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Drilling and cementing through surface sections with highly active shallow gas formations is seriously challenging. In order to save employees lives and maintain a safe working environment, wells are designed with careful consideration of mud weight, drilling and mud parameters, while utilizing advanced monitoring techniques during well execution. While drilling, if well flow situation occurs, killing the well using industry accepted killing methods is not always possible as proper blowout prevention systems are not yet installed at the wellhead. For such wells, a diverter system must be utilized, enabling closing the well and diverting the flow to a safe distance away via two large sized vent lines for the rig crew to evacuate the work area safely. In cases where shallow gas contains flammable hydrocarbons, evacuation is mandatory. In some places in the Middle East however, the shallow gas has different properties, being inert nonflammable Nitrogen (N2) gas, eliminating both explosion and fire risks. Nevertheless, handling high pressure blowouts introduce severe risks to crew members, equipment, and the environment. In case drilling is decided to be initiated in the high-risk shallow gas area, all precautions must be taken to prevent gas flow whilst drilling. This is possible by adequate mud weight selection allowing sufficient overbalance throughout the complete drilling, casing, and cementing operations ensuring shallow gas zone is properly sealed and isolated before drilling deeper intervals. In a large Middle East field, a large number of wells have been drilled with some areas having shallow nitrogen (N2) gas at high pressure in formations lying at 200-800 ft depth. In such areas, having shallow N2 gas flow is inevitable. Due to the limitation of surface locations, and in order to develop all field recoverable reserves, the only way is to drill wells in the highly active shallow N2 gas areas. The field is developed by drilling costly horizontal multilateral wells with maximum reservoir exposure. Wells typically start by drilling two large sized surface sections, a 28-in which extends to +/-160 followed by a 22-in wellbore sections. While drilling the 22-in section only extends to +/-1200-1800 ft, however, multiple drilling challenges have been observed as a result of the presence of a highly active shallow N2 gas formation. Gas influx while drilling, well flowing and gas channeling after cementing, and continuous gas bubbling in the cellar represent the main challenges experienced in the 22-in surface section. This paper will present details of the engineering analysis performed and the procedure developed to enhance well design and cementing through the highly active shallow N2 gas surface section. |
