Well Maintenance and Rehabilitation (Session 32-34)

185 Pages·2008·3.03 MB·English

SESSION 32
WELL MAINTENANCE AND
REHABILITATION
I
Dr Amjad Aliewi
House of Water and Environment
Email: [email protected] , Website: www.hwe.org.ps
Objectives
(cid:190)
Well Development has two broad objectives: (1) to repair
damage done to the formation by the drilling operation so that
the natural hydraulic properties are restored, and (2) to alter
the basic physical characteristics of the aquifer near the
borehole so that water will flow more freely to a well.
(cid:190)
All new wells (Figure1) should be developed before being put
into production to achieve sand-free water at the highest
possible specific capacity.
Objectives
(cid:190)
Well development is confined mainly to a zone immediately
adjacent to the well, where the formation materials have been
disturbed by well construction procedures or adversely affected
by the drilling fluid.
(cid:190)
In addition, the undisturbed part of the aquifer just outside
the damaged zone may be reworked physically during
development to improve its natural hydraulic properties.
Objectives
(cid:190)
Well rehabilitation objectives are:
(1) to reclaim existing wells than simply abandoning them to
start over. The main reason usually is sometimes very high cost
of finding a new location (getting it approved), drilling and
testing, and the logistics of powering a pump and piping water.
(2) A well that is failing was put in that particular spot for a
good hydrogeological reason and therefore it is not always
possible to identify a successful alternative site, hence
rehabilitate such wells.
(cid:190)
Older wells often require periodic redevelopment to maintain
or even improve the original yield and drawdown conditions.
Maintaining a high specific capacity assures that the well will be
energy efficient.
Figure 1: Typical set-up of a well
1. Factors That Affect Development
1.1 Well Completion Method
(cid:190)
There are two major completion methods - natural
development and filter packing. In natural development, a highly
permeable zone is created around the screen from materials
existing in the formation. Creation of this zone is best
understood by visualizing what happens throughout a series of
concentric cylindrical zones in a sand aquifer surrounding the
screen.
(cid:190)
In the zone just outside the well screen, development
removes most particles smaller than the screen openings,
leaving only the coarsest material in place. A little farther out,
some medium-sized grains remain mixed with the coarse
sediment.
1. Factors That Affect Development
(cid:190)
Beyond that zone, the material gradually grades back to the
original character of the water-bearing formation. Finer particles
brought into the screen in this process are removed by bailing or
pumping. Development work is continued until the movement of
fines from the formation becomes negligible.
(cid:190)
In filter packing, a specially graded sand or gravel having high
porosity and permeability is placed in the annulus between the
screen and the natural formation.
1. Factors That Affect Development
(cid:190)
By creating this succession of graded zones around the
screen, development stabilizes the formation and prevents
further movement of sediment.
(cid:190)
After development, water moving toward the screen
encounters sediment with increasing hydraulic conductivity and
porosity. Improving the hydraulic conditions around the well will
increase the specific capacity and efficiency.
(cid:190)
Thus, more water can be obtained from the well, and for any
yield the cost of lifting the water to the surface will be
minimized.
1. Factors That Affect Development
1.2 Open Area And Slot Configuration
(cid:190)
All development methods work best in wells equipped with
screens having both maximum open area and the type of slot
configuration that permit hydraulic forces exerted inside the well
screen to be directed efficiently into the formation. Both factors are
equally important in successful development.
(cid:190)
Screen open areas vary typically from a low of 1 percent for
perforated pipe to more than 40 percent for continuous slot, wire-
wound screens.
1. Factors That Affect Development
(cid:190)
Screens with high open area can be developed more effectively
because more of the development energy can reach the formation.
(cid:190)
Slot configuration also controls how much development energy
reaches the formation, and the percentage of the formation that this
energy can affect.
(cid:190)
Thus, more fine material can be removed more quickly if all the
available energy can be directed at most or all of the surrounding
formation.