Abstract
Azeri-Chirag-Gunashli (ACG) is a giant field located in the Caspian Sea, Azerbaijan. The major reservoir zones are Pereriv sandstone formations with 20–25% porosity, permeability 100–1000md, and oil column up-to 1000m. These formations are weakly consolidated where Open Hole Gravel Pack (OHGP) completions have become the standard design for production wells. Development began in 1997 and to date more than 70 high rate (up to 45mbd per well) OHGPs have been installed.
Wellbore stability issues require OHGP screens to be run in Oil Base Mud (OBM). Despite excellent initial success a number of sand control failures began to occur in 2008. A detailed gravel pack evaluation using multiple wash pipe gauges have revealed that earlier installations experienced screen plugging on lower section during the installation process. This leads to an incomplete pack in the toe area and subsequent screen failure as depletion increases or once water breakthrough occurs. The ultimate risk is of lost production rather than well control or loss of containment.
Analysis was done to understand the root causes of screen plugging and to develop solutions for each. The work resulted in five key changes being made to the OHGP completions.
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Revised TD criteria for the open hole section.
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Modified OBM conditioning procedures.
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Modified wellbore-clean-out procedures.
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A modified screen BHA design.
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The use of Ultra-Fine-Grain Barite in the OBM to reduce barite sag and the amount of large solids in the fluid system.
These changes have resulted in less screen plugging, and hence increasing pack efficiency across the productive interval. This has resulted in a step change in OHGP reliability in the last 3 years with zero sand control failures over the last 24 completions. The detailed understanding of the failure mechanism also facilitated a successful intervention campaign to remediate several failed OHGP wells pre 2008. These efforts have delivered ~60mbd reduction in production losses over the past 2 years.
