Contents preface (VII) introduction 1—37

Fig. 4.13 Filter packing of wells
4.11. DEVELOPMENT OF WELLS
Drilling operations for well excavation change the hydraulic characteristics of the formation materials in the vicinity of the borehole. Very often, these changes result in the reduction of the hydraulic conductivity close to the borehole. When a well is drilled with a cable tool rig equipped with a casing driver, the repeated blows on the casing rearrange the grains in the vicinity of the casing. In rotary drilling methods, the drilling fluids containing clay may flow into the aquifer for some distance and thus plug the pore spaces of the permeable formation. Before commissioning the well for use, it is, therefore, necessary to repair the damage done to the aquifer by the drilling operations. Besides, there is also a need to improve the basic physical characteristics of the aquifer in the vicinity of the well screen so that water can flow more freely into the well. A well is, therefore, ‘developed’ in order to attain these two objectives, and thus, maximise well yield. Well development involves applying some form of energy to the water-bearing formation in the vicinity of the well so as to remove fine materials (including drilling mud) from the aquifer and rearrange formation particles so that the well yields clear sand-free water in maximum quantity with minimum drawdown. Well development serves the following beneficial purposes :

(i) It increases the permeability of the aquifer material surrounding the well and filter pack (if present) by:

158 IRRIGATION AND WATER RESOURCES ENGINEERING
(a) reducing the compaction and intermixing of grains of different sizes during drill-ing by removing fine grains,
(b) removing the filter cake or drilling fluid film that coats the borehole, (c) removing much or all of the drilling fluid which has entered the aquifer, (d) breaking sand-grain bridging across the screen openings, and
(e) increasing the natural porosity of the previously undisturbed formation near the borehole by removing the finer fraction of the aquifer material.
(ii) It creates a graded zone of aquifer material around the screen in a naturally devel-oped well. This effect stabilises the formation so that the well will yield sand-free water.
(iii) It reduces the head loss near the well screen. (iv) It increases the useful life of the well screen.

(v) It brings the well to its maximum specific capacity, i.e., the maximum yield at mini-mum drawdown.

The methods usually adopted for well development are as follows (6): (i) Overpumping,

(ii) Backwashing.

(iii) Mechanical surging,
(iv) Air surging and pumping, (v) High-velocity jetting, and

(vi) High-velocity water jetting combined with simultaneous pumping.

There are several variations of most of these methods (16). Only the main features of these methods have been described in the following paragraphs.