Visitors to this section are encouraged to contact Roscoe Moss
Company representatives directly for additional information regarding this
section. Readers are also encouraged to examine these issues and are invited
to contact us if a discussion or clarification is desired.
Q1. What is the
relationship between open area and well efficiency?
A1. This long contested issue is
best addressed by examination of the results of experiments, aquifer model tests,
and actual well performance comparisons where discharge rates and drawdown levels
were measured on a variety of well screens with various percentages of open area.
Although conducted at different times and at different locations, the tests and
comparisons made by professionals in the ground water and petroleum fields arrived at
similar conclusions. The results of the tests clearly show that screens with
open areas of (1%) perform as well as screens with greater open area (36%).
Results further indicate that the major contributor to well inefficiency
is not the amount of screen open area but the near well losses caused by inadequate
repair of the damage zone or incomplete removal of mud and fine-grained materials
during development. The conclusions of these tests stress the importance of
proper gravel pack and slot size selection combined with thorough well development
techniques. These factors will contribute far more to the overall efficiency
of a well rather than simply increasing the open area of the screen.
Q2. What is the
most effective well development technique and is its relationship to screen
A2. Prior to drawing any immediate
conclusions about the relationship between open area and development, one must
examine the objectives of well development, determine and utilize the most effective
method, and establish a minimum requirement in terms of completeness.
The objectives of well development are; (1) to remove the drilling fluids and
fine-grained materials from the well and near well zone and; (2) to repair the
damage zone (the boundary between the well bore and the water-bearing formation)
which may have been invaded by fine material and drilling mud during the drilling
Development methods that are commonly employed include air or
water jetting, single or dual swabbing, and dual swabbing with simultaneous
airlifting. Of these, dual swabbing with simultaneous airlift method is the
most effective. Mathematical analysis and laboratory models have confirmed
this. The advantages of dual swab-airlifting development are two-fold.
Snug-fitting swabs are able to direct energy beyond the screen
through the gravel envelope and ultimately to the pack/aquifer interface and to
the damage zone. Secondly, the simultaneous airlift removes particulates from
the well and more importantly from gravel envelope and aquifer. Without
removal, these particulates will clog the filter pack thereby reducing its hydraulic
conductivity and causing poor well efficiency.
Considering development effectiveness and open area, the
interior structure of the well screen needs to taken into account. Which
screen type can best accommodate the dual swab airlift method? Because
the swab requires a snug fit within the full interior circumference, a screen having
smooth walls with no obstructions, such as shutter screen, is best suited.
The effectiveness of the method is limited in a continuous wire-wrap screen due to
the internal array of rods. The preferred method for development of wire wrap
screens is jetting. Jetting has been shown to be effective in cleaning the
interior of wells but tests have shown the energy of the water jet to dissipate due
to turbulence within 1 to 2 inches beyond the screen. The typical thickness
of a gravel envelope is 4-5 inches, therefore complete development of the envelope
and repair of the damage zone is uncertain.
To summarize, there is not a simple direct relationship between
screen open area and development effectiveness. The effectiveness of any
development program is governed primarily by the development method employed,
how it is applied and the interior structure of the screen it is used in.
Q3. What studies
have been done that demonstrate the relative corrosion resistance of common casing
and screen steels?
A3. A recent study was conducted by
Geoscience Support Services that compared the corrosion rates of 5 commonly steels
used in the manufacture of well casing and screens. The steel types included
316L and 304 stainless steels, corrosion resistant, high-strength, low-alloy steel
(ASTM A 606 Type 4), copper-bearing steel, and mild steel. Coupons prepared
from each of the steels were placed in an inactive production well which had been
removed from service due to problems related to corrosion. Three sets of
coupons were placed in the well and were removed at various time intervals over an
eleven month (1998 - 1999) test period. The coupons were then weighed and the
corrosion rates for each type were calculated. The test results are
shown on the table below along with relative cost comparisons.
* Steel costs represent well screen cost.
316 L stainless
ASTM A 606 type 4
The results of this study certainly verify the increased
longevity of steels designed for corrosion resistance in actual water well
applications. General conclusions have been drawn from studies of intake
structures and partially submerged culverts. The results of those studies
have merit for these particular environments in which they were tested, however
they should not be accepted as evidence for behavior under all conditions.
Q4. Are there any well
designs that can mitigate well structure damage caused by subsidence?
A4. One successful design to
mitigate structural damage to the well incorporates the use of a of a specially
constructed compression section installed in the upper blank cased section of the
well. The compression section consists of three sections of blank casing with
the center section being 2" larger in diameter. With special couplings
holding the sections together, the two end sections can slide within the center
section. The entire compression section is approximately 18-feet long and can
be made in any diameter up to 24 inches. The unit is typically placed at
the depth where subsidence is likely to occur. Under for these conditions,
compression sections have adjusted to the additional collapse and tensile pressures
exerted by the subsiding geologic units surrounding the well. Specific
information regarding the construction and application of the compression section
can be obtained by contacting the Roscoe Moss Company 's Los Angeles office.
Q5. Can a well be
designed to utilize water from a shallow water table or from a water-bearing zone
prone to falling water levels without exposing the well screen or inducing cascading
water with entrained air?
