Colmatage microbien des forages et circulation de l'eau - Résultats d'un « colmatomètre » expérimental

Un modèle physique est constitué de quatre éprouvettes de sable percolées à des vitesses de flux différentes par L'eau d'un forage colmaté. On a pu ainsi obtenir, en quelques semaines, un colmatage différentiel de deux éprouvettes percolées à des vitesses supérieures à 1 cm/s, et confirmer les hypothèses émises sur l'influence de La vitesse réelle des filets liquides dans L'apparition du colmatage, et sur ta nature bactérienne du processus colmatant. Ce modèle constitue un véritable "colmatomètre" qui a donné lieu au dépôt d'un brevet (BOURGUET et al., 1985). Avant La réalisation d'un champ captant nouveau, L'emploi d'un tel colmatomètre devrait permettre d'apprécier les risques de colmatage spécifiques au site, et de définir Les normes de crépinage et d'exploitation permettant de Les éviter, ou du moins de les minimiser. INTRODUCTION A study on 300 wells drilled in the Ypresian aquifer, over 60 % of which were clogged, revealed that there were significant statistic relationships between the frequency of ctogging, the high velocities of the water close to the well screens and the presence of hydrogen sulfide in the water; the latter means the existence of sulfato-reducing bacterial activity. The experimental model described here was designed to check the effect of the velocity of the water in the aquifer on the creation of clogging, as well as its relationship with the development of the bacterial population induced by a sufficient nutrient flow rate. This model, comprising sterilized sand, through wich water from a clogged well percolated, was set up prior to a model which will soon be built, consisting of cores of Ypresian sand containing its own microorganisms. APPARATUS AND METHODS Four stainless-steel tubes (length 14 cm, inner diameter 26 mm) were filled with sterilized siliceous sand, the initial porosity of which was 35 %. The particle size distribution (40 to 140 µm) and the permeability were similar to those of the Ypresian sand. Tube seeding was carried out for a week by percolation with water from a clogged well; no clogging in the tubes was observed. This water was then percotated simultaneously through the tubes (figure 2) for 58 days at rates, maintained by weekly corrections, close to 1, 3, 12 and 30 mm/s respectively (figure 3). The microbiological analyses consisted in : direct numeration of bacterial bodies by epifluorescence microscopy, indirect numeration by counting revivifiable heterotrophic aerobic bacteria after 15 days incubation