Since the beginning of Tunu field development in the late 80’s, shallow gas hazard has always been considered the major operational risk during the surface drilling operation. During the last ten years, 5 shallow gas events were reported; all have been safely controlled (no injured personnel) and the main consequences were material loss and non productive time.
With the objective to reduce shallow gas reservoir crossing, a high resolution seismic survey was performed in 2005. The survey results have been used to early identify potential shallow gas hazards at planning stage by 1) checking the field history of the selected future platform area and 2) systematically reviewing the seismic data on a well by well basis. Despite of the above described mitigation measures it appears that during 2007/2008, amongst 88 wells drilled, 43 shallow gas reservoirs (thickness above 6m) have been crossed on Tunu field. This phenomenon is directly linked to the geological fluvial deposit specificities of the Mahakam delta.
The shallow gas horizons (500-1200m TVD) became recently a source of interest for the geoscientists who mapped the gas in place yielding substantial reserves. After the potential has been confirmed, the drilling engineering studies started in order to design a well being able to produce shallow reservoirs. It is worth to remind that shallow gas drilling engineering is not a routine job…What kind of well architecture? Do we set a surface casing? Where do we set it? How will we manage the extremely low LOT at casing shoe? Which mud type has to be used? What will be the required mud density? How do we manage potential unconsolidated sand production....? So many questions looking for accurate technical answers in order to convert a hazard into a well.
In 2010, a specific development plan has been proposed and approved for constructing 17 shallow gas wells, showing highly encouraging production results to date.