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

Abstract

A large number of movable grains and complex grain size composition exist in unconsolidated sandstone reservoir (USR), thus, knowledge of the clogging caused by the coupled effect of grain migration and compaction of in-situ stress and the effect of grain size on grain migration are significant for groundwater recharge of the ground water-source heat pump. For this, a series of seepage experiments were conducted under in-situ stress for the unconsolidated sandstone samples with different grain compositions. Coupled effect of grain migration and compaction of in-situ stress is one of the important reasons for the recharge clogging for USR. Firstly, the original fine grains are migrated along seepage direction, and the pores are created by grain migration. Then, the skeleton structure constructed of coarse grains is reconstructed under the compaction effect of in-situ stress, and the stress field of coarse grains is redistributed, resulting in the coarse grains are crushed into fine grains. Furthermore, the migration of original and secondary crushed fine-grain blocks the seepage channel. An obvious threshold of grain size, 0.5 mm, is exhibited for grain migration and transportation property, where the apparent permeability experiences a transition from stability to decrease for samples composed of smaller grain (< 0.5mm), whilst the apparent permeability skips a stability stage and goes directly into the decrease stage, and even that exhibit a slight increase for samples with a larger grain (> 0.5mm). A unique failure mode, diameter shrinkage in the upper part of the sample, is presented due to the coupled effect of grain migration and the compaction effect of in-situ stress. Some measures taken in the field are exhibited, and their advantages and disadvantages are given. Furthermore, the applicability of the common methods in solving recharge clogging caused by grain migration is discussed. Fortunately, methods that regular alternate injection and extraction of flows in the well could be a potentially effective measure. The testing results in this context could facilitate our understanding of the clogging caused by coupled grain migration and compaction effect during groundwater recharge for USR in ground water-source heat pump, and the potentially effective measure could be provided in the field.