| dc.description.abstract | Minimizing well clogging by particles is considered one of the most important aspects of well design. Well design standards restrict the attraction of particles by limiting the flow velocity at the borehole wall. The corresponding limitation of the well capacity is especially problematic for the feasibility of projects in aquifers consisting of fine sand. To investigate the possibility of extending the current design standards, the real flow velocities in a number of wells were analyzed, using data from vertical flowmeter logs. Groundwater models were used to deduce a simulated flowmeter log. Comparing the simulated flowmeter log to the measured flowmeter log, provides information on the performance of different parts of the well screen. For a number of wells, the measured and simulated flowmeter logs agree quite well, which gives confidence in the approach. In a number of other wells parts of the well screen did not contribute or appeared to underperform significantly (in most cases the lower part), which is in accordance with experiences from contractors that perform vertical flowmeter logs. Since underperformance of one part of the well screen must be compensated by the other part, design flow velocities are more likely to be exceeded in the other part in that case (especially in the upper screen section). Design exceeding in part of the well screen could not be correlated with sand production. It is more likely that sand production is caused by the (unintentional) screening of sand layers that are too fine in comparison to the grain size of the gravel pack.
Possible causes for an uneven flow distribution were investigated. Partial well screens produce more water from the end of the well screen than from the middle part, because water can approach the well screen from more directions (i.e. laterally and vertically). The potential influence of head losses due to friction is usually minimized by choosing a sufficiently large well screen diameter. The most important reasons for underperforming well screen sections appear to be differences in the grade of clogging of the concerning part of the borehole. These differences are initiated during the procedure of well construction and well development. Due to an increase in the overpressure with depth inside the borehole during the drilling procedure (density of drilling fluid with suspended sediments is relatively high), more persistent filter cakes are formed in the deeper parts of the well. Well development techniques that treat the whole well, are therefore likely to be more successful in the upper part. This is enhanced when extreme flow velocities occur during well development, leading to significant head losses due to friction and thereby reducing the effectiveness in the lower part of the screen. When partial removal of the filter cake is achieved, preferential flow paths are created which subsequently complicate the treatment of other parts of the borehole. This can explain why techniques like overpumping and surging tend to be more effective in the upper part of the well screen, especially when long well screens are concerned. Wells in aquifers consisting of fine sand seem to be more likely to end up with an uneven flow distribution than wells in coarse sand. In one well, sectional surging proved to be effective to treat the underperforming lower part of the well screen, suggesting a more frequent use of this technique. | |
