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.