Schlumberger

Case Study: Waste Injection Completed in Argentina under Challenging Operating Conditions and Onset of Rainy Season

Challenge: An operator in Argentina’s Tartagal area needed to dispose of a fixed volume of drill cuttings and drilling waste in a short time before the start of the rainy season. The available disposal well had a completion type considered particularly risky. Because of the critical short lead time, a Front‐End Engineering Design could not be conducted. M‐I SWACO had successfully injected 42,000 bbl of drill cuttings into the selected formation in 2001. The type of completion on the disposal well could potentially make injection particularly risky. The well has two intermediate casings, 9⅝ and 7 in., respectively. The 7‐in. casing has two perforated sections of 23 ft (7 m) at approximately 2,323 and 4,596 ft (708 and 1,401 m), true vertical depth. The lower perforations were fired through the 3‐in. production tubing. To inject through the 3‐in. tubing, an R‐2 packer was set at 5,331 ft (1,625 m), isolating the 7‐in. section from the lower part of the well. Initially the target zone for injection was the Tonono formation, where the lower perforations were placed at 4,596 ft (1,401 m). The upper perforated interval at 2,323 ft (708 m) belongs to the Tarija formation, which prior to injection presented a negative pressure. This producing formation is currently depleted and both perforated intervals are connected through the 7‐in. annulus. During an injection operation, differential pressure through these connected perforation intervals could create a casing collapse and potential loss of the injector well. Other risks associated with an injection operation include possible screenout during injection as a result of negative pressure in the annulus and waste breach to surface as a result of upward fracture propagation.

Solution: M‐I SWACO engineers analyzed all potential challenges involved with this high‐risk project. Various solutions were presented, including a workover to isolate the upper and lower perforations. The workover solution was rejected due to limited logistics and cost. Because normal operation procedures could not be applied and the existence of equipment restrictions, the M‐I SWACO team performed a specially designed, detailed injectivity test. A step‐rate test was conducted and the results showed that the injection was feasible. Based on the injectivity test results, meticulous operational procedures were identified. The injection rates, displacement volume, solids concentration, slurry rheology, and other parameters were well established. Because the completion limited the maximum surface injection pressure to 1,750 psi a regular flush of the annulus with freshwater was recommended to displace solids away from the wellbore and maintain low injection pressure.

Result: Since the beginning of the operation a daily report has been sent to the M‐I SWACO Waste Injection Subsurface group to conduct the pressure monitoring and analysis. Any possible anomaly in pressure behavior is identified. Project results surpassed initial volume objectives. More than 56,000 bbl of slurry waste have been injected, containing approximately 18,000 bbl of drill cuttings. Cuttings from other locations were transported to this well for disposal. Start of the injection of this waste was achieved before the rainy season began and the injection has continued without anomalies. Injection will be interrupted to accommodate rain effects and will resume as weather allows.

Summary

The proposed waste injection process in a disposal well in Argentina presented both the client and M‐I SWACO with significant risk that involved mechanical restrictions in the completion assembly and the onset of the rainy season. A particular concern was the possibility of a waste breach to surface due to fracture propagation upwards, more pronounced in the shallow injection zones.

Armed with previous experience on and knowledge of this disposal well, the M‐I SWACO operations team and WI Sub‐surface group designed and performed a step‐rate test to guarantee project success.


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