A recent request about the topic of geothermal well cementing brought me memories of a project I led back in 2008. It was my first time in a geothermal well construction project. Yes, there was a rig, there was drilling, drilling fluids and cementing but, it was entirely different from the oil and gas business. The most significant difference that made me feel at first out of place was the perception of time. In that project, there was no rush, and the concept of NPT was not that severe.
It took me some time, but then it was good, and we managed to work well with the company in charge. The problematic part, later on, was to return to the dynamics of the oil and gas when the project ended. Nice experience nonetheless.
Below are some things that I learned from this time that apply, to help answer this request.
Cement Slurry considerations:
1. Lightweight because of losses, it is a common issue in igneous rock. Losses are likely difficult to cure due to caves and vugs, and the casing gets cemented to the surface.
2. At about 230F it needs silica to prevent cement retrogression and effect of CO2 (if dissolved in the produced fluids), At about 350 – 400F it is better to use silica flour with an average size of 10-20 micros.
3. We used Ceramic/glass-based products, such as cenospheres or cementitious material such as fly ash can as extenders, stabilizing cement phases at high temperature.
4. Very Low permeability in the cement matrix is a requirement. (< 0.1 mD).
5. The main issue is not temperature but cycling (temperature variations and cycles when the well in energized).
6. Magnesium and Sulfate Attacks kinetics increases = accelerated aging.
Comparison of High Temperature (BHST) applications of cement:
1. Deep well with normal geothermal gradients, HPHT or HTLP. BHST 160 C – 180C/Class G, H with 35 to 40% silica content.
2. Steam injection (Low API oil) to increase production: Temperature during injection is about 315C/Class G, H with 35 to 40% silica content.
3. Geothermal wells temperature as high as 370C/Class G, H with 35 to 40% silica content
4. Combustion in situ or fireflooding as high as 900C (cement can be used up 425C) /CAC or calcium aluminate cement, also known as calcium aluminate cement.
Now, Talking about high BHCT (circulating temperature during cementing)
Only, one and three can have relatively high BHCT when near the reservoir zone, but in geothermal applications, the well is cooled down before cementing. For BHCT above 90 C some API cement tests are not applicable (check slurry stability).
In a geothermal well, typically they would require a 1000 psi CS to be enough and casing are cemented to surface to prevent temperature-induced casing enlargement.
The BHST looks doen’t matter either it is 110 or 370 DegC anyway 35-40% of Silica?
Oleg, Thanks for your question
The reason to add silica is to lower the bulk lime-to-silica ratio (CaO/SiO2 ratio) from around 1.5 to 3 in Cement to at least 1. At 110 oC, a CaO/SiO2 ratio of > 1.5 would allow the formation of a denser phase (α-C2SH) resulting in loss of compressive strength and increased permeability. If the CaO/SiO2 ratio is reduced to around 1, α-C2SH will not form and a phase called Tobermorite will form preventing strength retrogression. If you follow in the diagram attached the phase forming at higher temperature for a CaO/SiO2 of 1, You will see Tobermorite, Xonolite and truscottite. All these are OK to preserve compressive strength up to 400 – 450 oC where complete dehydration will lead to disintegration of the set cement.
From the explanation above and to answer your question, The % of silica to add does not depend on the temperature, as you can see, so 35 to 40% should be Ok up to 370 oC. However, the percentage of Silica (SiO2) does depend on the cement composition. From your pure Cement QA/QC you will have the % of the cement phases, these are: (CaO)3SiO2, (CaO)2SiO2, (CaO)3Al2O3 and (CaO)4Al2O3Fe2O3. The average % for these varies and the QA/QC sheet from the cement manufacturer will include this information. You can see all 4 contain CaO and only the firsts two have SiO2, so using the % of the phases you should be able to estimate the required % of SiO2 to add (if you need to know how, please send me an email) to lower the CaO/SiO2 ratio to around 1. In some cases up to 60% of silica could be required.
A couple of final comments:
1) A higher temperature, above 200 oC, silica flour with smaller particle size of +/- 10 -20 microns is preferred
2) The problem of strength retrogression is not so much the loss of compressive strength but the increase in permeability (around 40 times more). Why? because it would be easier for formation fluids containing Mg + Na Sulphates and other to enter the cement matrix making the cement even weaker (aging) and lost of zonal isolation.
If you need further detail please contact me
Cheers
L. Diaz
Thanks Lenin for your comments mentioned regarding the % od Silica Flour in cement blend for HPHT to avoid the Strength retrogression and permeability. But in some articles mentioned the % of Silica Flour should be from 210 DegC not less than 60% even 80 or 100%. That’s why I wonder. But you say that the size of Silica should be 10-20 microns or 1250 – 625 mesh. It is almost the Silica Fume and it will make the skurry very viscous. What density did you use for this size of Silica?
Hello Oleg, Thanks again for your comments and questions.
As I mentioned, There is no need to increase the % of silica at higher temperatures. % of silica depends on cement composition (% of cement phases, content of CaO and SiO2) and not on temperature. I have seen as high as 60% of silica, but it wasn’t because of temperature.
You are correct that this much silica flour (10 – 20 microns) would make the slurry viscous, and that is something to be adjusted at the lab. An alternative, in case of high rheology is to combine silica flour with silica sand (average 100 microns) to reach the required total percentage of silica. Another alternative for geothermal wells is to add glass/ceramic cenosperes (300 microns) which would help with lowering slurry density as well, in case of severe losses very common in geothermal wells.
In your case, HPHT I would opt for a combination of silica flour or silica sand if BHST > 200 oC, with lowest possible addition of dispersant, sufficient fluid loss additive to stabilize the slurry and the required amount of retarder. The right combination of all these would prevent annoying 3ary and 4ry gelling problems or slurry destabilization at bottom
Thanks again Lenin for your advise. The Silica Flour size 325 mesh is good enough? Or you can use as well 200 mesh? Or combination. As with 325 mesh size of silica you need the antisettling agent to add. The BHST is 250-280 DegC. Or what would you suggest?
Oleg, good questions! Thanks.
325 and 200 mesh would provide similar performance in terms of preventing cement strength retrogression. At such a high temeperatures 280 oC and deep well, as I belive you mention HPHT, cement slurry stability is always an issue. If you only have 325 and 200 microns, I would choose only 325 mesh to add little help with settling and depending on cement quality the % of silica, as mentioned before.
Good. There is also horizontal well cementing in SAGD (mostly in Canada – heavy oil sand) where cycling is also a big issue. Check out those lessons learned.
Thank you Ember, Yes SAGD and steam injection wells are good reference. In Steam injection and geothermal wells the combination of high temperature and temperature cycles is the main problem.
Cheers
L. Diaz
Why the monkeys???
It means “see no evil, hear no evil, speak no evil” which sometimes is referred to pretending ignorance to what other do wrong, for me it is actually the opposite, it means to provide help to prevent what is or can be worng. If you don’t allow “evil” to enter your life you will have only good actions.