RECOMMENDED DRILLING PRACTICES 1. General Practices

Shallow gas guidelines for drilling operations are discussed in the following sections.

Where possible, floating rigs should be utilised to operate on locations with possible shallow gas because, on balance, the floating drilling unit is safer than a bottom supporter rig when dealing with shallow gas.

Recent experience shows that it is also possible to drill a pilot hole with jack-up rig in a floating position to permit a very fast move off from dangerous area in case of shallow gas blow out.

The procedures to move a vessel off location in case of a subsea gas blow-out depend on many factors (rig type water depth riserless drilling or drilling with marine riser etc.) It is imperative to specify these procedures for each rig and each well. When drilling riserless and a subsea gas blow-out is experienced, it may be possible to pump kill mud and simultaneously move rig off location a safe distance away from the bubbling gas area. The drill string should be disconnected and released immediately, if it prevents the vessel from moving off location and or when it endangers the rig structure.

The drill string disconnecting and releasing procedure should be available and known to all relevant personnel in order that they are carried out efficiently without causing any delays in moving off location.

a) A pilot hole should be drilled in areas with potential shallow gas, as the small hole size will facilitate a dynamic well killing operation. The probability of encountering a kick, the severity of the kick and the chance of dynamically killing the well, determine the pilot hole size to be drilled. Small pilot holes will enhance the dynamic well killing capability, and improve log quality. Generally, it is recommended that a drill 12¹/4” or smaller pilot hole is drilled.

b) Restrict the penetration rate (recommended ROP = one joint/hr). Particular care should be taken to avoid an excessive build-up of solids in the hole which could cause formation breakdown and hence losses. Drilling with heavier mud returns could also obscure indications of drilling through higher pressured formations and the well may kick while circulating the hole clean. Restricted drilling rates also minimises the penetration into the gas bearing formation which in turn minimises the influx rate. An excessive drilling rate through a formation containing gas reduces the hydrostatic head of the drilling fluid, which may eventually result in a flowing well.

c) All efforts shall be made to minimise the possibility of swabbing. Pumping at the optimum circulating rate, is recommended for all upward pipe movements (e.g. making connections and tripping). In larger hole sizes especially (i.e. larger than 12¹/4”) it is important to check that the circulation rate is sufficiently high and the pulling speed sufficiently low to ensure that no swabbing will occur. A top drive system will facilitate efficient pumping while tripping out of hole operations. The use of stabilisers will increase the risk of swabbing, hence the minimum required number of stabilisers should be used.

a) Accurate measurement and control of drilling fluid is important in order to detect gas as early as possible. Properly calibrated and functioning gas detection equipment and a separate flowmeter are essential in top hole drilling. Flow checks must be made before tripping, any time a sharp increases in penetration rate or tank level anomaly is observed. When any anomaly appears on the MWD log (if a MWD data transmission system is used) and at any specific depth referred to in the drilling programme (taken from the shallow seismic survey), it is recommended to flow check at each connection.

b) A float valve must be installed in all bottom hole assemblies which are used in top hole drilling. The float valve is the only down hole mechanical barrier available.

c) Shallow kick-offs should be avoided in areas with probable shallow gas. Top hole drilling operations in these areas should be simple and quick, to minimise possible hole problems. BHAs used for kick-off operations, have flow restrictions which will considerably reduce the maximum possible flow through the drill string. Dynamic well killing operation will then be very unlikely.

d) A stock of kill mud based on hole size, and for off-shore rigs, water depth and riser size shall be prepared before commencement of drilling. The mud weight held in readiness should be slightly less than the fracture gradient from the sea floor to the shoe of the initial casing string. The correct mud weight must be determined for the particular area is being drilling.

e) Before spudding the well, a meeting should be held in order to alert key personnel (Drilling Contractor personnel, mud engineer, mud logging operator included) of the following issues:

• Risks related to a possible shallow gas blow-out.

• Considerations on blow-out development times.

• Requirement for quick action to be implemented by personnel involved in operations.

• Drilling control (parameters, levels, gas detectors, tripping, etc.) should be strengthened for this phase.

• For off-shore rigs; emergencies procedures for shallow gas blow-outs (specific procedures for each rig) must be available and also included, for movable rig, procedures for moving off location.

• For off-shore rig: alert the supply/standby vessels and on-shore base in order to facilitate fast movement off location and, if necessary, the evacuation of personnel.

• Duties and responsibilities.

Note: The above drilling practices may be modified for development wells where it is confirmed that no shallow gas is expected.

