Some areas are notorious for the presence of hydrostatically pressured gas pockets at relatively shallow depths.
1 Prediction of shallow gas
Although the location of gas pockets is difficult to predict, high resolution seismic data acquisition, processing and interpretation techniques increase the reliability of the shallow gas prognosis and therefore, surveys are to be recommended.
Well proposals should always include a statement on the probability of encountering shallow gas, even if no shallow gas is present. This statement should not only use the "shallow gas survey", but include an assessment drawn from the exploration seismic data, historical well data, the geological probability of a shallow cap rock, coal formations, and any surface indications/seepages.
2 Shallow gas procedures
Based on the shallow gas statement in the well proposal, practical shallow gas procedures should be prepared for that particular well. The following guidelines should be adhered to:
·avoid shallow gas where possible;
·optimise the preliminary shallow gas investigation;
·the concept of drilling small pilot holes for shallow gas investigation with a dedicated unit is considered an acceptable and reliable method of shallow gas detection and major problem prevention;
·surface diverter equipment is not yet designed to withstand an erosive shallow gas flow for a prolonged period of time (and probably never will). Surface diverters are still seen as a means of "buying time" in order to evacuate the drilling site;
·diverting shallow gas subsea is considered to be safer as compared to diverting at surface
·a dynamic kill attempt with existing rig equipment may only be successful if a small pilot hole (9 7/8" or smaller) is drilled and immediate pumping at maximum rate is applied in the early stage of a kick;
·riserless top hole drilling in floating drilling operations is an acceptable and safe method.
The procedure to handle a shallow gas kick with a floater or with a land or bottom supported rig is presented in Appendices Section 6.23 and Section 6.24.
3 Identification of shallow gas pockets whilst drilling
Whilst drilling at shallow depth in a normally pressured formation, no indication of a gas pocket other than higher gas readings in the mud returns can be expected. Since the overbalance of the drilling fluid at shallow depths is usually minimal, pressure surges may cause an underbalanced situation which could result in a kick. Therefore, every attempt should be made to avoid swabbing.
4 General recommended drilling practices in shallow gas areas
Common drilling practices which are applicable for top hole drilling in general and diverter drilling in particular are summarised below. Recommendations are made with a view to simplify operations, thereby minimising possible hole problems.
·A pilot hole should be drilled in areas with possible shallow gas, because the small hole size will facilitate a dynamic well killing operation.
·Restrict the penetration rate. Particular care should be taken to avoid an excessive build-up of solids in the hole which could cause formation breakdown and losses. Drilling with heavier mud returns could also obscure indications of drilling through higher pressured formations. The well may kick while circulating the hole clean. Restricted drilling rates also minimise 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.
·Every effort shall be made to minimise the possibility of swabbing. Pumping out of the hole at optimum circulating rates is recommended for all upward pipe movements (e.g. making connections and tripping). Especially in larger hole sizes (i.e. larger than 12") it is important to check that the circulation rate is sufficiently high and the pulling speed sufficiently low to ensure that no swabbing will take place. A Top Drive System will facilitate efficient pumping out of hole operations. The use of stabilisers will also increase the risk of swabbing, hence the minimum required number of stabilisers should be used.
·Accurate measurement and control of drilling fluid is most important in order to detect gas as early as possible. Properly calibrated and functioning gas detection equipment and a differential flowmeter are essential in top hole drilling. Flow checks are to be made before tripping, at any time a sharp penetration rate increase or tank level anomaly is observed, when any anomaly appears on the MWD log, and at any specific depths referred to in the drilling programme (taken from the shallow seismic survey). It is recommended to flow check each connection while drilling the pilot hole in potential shallow gas areas. Measuring mud weight in and out, and checking for seepage losses are all important practices which shall be applied continuously.
·A float valve must be installed in all bottom hole assemblies (BHAs) which are used in top hole drilling in order to prevent uncontrollable flow up the drillstring. The float valve is the only down-hole mechanical barrier available. The use of two float valves in the BHA may be considered in potential shallow gas areas.
·Large bit nozzles or no nozzles and large mud pump liners should be used to allow lost circulation material (LCM) to be pumped through the bit in case of losses. Large nozzles are also advantageous during dynamic killing operations, since a higher pump rate can be achieved. As an example, a pump rate of approximately 2700 ltr/min at 20,000 kPa pump pressure can be obtained using a 1300-1600 HP pump with 3 x 14/32" nozzles installed in the bit. By using 3 x 18/32" nozzles, the pump rate can be increased to around 3800 ltr/min at 20,000 kPa. The use of centre nozzle bits will increase the maximum circulation rate even further and also reduces the chance of bit balling.
·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 also have flow restrictions which will reduce the maximum possible flow through the drillstring considerably. A successful dynamic well killing operation will then become very unlikely.