRF show that the carbonate minerals are readily dissolved due to the rock/fluid interaction. The reactive front, measured by the distance of calcite removal from the inlet face of the rock sample, is shown to follow the Lucas-Washburn equation and is proportional to the square root of time. The fluid time series shows Ca²⁺ being readily added to the fluid at the start of the rock/fluid interaction, and then at a slower rate. The SEM scans show the removal of calcite from the rock surface, exhibiting an increase in porosity, as well as reprecipitation of calcite in different spatial locations. This work shows the physicochemical changes occurring in the rock matrix and quantifies the depth of penetration of the reactive fluid during the post hydraulic fracturing shut-in period. This can help in understanding the fate of the hydraulic fracturing fluid and the chemical evolution of the rock surface in the near fracture zone, and subsequently modeling the changes occurring in the near fracture zone during the shut-in period.