Experimental Investigation of Well Cement Integrity in Decommissioned Oil and Gas Wells – The Effect of Water and Oil Contamination
Bibliographic record
Abstract
Abstract During the cement plug placement phase of well decommissioning, the cement slurry is at risk of intermixing with the wellbore fluids, which may impact the performance and isolation capabilities of the resulting cement plug. In this study, we investigated the sealing performance of the cement plugs, which are contaminated with different wellbore fluids. Two cement blends were used: neat API class G cement and an engineered API Class G cement with additives for optimal performance. Two types of contaminants were investigated: water to mimic fluids used by decommissioning rigs and light crude oil to mimic residual produced hydrocarbons. The contaminants were added to the slurry at varying levels (12.5% and 25% v/v for water and 2.5% v/v for light crude oil). The contaminated blends were cured inside a bench-scale wellbore simulator under ambient temperature and 6.89 MPa for 7 days. The isolation performance of the contaminated plug was assessed by conducting water flow tests. For the neat API Class G cement plug, the water flow rates through the 25% water contaminated plug were an order of magnitude higher than the flow rates through the uncontaminated plug. Visual inspection of the possible flow paths by Nitrogen injection revealed that Nitrogen gas was flowing at the cement-casing interface. For the API class G cement plug with additives, water flow rates through 25% water contaminated plug were 2 to 3 orders of magnitude higher than that of the uncontaminated cement plug. Contamination with water in this blend created a different failure mode with Nitrogen flowing through the matrix of the cement. An oil contamination of 2.5% of both the neat API class G and API class G+ additives cement had little to no effect on their isolation capabilities as long as the fluid injection pressure and confining pressure are the same and kept at the level of curing condition (i.e. 6.89 MPa, no change in effective stress). When the effective stress on the cement plug is changed, (by reducing the confining pressure down to atmospheric pressure, while keeping the injection pressure constant at 6.89 MPa), sealing ability of both the original and the uncontaminated cement plugs deteriorated. The contaminated plugs lost more of their sealing capabilities when compared to the uncontaminated plug. These results indicate that the effective stress (not the oil contamination) was the main controlling factor for the deterioration of cement plug sealing performance with oil contamination only having a small effect.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".