9.3.2. Logging

Information about the presence and depth of possible hydrocarbons can be obtained from electric wireline logs or MWD, the latter being the preferred method, since early detection obviously enhances the safety of the operation.

MWD is the only currently downhole tool capable of shallow gas detection by means of resistivity and gamma ray recording.

In the presence of normal gradient formation and gas bearing sands the use of MWD, with almost immediate surface readout, can be valuable in confirming the requirements for specific measures and procedures to be adopted.

However, in the presence of over-pressured gas pockets, a downhole tool may be of limited practical value due to the very fast development of a blow out.

In development drilling, where there is sufficient geological information available from surrounding wells to determine that there is no shallow gas, logging may not be required, which avoids pilot hole drilling and hole opening operations. However, the use of a diverter system is still recommended if there is a risk of colliding with another well or there is a possibility of penetrating charged sands from leaking or poorly cemented casing strings.

Shallow gas detection, with electrical wireline logging or MWD, is not always reliable or conclusive. Excessive hole size and the presence of fresh formation water may mask the shallow gas effect during recording.

9.3.3. Losses

Losses should be avoided during drilling with a diverter system installed. If losses are encountered, they are to be cured quickly using Lost Circulation Material (LCM) or cement.

Full returns are to be regained before proceeding to drill ahead. If the losses cannot be cured, possible courses of action include plugging back with cement, either to set casing high or to abandon the hole.

9.3.4. Cementing Operations

The most important item to prevent shallow gas blow-out during cementing job, is an accurate and correct cement programme.

In addition, and where applicable, it is recommended that the BOP stack remains nippled up with a small annular pressure maintained during WOC time.

There is no available data to determine the effect of gas blocking agents to stop the gas flow.

9.3.5. Drilling Procedures 30” Casing

There is no protection against shallow gas in this hole section apart from the dynamic killing option. Running and cementing the 30” casing in a pre-drilled hole, after having drilled a pilot hole, is the recommended technique in areas where shallow gas might be encountered.

An important aspect, which always need to be considered, in floating top hole drilling operation utilising a marine riser, is the formation strength at the shoe. If the formation strength at the 30” shoe is considered insufficient the use of the marine riser and diverter system has to be ruled out and riserless drilling should be employed.

20” Casing:

There are three main methods used to drill 26” hole in shallow gas area:

• Drill pilot hole and open the hole without riser .

• Drill pilot hole and open the hole through the riser with underreamer.

• Drill pilot hole, pull the riser and open the hole.

• Drill pilot hole through a marine riser with return to seabed via a sub sea exhaust valve (or dump valve) or subsea diverter.

Operation Without The Riser

Riserless drilling is considered to be the safest way to cope with the shallow gas problem since the vessel can quickly move away from a subsea blow-out. The risk of riserless drilling increases with decreased water depth. The presence of the water ensures that some hydrostatic pressure is always available to act against the shallow kicking formation. The possibilities of riser collapse and borehole unloading are eliminated. The primary disadvantage involved with drilling in shallow water, without a marine riser, is that a gas kick may result in reduced rig buoyancy due to the presence of a gas bubble in the water beneath the rig.

However, case histories show that the effect of buoyancy loss as a result of a sub sea gas blow-out does not represent a major risk to floating vessel. Also because ocean currents are usually sufficiently strong to carry all of the gas safely away from the rig.

Water depth has some influence on buoyancy loss, but it has greater influence on vessel instability, especially at very shallow water depth.

A minimum water depth cannot be given since many variable factors should be considered for each case.

Pilot Hole Opened Through The Riser With An Underreamer

There are various advantages, unrelated to well control, when using a riser. Using a riser allows treatment of the returning mud stream, considerable saving in time and money, formation samples and gas samples may be examined and evaluated at surface as drilling progresses, the mud gradient can help to control the well.

Use of a marine riser while drilling shallow holes does pose some disadvantages; the gas may cause the mud in the riser to unload, this results in decreased hydrostatic pressure acting against the kick which in turn allows the influx rate to increase. Higher pump rates are necessary to outrun kicks having higher rates of fluid influx. Unloading the mud in the riser also allows the hydrostatic pressure exerted by the water outside the riser to act against riser wall, thus raising the possibility of riser collapse. If we attempt to control a shallow kick by a column of heavy mud above the kick zone, the possibility exists that an excess of heavy mud in the riser may cause lost circulation and an even worsen the conditions.

The operative time necessary to run and to pull-out the riser.

When drilling with a riser consideration must be given to riser release and moving off location if a shallow gas blow-out occurs.