Abstract
Cementing operations in wellbores, especially for long casings, are often challenging and prone to deficiencies when not properly planned and executed. While exploring for and exploiting of geothermal resources at temperatures above the critical point of water was attempted in different drilling projects in recent years, the well design, and especially the procedure to run and cement long production casings became a key challenge for drilling engineers. For the first time, a reverse cementing job for a 2.97 km long production casing in a high-temperature geothermal well could be monitored and analyzed using a combination of permanently installed distributed fiber optic and electronic sensors as well as conventional well logging equipment. Data from the permanently installed sensors were used to monitor and evaluate the cementation process as well as the onset of the cement hydration. Based on the data, the understanding of downhole fluid dynamics during cementation could be improved. Our analysis suggests that the cement was diluted during cement placement and partly lost into the formation. These findings can help to better prepare for future drilling ventures under similar downhole conditions.
Highlights
Cementing a casing string is considered one of the most important operations during the construction of a wellbore (Allouche et al 2006)
Did the analysis show that the limited flow rate led to a free fall of cement blobs within the annulus, it could be shown that dilution of cement occurred that led to a dilution of the cement slurry which corresponds to a low cement bond-logs (CBL) reading and to the analysis of the : Top-of-cement (TOC) together with the pressure readings within the annulus
The low CBL reading in the depth interval below the 13 3/8” casing shoe is most likely related to the excessive amount of retarder
Summary
Cementing a casing string is considered one of the most important operations during the construction of a wellbore (Allouche et al 2006). Does the cement prevent short mixing of fluids from different geological formations, it gives the well construction its required strength to withstand hostile downhole environments. The completion of deep geothermal wells is challenging because of high pressure and temperature at depth and high radial thermal gradient during load changes (Teodoriu and Falcone 2009). High-temperature geothermal wells in Iceland are typically equipped with a conductor casing, a surface casing, an anchor casing, a production casing and a perforated liner. Such casings are typically cemented to the surface to withstand the extreme thermal and mechanical loads (except the liner which is usually just hanging). The practice to cement the entire casing gives rise to technical challenges during cementation of exceptionally long casings due to the differential pressure conditions
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