While drilling wells in the Total operated Franklin, Elgin and Shell operated Shearwater HPHT fields of the UK central North Sea, gas was often encountered over intervals in the Upper Cretaceous Hod Formation. In addition, gas continued to bleed occasionally into the well until the section TD. During the completion phase of a Franklin well, the annulus pressure started to rise and a temperature and gamma ray survey indicated that fluid was bleeding into the annulus from a thin zone within the Hod Formation. Given the common issues, the so-called "Hod Geohazard" was identified for further shared study between the two operators.

Open hole logs over the Hod Formation for three of the Franklin wells were used to quantify the hydrocarbon in this interval. Three thin limestone horizons were recognised from a review of the drilling gas and cased hole gamma ray and acoustic data. An overlay technique was applied to the gamma ray and acoustic logs, which showed that two of the limestones were tight, but the third, three meters thick, had apparent porosity.

The overlay technique was then applied to the development wells, where only gamma ray and acoustic data were available. This allowed a qualitative estimation of the degree of porosity and permeability for those wells. The results indicated that the three limestone horizons had a regional distribution, although with varying degrees of porosity. This then provided a predictive tool to assess which wells were likely to have live annuli when turned over to production.

Identification of this Geohazard as a tight, overpressured gas bearing horizon has been important. It was realised that cemented casing was unlikely to hold this gas back during the production lifecycle of the wells. Consequently, casing annulus pressure management systems were installed on the facilities. In addition, increased awareness of Hod gas influx during drilling was included in the planning of subsequent wells in the region.

The key to these improvements was the collaboration of operators sharing experiences. It resulted in improved drilling and completion practices for both companies during their projects. The lessons learned can be applied by other operators in similar challenging HPHT environments.

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