My sand analysis results and question for slot/gravel pack.

Discussion in 'Pumps and Tanks Well Forum & Blog. Water is life.' started by jd87n, Oct 10, 2018.

  1. jd87n

    jd87n New Member

    Joined:
    Oct 1, 2018
    Location:
    Portland
    Thank you to everyone that has helped me in the past. Based on the recommendations I sent a sand sample to a screen company and they told me that a casing pipe with a .020 screen with 16x30 Colorado Silica gravel pack or equal would be the ticket to keep sand out of my well.

    Couple questions:

    1. How much annular space around the casing pipe should I have to fill in the this 16x30 Colorado silica gravel pack?

    3. If I don't go with a casing pipe and I hammer a sand point in the soil, should the sandpoint slot size be the next size down? My theory here is that the sand point doesn't have the extra filtration of the Colorado silica pack it would need a tighter screen to keep the sand away. Or is it possible that I am overthinking this and the .020 size will keep sand out.

    Im uploading my sample results for anyone that is interested and Silca pack graduation curve.

    Also, here is a brochure that I found
    regards to silica packs.

    http://premiersilicasf.blob.core.wi...es/water_well_gravel_pack_design.pdf?sfvrsn=4

    Here is the Sample results of my test:

    sand graph.png in
     
    Last edited: Oct 10, 2018
  2. jd87n

    jd87n New Member

    Joined:
    Oct 1, 2018
    Location:
    Portland
    Forgot to upload the Silica graduation curves.

    col.jpg
     
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  4. VAWellDriller

    VAWellDriller Active Member

    Joined:
    Sep 8, 2012
    Occupation:
    Mud rotary well driller, pump installer
    Location:
    Richmond, VA
    Every well I drill is screened and gravel packed. To answer your questions:

    I stick with 1.5 -3" annular space.....1.5" would be fine for what you want to do.

    Yes, if you do not install artificial gravel pack, and drive a point, it is common to use a smaller slot size. In a gravel packed well, the screen is designed to work with the gravel pack grain size....In naturally developed well, the screen must match the formation grain size. I would ask this question to the folks that did your sand analysis...they will likely have 2 different slot recommendations. If you do not install gravel pack, I would get stainless wire wound v slot screen....it is much less likely to get blinded by sand.
     
  5. jd87n

    jd87n New Member

    Joined:
    Oct 1, 2018
    Location:
    Portland
    Thanks for the response.

    As far as the well point goes they did give me another size after I asked. The response was interesting and something I didn't think about. Their theory was that the well point with no gravel pack or a well that you naturally develop should have a screen that retains 40% of the material. At first I thought they were crazy and didn't understand why I would want 60% of sand coming in to the well and only 40% of the sand from being blocked.

    But then it hit me. The 40% of the material that is being blocked is going to form a natural gravel pack over time around the screen. This natural gravel pack is going to let more water flow and eventually block the 60% finer particles. All this time I was just thinking about going crazy and getting the smallest screen that would block absolutely everything from day 1. (Not thinking about also restricting water flow)

    The break down turned out to be.

    "When designing a screen, the rule of thumb is to select a screen slot size to retain about 90%" (Casement screen)

    That turns out to be for me:

    dug casement:
    .025 slot screen with a 16x30 pack of silica sand will retain 90% (Liberal)
    .020 slot screen with a 16x30 pack of silica sand will retain 98% (Med)
    .016 slot screen with a 16x30 pack of silica sand will retain 100% (Conservative)

    Sand point:
    .008 40% retention.

    Again these are based on my particular natural sand size in my soil. If someone is reading this you can look above at my charts and if you know your sand size make a guess if these numbers match.

    Like I said this was a real eye opener as I was to focused on restricting 100% of the sand in my well when talking about a sandpoint and not using the sand thats there as a natural gravel pack. Now I just will need to be patient creating my natural gravel pack on the sand point well and getting the correct casement for the one I dug. Not many people seem to like to sell casement screens to Johny home owner.
     
    Last edited: Oct 11, 2018
  6. valveman

    valveman Cary Austin Staff Member

    Joined:
    Mar 15, 2006
    Occupation:
    Pump Controls Technician
    Location:
    Lubbock, Texas
    Also remember the smaller the slot and gravel size the more it restricts the flow. A well with big slots and pea gravel will make sand but might produce 30 GPM. Where a small slot and gravel pack will prevent any sand from entering the well, it may drop the production rate to 5-10 GPM. The trick is to find the largest slot and gravel that will still prevent the sand from entering the well.
     
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