It’s important to be exact and get what you need.
By Gary Shawver, MGWC
I’ve focused on grouting of water wells in a couple of my last columns.
I’ve given an overview of the benefits of grouting wells vs. ungrouted wells, and outlined there is a place for neat cement as well as one for bentonite-based grouts.
A while back I saw a long online discussion focusing on various grouts. It’s clear this topic has a lot of interest in the drilling industry. With that, I want to go over what my company has found using neat cement.
One of the topics discussed online was how to seal a geothermal re-injection well. For those of you who may not be familiar with this term, first off, a geothermal injection well is water returned to an aquifer from a water supply well after the heat is taken out for heating (or heat is put into the water on the cooling cycle). Many states do not allow this, and that will be a topic for another column.
When water is re-injected back into an aquifer, and preferably a consolidated aquifer (limestone, sandstone, etc.), it is often injected back into the aquifer under pressure. Therefore, sealing the casing is of utmost importance so the water does not follow the annulus back to the surface and create a huge problem for the owner and drilling contractor.
Many of our “pump and re-inject” geothermal projects have been large-volume wells (up to 1100 gpm). Some have had re-injection pressure up to 40 psi. That may not sound high, but any pressure is a concern.
While there seems to be an ongoing discussion about neat cement shrinking and cracking—all I can tell you is we’ve not had issues with neat cement failing and causing contamination to enter the well, and conversely we’ve never had issues with grout failure for either a re-injection well or a naturally flowing well when using neat cement grout.
Mixing neat cement grout properly and getting it properly in place is one of the keys to using the grout successfully. When mixing on site, it’s imperative to get the water volume with cement powder content right.
Most specifications allow for a maximum of 6 gallons of water per 94 pounds (one bag) of Portland Type 1 cement powder. We try to mix it, and depending on the depth, pump it to the size of tremie pipe used at a ratio of 5⅝ to 5¾ gallons of water per bag. Mixing and pumping at less water than this can be a real trial.
It’s imperative to get the water/powder ratio right. You can’t just pump some water into the grouter, look at the “line in the grouter,” and put the bag of Portland in. Get it exact!
The properties of neat cement when it is set have a much higher structural strength integrity than does a bentonite-based grout. Again, this is not to say bentonite-based grouts don’t have their place. They do and we use them daily in our business.
It’s simply the structural integrity of neat cement-based grouts are necessary when dealing with pressures of re-injection and flowing wells. There are other areas of well construction where neat cement is important to use as well.
Getting It Right
When we use a ready mix company to bring our grout to the site on larger jobs, here’s some things we do to ensure the project goes well.
First, we make sure the ready mix company understands what neat cement is. If we haven’t worked with the company before, we go to the plant and talk to its supervisor in person. Why? To get everyone’s attention.
I don’t know how many times I’ve talked to someone on the phone who says, “Yeah we can do that,” but then the mix gets to the site and it’s not what it’s supposed to be. It is either lumpy, too thick (they can’t imagine we really want it liquid), or too wet. There have been times the truck was not cleaned well from its previous job and still had stones or sand in it. Most grout pumps don’t like those.
So we talk to them and give them a paper stating exactly how we expect the mix or we won’t pay for it. We point out we want a clean truck washed out before they mix it. We give them the pounds of powder and the gallons of water to be mixed with it. For example, if we want 100 sacks, we want 9400 pounds of powder, mixed with 575 gallons of water (5¾ gallons per sack). Many ready mix plant operators don’t know what a sack is or they want it in “yards.”
The long and the short is this: Go to the place of business and sit down and explain what you do and what you need. Educate the ready mix company and then you’ll most likely get what you want and need. Since we’ve done this, our problems with large grouting projects have dropped dramatically.
One of the things we do is always try to measure the weight of the cement with a mud scale. For example, if you want a 6-gallon per sack mix, the weight of the cement per gallons should be 15 pounds per gallon.
There is a formula for figuring it out. Halliburton Cementing Tables, known to many in the industry as the Halliburton Red Book, has information on it. The book is not easy to find in print anymore, but it is accessible on the Halliburton website. Go to www.halliburton.com and type in “Redbook software” in the search box. If you grout with neat cement, you should definitely become familiar with this information.
If you want to be sure your mix is right when you are done pumping, then weigh the cement as it comes out of the top of the annulus. Most likely when you begin to see it, the weight will be less as you are typically pumping it through water. Eventually if you have enough mixed right and say a 5% to 10% overage of cement on your load, you will get it to the right weight.
On larger projects we typically document what the weight was when it got to the site and what it was when we were done pumping. We use our own “grout report” to ensure we have done our diligence on our grouting. If it is an engineered project, we provide this report as a submittal when completed.
There is so much more to talk about on neat cement grouting than space allows for here. I will cover more about it in my next column. Until then, if you have specific questions, email me and I’ll try to get back to you in a timely manner.
Gary Shawver, MGWC, is president of Shawver Well Co. Inc. in Fredericksburg, Iowa. He has been in the water well industry for 40 years and is a Master Groundwater Contractor. He has served as president of the Iowa Water Well Association, the Iowa Groundwater Association, and most recently served on the NGWA Board of Directors. Shawver is semi-retired, having sold his business to his employees. He contributes to NGWA’s member e-publication and can be reached at firstname.lastname@example.org.