This post provides additional information for those who need more detail regarding cement slurry contamination. This is a follow-up post to previous posts where we covered fluids intermixing (as in liner cementing and rheology in primary cementing).
What are the consequences of contamination?
Poor Cement Log Evaluation:
Cement log evaluations can show poor cement in Liner jobs (typically 7” or less) more commonly ranging from 3,000 to 4,500 meters in depth.
Cement slurry volumes will range from 30 to 80 bbls using only top plug even after having successfully executed cement jobs.
Gelled Contaminated Residues on Top of the Liner Hanger:
Strong contamination due to in-pipe fingering and annular channelling of the cement slurry, spacer and mud. This can be affected by the type of mud, mud salinity, density, the sufficiency of the centralization and whether the mud/hole is conditioned or unconditioned). This can lead to potentially dangerous gelled fluids/residues being left on top of the liner.
You’ll find lots of useful reference information on this table below. You may want to save it and keep it as a kind of cheat-sheet for your engineering work or studies.
|Optimized Centralization, based on: software simulation with detailed well (Survey, Caliper, ) Configuration; Fluids Definition (OBM, Spacer, Cement Slurry rheology) and ECD management (known or predicted formation pressure and FG).||High.|
|Reduction of contamination between slurry, spacer and mud by proper rheology hierarchy (mud displacement efficiency) during the Job design.||Medium to Low. It lowers as liner depth goes deeper.|
|Cement Slurry / Spacer / OBM compatibility Tests: Rheology (API), Thickening Time and SGSA (suggested contamination profile 50% Slurry + 25% OBM + 25% Spacer or based on mud displacement efficiency simulation). Actions to be taken based on results.||Medium to Low. It could be difficult and time-consuming to obtain a balance between the slurry designed characteristics for its primary purpose (safe annular placement and successful zonal isolation) and the resulting modifications performed to overcome spacer/mud incompatibility. This incompatibility can be manifested in rheology, thickening time, static gel and/or CS development.|
|Laboratory UCA test to be performed according contamination profile (select educated-worst-case scenario as per computer simulation in mud displacement efficiency). Provide contaminated slurry characteristics (Acoustic Impedance, etc.) to logging company for Cement Log Evaluation.||Medium to Low. It could be time consuming for the lab and it will only has an effect on a better understanding of the cement evaluation log.|
|Use of Saline Slurry for OBM salinity higher than 180,000 ppm (This could also be influenced by BHT and OBM density).||High to Medium. Applicability is case-specific.|
|Reduction of contamination between slurry, spacer by proper rheology hierarchy (mud displacement efficiency) during the Job design.||Medium to Low. It lowers as liner depth goes deeper.|
|Possibility of using a physical barrier (bottom plug).||High. It requires adouble-plug liner Tool system. It can reduce total slurry contamination (total contamination being, in-pipe + annular fluid mixing) in as much as 60%, specially for casing liners deeper than 4,500 meters. It could increased associated job risk related to tool failure or float equipment plugging.|
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