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.

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Q2.   What is the most effective well development technique and is its relationship to screen open area?

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 process.

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.

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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.

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.

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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.

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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 operation.

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.

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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.

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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 (d50) size is then multiplied by a factor of 4 to 6 to establish the d50 of the gravel pack.  Through the points defining the 4x d50 and the 6x d50 two 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 d50 of the formation.

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References

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