Schlumberger

Case Study: Efficient Injection Exceeds One Million Barrels of Drilling Waste Offshore Sakhalin Island

Challenge: An operator offshore Sakhalin Island made the decision to apply waste injection (WI) because of challenging climatic conditions with a limited ice-free window for navigation and with no established logistics or infrastructure for drilling waste management onshore. The dedicated injection well was the first of the drilling campaign in this field.

Solution: The decision was taken to perform injections from the start of the drilling project. Injections were performed through the 4½-in. tubing with perforations at 7,930 to 7,963 ft (2,417 to 2,427 m) total vertical depth. Continuous 24/7 injection monitoring was launched from offices in Moscow and Yuzhno-Sakhalinsk. Daily monitoring and analysis of waste injection performance was provided to identify potential problems during the subsurface injection procedure and offer early warning signals in addition to give additional understanding of injection performance and increased overall process assurance.

Result: Value of the injection analysis and monitoring service was demonstrated when abnormal pressure decline was observed for the first time, and analysis revealed that injection operational parameters and fluid rheology did not show considerable variation during the injection process. Comprehensive pressure analysis was carried out to identify the subsurface reasons behind specific pressure patterns, predict the impact on future injection and reveal associated risks. The observed straight-line decline pattern does not represent the normal fracture response and could have been driven by an induced restriction at the injection point. Review of injection and surrounding lithology revealed possible pinching created by the higher in-situ stress layer at the injection point. Solids packing in this pinching area could lead to limited communication with the primary disposal sandstone layer. Near-wellbore packing would increase the surface pressure required for injection cycles; the stress re-orientation in the near-wellbore would induce tortuosity effects that would contribute to increase of the pressure placement. And finally inadequate solids placement would corrupt the disposal capacity of the injection target. It was immediately recommended to increase the displacement seawater volume step by step (25, 50, 73 and 100%) until improvement in the pressure-decline signature could be seen. Viscous pill volume was also increased by 50% to enhance the solids displacement from the high stress area around the perforations. When displacement volume increased to 50% the normal fracture response at shut-in was restored.

The Results

As a result of vigilant injection monitoring and prompt adjustments in the displacement procedure, the early warning signals of near-wellbore packing were eliminated and optimized placement of solids was achieved. Enhanced understanding of the subsurface injections allowed the operator to manage and maintain a particularly important injection well, avoiding downhole problems. Pressure monitoring allowed the operator to ensure seamless injection operations while mitigating potential injectivity and subsurface risks and meeting the project’s high environmental standards.


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