Well Casing

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Part 2. Casing materials standards and strengths.

By Thom Hanna, PG, and Mike Mehmert

We covered basic nomenclature in part one of this two-part series in the September 2023 issue. So, now that we are all talking about the same thing, let’s look at the standards and strengths of various common casing materials.

Standards were developed to ensure that when purchasing and using materials, those materials meet certain universal minimums.

Pipe materials used for constructing water wells are manufactured to the standards developed for two major pipe markets: the water, chemical, and gas transmission markets by the American Society for Testing Materials (ASTM standards) and the oil and gas industries by the American Petroleum Institute (API standards).

Most wells constructed today in North America reference specifications conforming to ASTM or API standards which cover such properties as: type of materials (steel or PVC); chemical composition; tensile and bending variations in composition, dimensions, weights requirements; pipe weights; end finish; coatings; and quality control (Table 1).

Selection of Casing Materials

Selection of casing materials is typically based on water quality, depth, well diameter, drilling method, costs, and project compliance requirements.

The most common water well casing materials are low carbon steel (LCS) which include low carbon steel and high-strength low alloy, thermoplastic PVC (PVC), and stainless steel (SS). Type 304 or 316 stainless steels are commonly specified for municipal wells for long service life.

The addition of chromium and nickel (for 304) and chromium, nickel, and molybdenum (for 316) to regular steel produces SS grades resistant to most water well environments. Occasional duplex stainless steels are used in very corrosive environments.

Recently, PVC casing has become more popular in well applications where its strength properties meet requirements, and its superior corrosive resistance and ease of handling are attractive.

However, there can be site-specific conditions, like the necessity of driving or pulling back casing or geothermal applications, that can prohibit using PVC. A final material consideration is traceability.

There can also be requirements for materials’ traceability as to country of origin or specific chemical composition, depending on the project scope and funding. While these conditions are not typical, they should be identified and understood up front to avoid frustration, unforeseen time requirements, possible downtime, and worst of all, fines or penalties.

Well Casing Strength

Well depth and diameter will directly influence the tensile and collapse strength requirements needed for a specific application. PVC pipe, for instance, may be ideal for a corrosive environment, but might be excluded due to depth or diameter requirements because PVC has approximately one-fifth the yield strength of steel pipe. PVC has a yield strength of 7000 psi vs. mild carbon steel that is 35,000 psi (Table 2).

Casing wall thickness must be sufficient to withstand full hydraulic loading if the casing is pumped dry. This is especially important when estimating differential loading during cementing operations.

Always remember with any casing material to include adequate safety factors (typical minimums are 50% to 100%). Especially keep in mind that there can be additional dynamic forces applied to the casing string during installation and development that are difficult or impossible to accurately quantify.

The most widely used collapse formula used to calculate steel casing design collapse is the Timoshenko equation:

where:

Pd = design collapse (psi or kPa)

Yp = material yield strength (35,000 psi or 244,000 kPa for steel, 30,000 psi or 207,000 kPa for stainless steel)

D = outside diameter of casing (in or cm)

t = wall thickness (in or cm)

e = eccentricity constant (typically 1%)

Pcr = perfect cylinder collapse (psi or kPa) (Equation 9.2)

E = Young’s modulus (2.1 × 108 kPa or 3.0 × 107 psi for steel; 1.9 × 108 kPa or 2.8 × 107 for stainless steel)

u = Poisson’s ratio (0.28 for steel, 0.3 for stainless steel)

E and u are used in Equation 9.2 which is used to estimate the collapse of a perfect cylinder Pcr, which is part of the Timoshenko equation:

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It is not widely published but casing wall thickness can vary by as much as 12.5% and still meet manufacturing standards. This is most commonly realized with stainless steels due to the higher cost per pound.

A reduction of 12.5% in nominal wall thickness can result in more than a 25% reduction collapse. This can especially be an issue during cementing. Table 3 illustrates the potential collapse reduction due to wall thickness variation.

As manufacturing processes have become better through the years, casings are being made closer to the minimum acceptable wall thicknesses, so be aware of this and account for the thinner wall casings. We have seen casings collapse at the exact minimum wall thickness in some deep wells where the pressure differential between the grout in the anulus and the fluid inside of the well exuded the minimum collapse strength.

There are two published formulas for calculating PVC collapse. The Kurt equation yields results approximately 25% less than the ASTM equation as it makes allowances for eccentricity and wall thickness variation from nominal wall vs. minimum wall.

This conservative approach is a good first cut at design requirements.

(ASTM equation) Pd = 2E / [ (1 – u²) (OD/t) (OD/t – 1)² ]

(Kurt equation) Pd = 0.75 2E / [ (1 – u²) (OD/t – 1)³ ] where:

Pd = design collapse no safety factor

0.75 = empirical allowance for dimension variations

E = Young’s modulus as 4 × 105

u = Poisson’s ratio as 0.4

OD = outside casing diameter in inches

t = wall thickness in inches

Casing Diameter

The first place to start when designing a well is to determine what size pump you will need. It is impossible to put a 12-inch pump bowl in an 8-inch well. The size of the pump to be installed is typically the primary determining factor and a rule of thumb is that the casing PS should be 2 pipe sizes larger than the pump (Table 4).

Adequate annular clearance between the pump and casing ID is particularly important with submersible pumps for adequate motor cooling. Pump manufacturers can provide annular flow rate requirements for a specific motor horsepower.

Additional considerations for casing diameter could come when utilizing a telescope design whereby the production liner telescopes inside a surface or intermediate casing string requiring allowance for completion tooling, packers, or the possibility of future retrieval.

Casing Connections

Casing connections vary depending on how the contractor likes to handle the casing and other well completion considerations. Typical end fittings are plain end, plain end-beveled, threaded and coupled, and slip collars.

Threaded and coupled is typical in smaller diameters. Care should be taken when considering threaded stainless steel for larger diameters as the material can be easily cross threaded without adequate tooling.

Most large diameter water well casings are welded using beveled pipe or slip couplings, so ovality and end finish beveling should meet standards. Welded casings also require slip couplings or lifting lugs for proper handling. Care should be taken to align lifting lugs to assure even weight distribution and safe handling.

Always make allowances for actual OD of any connections, external centralizers, baskets, or tubulars (permanent or temporary) when making final borehole size determinations or ID clearances in the event of a telescoping installation.

As always there can be site-specific situations which prohibit normal field procedures from being followed. One example would be casing and screen installation in a horizontal collector well where the individual length of pipes is often less than 10 feet and fittings need to be made so that they are not welded in the field or threaded.

While these may be infrequent, they must be understood to properly evaluate cost estimates of labor and additional fittings.

Conclusions

It’s still true today that the devil is in the details, so always be thorough in evaluating site conditions, regulatory requirements, technical specifications, and documentation needs when undertaking your next project.

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Thomas M. Hanna, PG, is a technical director of water well products/hydrogeologist for Johnson Screens where he works in areas of well design, development, and well rehabilitation. He is a registered professional geologist in Arizona, Kentucky, and Wyoming and has worked for several groundwater consulting firms. Hanna can be reached at thom.hanna@johnsonscreens.com.

Mike Mehmert is the former director of sales and marketing-well products (North and South America) for Johnson Screens. He was honored with a 2019 Life Member Award from the National Ground Water Association and was The Groundwater Foundation McEllhiney Lecturer in 2010. His professional career included consulting, contracting, and manufacturing.

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