Clogging caused by coupled grain migration and compaction effect during groundwater recharge for unconsolidated sandstone reservoir in groundwater-source heat pump

Abstract

In unconsolidated sandstone reservoirs, presence of numerous movable grains and a complex grain size composition necessitates a clear understanding of the physical clogging process for effective groundwater recharge in groundwater-source heat pump systems. To investigate this, a series of seepage experiments was conducted under in situ stress conditions using unconsolidated sandstone samples with varying grain compositions. The clogging phenomenon arises from the combined effects of grain migration and compaction, wherein the migration of both original and secondary crushed fine-grain particles blocks the seepage channels. Notably, grain composition influences the migration and transport properties of the grains. For samples composed of smaller grains, the apparent permeability demonstrates a transition from stability to decrease. In contrast, samples with larger grains experience a skip at the stability stage and directly enter the decrease stage, with a minor exception of a slight increase observed. Furthermore, a unique failure mode characterized by diameter shrinkage in the upper part of the sample is observed due to the combined effects of grain migration and in situ stress-induced compaction. These testing results contribute to a better understanding of the clogging mechanism caused by the coupled effects of grain migration and compaction during groundwater recharge in unconsolidated sandstone reservoirs used in groundwater-source heat pump systems.