A5. The casing path well was
designed over twenty years ago to address these problems. Unlike a
standard well, the casing path well is designed with a sealed screen chamber that
permits the extraction of water from a perched water zone or aquifer that
experiences fluctuating pumping levels where the screen section may become
exposed. During pumping a partial vacuum is created in the upper sealed screen
chamber. Water enters the chamber and flows downward between the well casing
and chamber wall and enters the inner production casing through windows placed
below the lowest estimated pumping water level. Because of the partial vacuum
a water level surface is created in the screen chamber which is higher than the
pumping level in the production casing. Since the water level in the screen
chamber is higher it means the drawdown impacts are lessened in the shallow zone
thus less more water can be safely produced from these zones without the problems
associated with cascading water and entrained air. The primary problems being
increased corrosion on the well casing and pump as well as inefficient pump
Several hundred casing path wells have been successfully
constructed and operated in California's Central Valley and in agricultural region
in west Texas. In each instance, the well owner achieved higher ground water
yields with casing path wells compared with wells of standard construction.
Due to the fact that the wells will produce at higher rates, the owner realizes an
immediate cost benefit and considerable savings by avoiding construction of
additional wells and pumping facilities.
Q6. What water
quality parameters are the most important indicators of corrosion or incrustation?
A6. The most important water
quality characteristics that influence rate of corrosion are dissolved gases,
particularly dissolved oxygen. Other common gases that can contribute to the
corrosion of steel are carbon dioxide and hydrogen sulfide. An increase in
the amount of dissolved oxygen increases the rate at which oxygen is transported
to the corroding metal surface. Most metals will exhibit an increase in
corrosion as the dissolved oxygen content increases up to 20 to 25 mg/l.
Above this level the increased oxygen content can promote passivation of the metal
thereby reducing the corrosion rate.
Carbon dioxide does not directly cause a corrosive reaction, but
it reacts with water to form carbonic acid. The carbonic acid in turn lowers
the pH creating conditions favorable to corrosion. Another effect of lowered
pH is increased solubility of calcium carbonate, which is the main compound
associated with well incrustation. Similar to carbon dioxide, hydrogen sulfide
does not by itself cause corrosion. Sulfide deposits will promote galvanic
attack, characterized by localized pitting, due to the electrical potential that
exists between iron sulfide and steel.
With regard to incrustation the most important chemical compound
as an indicator of scale formation potential is calcium carbonate. Chemical
indicators for the production of calcium carbonate would be a predominance of
calcium and carbonate. Waters saturated with calcium carbonate will tend to
form scale. These scales have been associated with plugging of gravel packs
and screen slots which results in lowered well efficiency.
Two indices that are commonly used to predict the tendency of
waters to be either corrosive or scale forming based on calcium carbonate saturation
are the Langelier Index (LI) and the Ryznar or Stability Index (RSI). These
indices are calculated by the relationship between the pH of the water to the pH of
the water saturated with calcium carbonate (pHs). For the Langelier Index
where LI = pH - pHs, negative values denote the water will dissolve calcium
carbonate and will be corrosive to steel in the presence of oxygen. Positive
values denote the water is supersaturated with calcium carbonate and is more likely
to form scale.
The Ryznar Index is a modification of the earlier derived
Langelier Index following the study of scaling and corrosion conditions in various
municipalities. For the Ryznar Index, where RSI = 2pHs - pH, values above 6.0
denote the water is corrosive and scale forming when the value is below 6.0.
According to NALCO, the Langelier Index is most useful in predicting corrosive or
scaling tendencies in a bulk system (where flow velocity is slow), such as a
reservoir or water treatment apparatus. The Ryznar Index is more hypothetical
and should be applied only to flowing systems, where the environment at the pipe
wall is quite different from that of the bulk water.
Q7. What are the
recommended spatial relationships between formation, gravel pack and slot size?
A7. Selection of the proper gravel
pack and screen slot size is one of the most crucial steps in designing water wells
with high efficiency. Although several gravel pack gradation selection
criteria exist, two of the most common techniques will be discussed here.
Basic Gravel Pack / Slot Size Selection
One of the more basic procedures involves performing a sieve analysis on
representative water-bearing formation samples and selecting the finest aquifer
gradation for determining the pack gradation. Using a typical sieve analysis
form, the 50% passing (d
size is then multiplied by a factor of 4 to 6 to establish the
50 of the gravel
pack. Through the points defining the 4x d
and the 6x d
curves are drawn which parallel the formation gradation curve. The percentages
passing of the recommended gravel pack are determined where the curves intersect the
lines corresponding to the standard sieve sizes. The gravel pack that is
selected should ideally fall within the two curves and be commercially available.
Slot sizes for gravel packs that have been determined by the
aforementioned method can be selected using a recommended 10 - 20 percent passing
of the pack at that opening.
Simplified Gravel Pack Selection
A simpler method for selecting gravel pack gradation involves matching the formation
gradation to a common gravel gradation and appropriate corresponding slot size.
The following table summarizes the pack gradation and slot size recommendation based
on the d
50 of the formation.
d50 Size (in)
Recommended Gravel Pack
Slot Size (in.)
0.094 - 0.125
0.020 - 0.030
0. 005 - 0.020
B with 20% 12 - 20 (where
100% passes #12 and 100% retained on #20)
0.050 - 0.063
Gradation Gravel A
U.S. Standard Sieve No.
85 - 95
25 - 35
5 - 20
2 - 10
Gradation Gravel B
U.S. Standard Sieve No.
95 - 100
70 - 80
15 - 25
0 - 5
Roscoe Moss Company. 1990. Handbook of Ground Water Development.
John Wiley & Sons
Williams, D.E. 1999. Corrosion Field Test of Steels Commonly Used in Casing and Screen.
Geoscience Support Services, Inc.
Jackson, P.A., Bikis, E. A., Ahmad, M.U. Laboratory and Field Studies of Well Design and Efficiency
List, J.E. PhD. Analysis of Development Methods for Gravel Envelope Wells
Williams, D.E. Modern Techniques in Well Design
Williams, D.E. Analysis and Comparison of Wells in the Pleasant Valley Area of Ventura County, California