Development of water wells

WATER WELLS
DEVELOPMENT OF WELL

To avoid damages and failures of well, well development matters a lot. The main objectives are

1. To clear mud cake formed on bore well wall against permeable zone
2. To clear invaded permeable zone and to remove material which give problem to yield
3. To minimize the skin effect and maximize well efficiency and specific discharge (Q) of the well.

Development Methods

Following two methods does the development of wells

1. Dispersed technique in which development forces allowed to act on the entire screen length at one time

2. Concentrated method in which the force is opposed on a reduced section of the screen at one time


The general dispersed methods are over pumping backwashing, mechanical surging and air development by surging and pumping. The concentrated methods are air surging and lifting and high velocity water jetting and airlifting.

Use of Chemicals

Using chemicals for well development in another efficient process. Most commonly used chemicals are polyphosphate- act as deflocculates and dispersion agent for clays and removal of mud cake. Acids which dissolve acid soluble parts of the formation, permits higher flow rate of water in to the borehole.

Development of Hard Rock Wells

The best technique in chemical treatment followed by air surging and lifting. These are aquifer development techniques in hard rock area where acids are used. Explosives are used hydro fracturing is also most effective to the aquifer yield.

Maintenance And Rehabilitation

The major components of a well maintenance are maintenance of the well and pump for that logbook and history book should be properly maintained to respective wells.

Pump Maintenance

Following points should be taken care of in pump maintenance. If yield falls about 10-15%, pumped should be examined. Any defect in pump observed, intermediate rectification should be done. Well bottom should be clear from filling well.

Well Sickness

The general types of well sickness are,

1. Sand pumping
2. Decrease in yield
3. Deterioration of quality.

Sand pumping due to various reasons like bridging of gravels, improper design, improper selection of gravel, inadequate development high rate of pumping, rapture in screen existence of gases in aquifer zone etc so for this proper understanding of the problem and proper remedies should be taken like lowering proper screen, proper gravel size etc if the problem is due to improper selection of screen and gravel.

Decrease in Yield

In some case, after the proper development, the yield of the well will be very low this is because of the change in regional hydro-geological parameters, for example, in cretaceous growth of bacteria, erosion structural failures or interference of another well nearby. For this hydro fracturing and chemical treatment, mechanical brushing, etc., needed.

Deterioration of Quality

Due to the contamination in surface water, inadequate flushing of aquifer, growth of bacteria etc. For this prevention measures like prevention of seepage of surface contamination and promotion of sufficient recharge should be done.

Ground Water Flow Equation

When the fluid mass flows through various porous medium it will follow the physical property and nature of media. With the combination of Darcy’s law and equation of continuity, we can describe the conservation of fluid mass during flow through porous medium

CONFINED AQUIFER

Steady State Saturated Flow

Here the flow is taking place in all the three directions. Hence the rate of inflow any elemental control volume is equal to the rate of out flow elementary control volume. Hence for the steady state flow through isotropic and homogeneous medium is given by following equation

This is also called as Laplace’s equation

Transient Saturated Flow

The main principle for the flow equation is the rate of flow into any elemental control volume is equal to the time rate of change of fluid mass storage within the element



In special case of horizontal confined aquifer of thickness b, S = Ss b and T = Kb, then the above equation simplified as



UNCONFINED AQUIFER

Transient flow in unconfined aquifer

Here the flow distributions govern by the water table shape. To find a solution Dupit’s given two assumptions

Flow line are horizontal, equi potential lines are vertical
The horizontal k is equal to the slope of the free surface and is invariant with depth



Solution for Ground Water Flow Equation

To solve the flow equation either analytical or numerical methods are used. In analytical methods the actual filed conditions are so complex, it becomes to obtain solution, whereas the numerical solutions are much more versatile and with widespread availability of computer, they are much easier to use than complex analytical methods

In general numerical methods, such as the finite element method (FEM) and Integrate finite difference method (IFDM) are most commonly used.

While solving the groundwater flow equation is more real time problem, boundary condition have to be considered. Basically three-boundary condition, such as variable head boundary, constant head and no flow boundary exists.


Application

Ground water is used to understand the quantities of flow and its direction. The main application is flow net analysis, water balance study, and Ground water flow modeling planning of ground water management strategies.

ELASTICITY AND COMPRESSIBILITY OF FORMATION

Rocks possessing void space are susceptible to volume change in response to external force (load) acting on it. The compressibility of the rock is expressed in term of bulk modulus of elasticity.


E - Bulk modules kg/m2
r - Mass in metric slugs/cm2
p - Pressure in kg/m2
V - Volume in m3

The reciprocal of bulk modulus of elasticity is called compressibility


It always expressed in m2/kg. In general b is individual grain in negligible and granular rocks with high porosity are more compressible than dense rocks

Relation between b and storage coefficient

In confined aquifer, change in the pressure head reflects changes in the pressure exerted on the aquiclude and the resulting elastic change in the aquifer system. The force at contact of confining layer artesian aquifer may be expressed as

Pa = Pw + ps

Pa - Total load exerted on a unit area of aquifer
Pw - Part of total load born by the confined water
ps - Part of total load born by the structural skeleton of the aquifer

During pumping in a confined aquifer water is discharged due toe the expansion of water and compression of rock formation. Conversely when pumping stopped, the pressure head builds up gradually due to the transfer of land water itself undergoes slight contraction.

Effect of Elasticity of Confined Aquifer on Water Level in Wells

In confined aquifer, changes in external load on them is reflected as variation in water levels in the wells, which are tapping them


Atmospheric Pressure

In confined aquifer, when the increase in the atmospheric pressure produces decreases in the water level and conversely. This is expressed as Barometric efficiency, which is



The barometric efficiency is interpreted as a measure of the competence of overlying confining bed to resist pressure changes.

Tides

The responses to the tides are recorded in sinusoidal fluctuations. Contrary to the barometric effect, in the case of ocean tides, as the sea level rises, ground water also increases. Tidal efficiency C is related it the barometric efficiency B by

C = 1 – B

Earthquakes

Earthquakes shocks produce small fluctuations, hydroseisms, in well penetrating confined aquifer. The passage of seismic wave through the confined layer of aquifer resulting compression and expansion of layer. Hence fluctuations appear after little more than one hour even from the most distant earthquake centers.

External loads

Changes in loading results in Change in hydrostatic pressure in confined aquifer because of its elastic property. For example in wells located near railway lines, passing train produce measurable fluctuations of the piezometric surface.