espadon ..drilling now?, page-6

I have not read the original text about "Shallow Gas". If it is shallow gas, this can be a problem for drilling especially with a drill ship which probably explains why it is a deviated well. Attached is an extract about shallow gas with some reference to drill ships.

ref : Michel Grepinet, Total Exploration Production Safety Division, Paris,
L. Flak, former Wright, Boots & Coots employee

Part 7-Shallow Gas Blowouts: After underground blowouts, shallow gas blowouts are the second most common type of blowout and the most common type of surface blowout. Loss of rig, platform and life have resulted from shallow gas blowouts. Invariably, the well is lost



Little time and effort has been expended on shallow gas hazards, although industry has put a great deal of time and effort into analysis of kick control and blowout disaster mitigation,. Preparing for shallow gas can save lives and rigs. This article examines the dangers of shallow gas, precautions and procedures for shallow gas drilling, and disaster mitigation.



DANGERS
Two factors make shallow gas drilling a difficult challenge. First, unexpected pressure at the top of the gas bearing zone, due most often to the "gas effect" dictated by zone thickness and/or natural dip, can be significant. This pressure is usually unknown, seismic surveys being often unable to give an idea either about thickness or in-situ gas concentration. In more complex situations, deep gas may migrate upwards along faults. In Sumatra for instance, an influx could not be stopped even with 10.8 ppg mud at very shallow depth; the bit had crossed a fault plane. Second, low formation fracture gradients are a predominant factor in shallow gas operations.

Reduced safety margins. These two factors result in reduced safety margin for the driller. Minor hydrostatic head loss (swabbing, incorrect hole filling, cement slurry without gas-blocking agent), any error in mud weight planning (gas effect not allowed for), or any uncontrolled rate of penetration, with subsequent annulus over-loading, will systematically and quickly result in well bore unloading.

Shallow gas flows are extremely fast developing events. There is a short transition time between influx detection and well unloading, resulting in much less time for driller reaction and less room for error. Poor quality and reliability of most kick-detection sensors worsens problems.

Severe dynamic loads. Past history has disclosed the magnitude of severe dynamic loads applied to surface diverting equipment, and the high resulting failure probability. One of the associated effects is erosion, which adds high potential for fire and explosion from flow impingement on rig facilities.

Rig cratering. The risk of cratering is a major threat for bottom supported units. As it is impossible to eliminate them (most shallow gas-prone areas are developed from bottom supported units) emphasis should be put on careful planning and tight monitoring during execution.



RIG SELECTION
When selecting a rig, two facts should be kept in mind. First, major shallow gas fields (in particular in Indonesia) have been explored, appraised and developed from bottom supported units.

Second, the most conspicuous disasters due to controlled shallow gas blowouts have involved drillships and semi-submersibles .

Two support types are particularly exposed and should be avoided: mat supported jackups and drillships. These rigs are not designed to ensure proper stability and/or to withstand heavy loads imposed by a boiling gas plume. They are, therefore, prone to take on water, catch fire, lose stability and eventually capsize.

Whatever the facility offered by semi-submersibles to move off location in case of uncontrolled gas flow, the questionable condition of the rig's mooring and release hardware, and the general unreliability of most semi-submersible surface diverting equipment, requires careful consideration.

Rig selection for shallow gas areas should take into account: Selection of a "safe" drilling location, according to seismic survey mapping Design of an engineered drilling program, with priority given to permanent hydrostatic control of the well

Comprehensive assessment of pre selected rigs, with final rig selection based on past experience and competence in shallow gas drilling, and also on diverting equipment fitness for purpose, layout, and structural ability to withstand a gas blowout long enough to permit a safe evacuation.