Below are answers to the questions asked during the Q&A section in the Webinar by Roscoe Moss, an introduction to Well Casings and Screens.
Q: Any additional considerations that should be taken into consideration when installing casings for geothermal well use compared to normal water wells?
A: Generally speaking, for deep geothermal wells constructed with a series of intermediate casings one should take into account the collapse forces induced on each casing string as cement is pumped in the annular space between them. The cement placement should be done in a continuous uninterrupted manner so as to avoid joints in the seal.
As such the casing at depth is required to have sufficient collapse strength to remain intact until the cement sets. Literature also suggests incorporating use of blowout preventers during the construction of geothermal wells in order to control unexpected releases of high pressure and temperature fluids.
Q: Do you think about considering dynamic forces on screens?
A: Absolutely, the dynamic forces imparted on well screens must be accounted for when designing the intake section of the well. Similar to the casing, the collapse and tensile strengths of screen must be calculated in advance of the installation to ensure the screen is capable of withstanding the stresses to which it will be subjected during the construction process.
There are a number of variables that contribute to the collapse and tensile strength of the screen. Those variables need to be identified in order to utilize them in the appropriate equations when solving for collapse and tensile strength for the particular type of screen you’re intending to use. Unlike steel casing or pipe, there are no standardized equations for determining the physical strength of well screens.
For our well screens, Roscoe Moss Company provides the tensile and collapse strength values derived from formulas developed by engineers recognized for their expertise in fluid dynamics and material performance. These formulas and calculators that utilize them are available on our company website (roscoemoss.com< technical resources ) and our free App (search under Roscoe Moss or RoscoeMoss).
Q: When it comes to iron precipitation or biofuling and resultant efficiency drops, is there a preferred screen design that is less susceptible to drops in efficiency from bacterial clogging?
A: All screens will be negatively impacted if biofouling occurs and none are excluded from losing efficiency when the openings are clogged by bacterial byproducts. This includes wire wrap, louvered, and slotted pipe that our company has either manufactured or provided. Well designers and owners should understand that in severe cases, bacterial problems are to be managed over the lifetime of the well.
Therefore it is critically important to select a well screen possessing an internal geometry that ;
#1, does not provide areas where the bacteria can attach themselves and proliferate, and
#2, has a smooth profile so that snug fitting rehabilitation tools (i.e. swabs) can make full contact with entire interior of the screen surface
It is important that the screen not only have these qualities but also be durable enough to accommodate rigorous physical and chemical development methods. Of the common screen types most commonly used today, the louvered screen provides the best option for wells constructed in areas of known bacteria-related problems because of its unobstructed interior that provides less opportunity for bacteria to adhere and its durable construction which makes it adaptable to the most aggressive rehabilitation methods .
Q: You said (if I understood it well) that the bridge slot screen type is not much used in the USA these days. Which are the reasons?
A: Compared to other perforated screen types bridge slot screen has been used on a very limited basis in the U.S. Most recently, bridge slot screen was primarily used for construction dewatering and not for commercial, industrial and municipal drinking water wells.
Another main reason was that the last remaining company manufacturing the bridge slot screen ceased operations several years ago. The dewatering and well industry now finds most of its needs are satisfactorily met with currently available screens such as louvered, wire wrap and slotted pipe.