Shallow Gas Summary

Sintef definttion of Shallow gas: Any gas zone penetrated before the BOP has been installed. Any zone penetrated after the BOP is installed is not shallow gas (typical Norwegian definition of shallow gas). ……………………………..

Shallow gas is defined as any hydrocarbon-bearing zone which may be encountered at a depth close to the surface or mudline. Generally it is not possible to close-in and contain a gas influx from a shallow zone because weak formation integrity may lead to breakdown and broaching to surface / mudline. This situation is particularly hazardous when drilling from a fixed installation or jack-up. …………………………………………………..

Shell shallow gas procedures Recommendations

Shallow gas investigation requirements of various departments should be combined as early as possible to optimise the collection of near surface formation data for  engineering - and casing string design requirements. Primarily, existing documentation (and experience) should be reviewed for the potential of shallow gas, after which other methods and techniques may be used for further  investigation. Modern techniques in site surveying should be utilised to obtain the most reliable shallow gas detection. This is especially relevant in shallow gas prone areas and in  places where where little little geological geological information information is available. available. Seismic Seismic surveys surveys are required required for all offshore exploration wells and a statement on the probability of encountering shallow gas should be included in each well proposal. Minimum standards of  acquisition are defined in Section 12 When the site survey indicates a possible shallow gas accumulation, the proposed surface drilling location should, if at all possible, be positioned such that drilling through the shallow gas accumulation is avoided. Depending on the size and geometry of the target(s) of the proposed well, it may be required to drill a deviated well, or to revise the target area tolerance, or to change the initially planned objectives of the well. Another option is to set an additional casing string just above the shallow gas zone if the objectives of the well can still be reached and if BOPs

can be used to drill through the shallow gas zone to the next casing point (i.e. if the formation strength is sufficient to allow secondary well control). Soil sampling/pilot hole drilling before the arrival of the actual drilling unit should  be considered for locations where offshore platforms are planned to be installed, in areas where little geological information is available and in areas with a high  probability, but unknown depth, of the shallow gas. The use of bottom supported rigs should be discouraged in probable shallow gas areas, unless pilot holes (pre-spud) have confirmed the absence of shallow gas. In offshore drilling, floating rigs are preferred to operate in shallow gas prone areas,  because they can move off location in case of a shallow gas blowout.

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Chevron well control

Pride Training center. It is unquestionable that the first thing to do is a high -resolution seismic survey which will help predict the presence of shallow gas pockets. The program of these wells must envisage the presence of shallow gas and give practical procedures specific to each well by taking account of a general philosophy based on :  Avoid shallow gas if possible. Optimize shallow gas research and prediction. Consider drilling a pilot hole, possibly with a special unit ; it will allow to detect shallow gas reliably and to control it more easily.  A surface diverter is not designed to undergo an erosive gas influx for a long period. Surface diverters are above all considered to be a time-saving means allowing evacuation of the site. Using a subsea diverter is considered to be safer than using a surface diverter.  A dynamic kill (control through pumping), considering the current equipments in the field, can succeed if : the diameter of the well is less than 9” 5/8 o pumping at the maximum flow rate is performed as soon as a kick is detected. o Drilling without a riser can be carried out in a floating offshore drilling configuration. • • •

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KCA Deutag Diverter or BOP Stack? The following criteria should be observed when making the decision whether  to drill with a diverter system or to use a BOP stack: (1)

Diverter drilling through formations with an increasing pore pressure regime should be avoided if losses are to be expected below the conductor shoe. The casing scheme should be changed accordingly and a normal BOP stack should be used when drilling through the zone of interest.

(2)

Diverter drilling through known hydrocarbon-bearing formations should be avoided if it is likely that the well is capable of flowing. The casing scheme should be changed to allow the use of a BOP stack.

…………………….. Shallow Gas General Drilling Guidelines

The following guidelines shall be adhered to while drilling : Consideration shall be given to drilling a pilot hole with the 8 ½” or smaller bit size when drilling explorations wells. The BHA design shall include a float valve and considerations given to deviation and subsequent hole opening. The major advantages of a small pilot hole are: The ROP shall be controlled to avoid overloading the annulus with cuttings and inducing losses. All losses shall be cured prior to drilling ahead. Drilling blind or with losses requires the approval from Head of Operations. Pump pressure shall be closely monitored and all connections (on jack-up) shall be flow checked. •

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Pipe shall be pumped out of hole at a moderate rate to prevent swabbing.