Drilling waste sub-surface disposal, via injecting into a pre-defined suitable formation, known as Waste Injection (WI) technology; has been selected as a primary drilling waste management tool for the development of a major oil field in the Caspian Sea. The field development has been utilizing WI technology since the beginning of development drilling operations. The main drivers were proven track record of the technology, meeting "zero-discharge" environmental requirements and elimination of logistical complications associated with onshore transfer of drill cuttings. A dedicated WI well was drilled and the surface equipment package to process and inject drilling waste was installed on each drilling and production platform.

WI failure was considered as one of the top ten risks to achieving full field production targets. Complexity of the injection horizon consisting mostly of massive mudstone, regional faulting and tectonic activity combined with a lack of local WI experience called for a detailed sub-surface engineering design and meticulous monitoring of the waste disposal domain.

Continuous pressure monitoring was recognized as a critical approach in providing sub-surface assurance and timely identification of any early-warning signatures. Unique to these WI wells, the injection wells were equipped with down-hole pressure-temperature gauges for accurate pressure measurement and subsequent analysis.

This paper discusses the execution, results and conclusions of monitoring this challenging project over a 5-year period, including subsurface complexities experienced during this period such as very high injection pressures, limited fracture-wellbore communication, enhanced solids settling, observation and explanation of long-term pressure trend behaviours. In addition solutions developed to prolong the life of the injection asset; including enhanced seawater "over-flush" injection strategy coupled with down-hole pressure measurement, careful monitoring and analysis.

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