Iron precipitation clogging of a recovery well following nearby deep well injection

摘要

A deep well injection experiment was conducted at a pilot plant in the Western Netherlands, using river Rhine water pretreated to various degrees, 2 injection wells, one monitoring well and one distant recovery well. The latter suddenly clogged after 728 days of operation, without noticeable build-up of hydraulic resistance in advance. Before clogging there was even an unexpected drop of the resistance at the bore hole wall, and a sharp increase of the resistance at the well screen. This could indicate that material which plugged the bore hole wall, for a yet unknown reason, suddenly remobilized and clogged the well screen. Chemical analyses of the clogging material revealed that iron(hydr)oxides (Fe(OH)_3 or ferric hydroxiphosphatcs (Fe_n(HPO_4)_m(OH)_(3n-2m)) prevail, and that hydroxyapatites (Ca_5(PO_4)_3OH) may have formed accessory deposits. Iron(hydr)oxide coatings and filamentous iron bacteria were detected by camera inspection, at the inner screen wall. The most probable cause of clogging is the mixing of (sub)oxic injected water with both anoxic injected water and deep anoxic native groundwater, around and in the well.