When penetrated from a platform or a jack-up, shallow gas reservoirs have the potential of being more hazardous. Since the conductor almost reaches the floor of the rig, any kick products discharge into the hazardous zone directly.
To direct the flow overboard, the diverter will close automatically when a shallow gas flow occurs. During a period of stress, the diverter system’s reliability is questionable which is why failure should always be considered.
If a restriction forms in the diverter line, a hazardous situation quickly develops on a bottom supported rig. Around the seabed’s casing, gas can actually broach as a result of the pressure build-up. Whenever this occurs, the risk of the seabed becoming fluidized increases and therefore so does the risk of a rapid reduction in spudcan resistance.
Shallow gas encountered on a jack up rig (Ref – officerofthewatch.com)
While the surface hole is open, several precautions need to be taken and these are listed below;
- When sudden losses in the annulus occur, the facilities need to be available for these to be filled quickly.
- On trips, pumping out of the hole should be considered.
- In the drillstring, a float valve should always be run.
- The annulus should never become overloaded with cuttings, so this needs to be monitored. After overloading, this can cause losses or liberated gas from cuttings and this can potentially lead to the annulus unloading. By limiting ROP, drilling the pilot hole, and circulating at a high rate, the drilled gas and cuttings can be distributed and problems prevented.
- The hole should be monitored and the facilities should be available to ensure it remains full while tripping.
- There should always be enough mud on the site to fill the volume of the hole twice over.
- The facilities, tools, and materials should be available to keep the hazardous zones free from the flow (without also imposing backpressure on the well or restricting the flow itself).
If the well begins to flow, the following can be used as a guideline;
- Start by maintaining the maximum pump rate.
- The lower kelly cock should end just above the rotary table after spacing out.
- Returns can be diverted overboard by opening the diverter lines, closing the diverter element, and closing the shaker valve.
- All non-essential machinery and equipment should be shut down and this will reduce the number of potential sites of ignition. Beneath the rig floor, the fire hoses should be deployed. In the meantime, all personnel not considered essential should be evacuated.
- Signs of gas breaking through the sea (outside the conductor) should be monitored. If evidence is detected, all personnel should be evacuated instantly.
References
Cormack, D. (2007). An introduction to well control calculations for drilling operations. 1st ed. Texas: Springer.
Crumpton, H. (2010). Well Control for Completions and Interventions. 1st ed. Texas: Gulf Publishing.
Grace, R. (2003). Blowout and well control handbook [recurso electrónico]. 1st ed. Paises Bajos: Gulf Professional Pub.
Grace, R. and Cudd, B. (1994). Advanced blowout & well control. 1st ed. Houston: Gulf Publishing Company.
Watson, D., Brittenham, T. and Moore, P. (2003). Advanced well control. 1st ed. Richardson, Tex.: Society of Petroleum Engineers.
Watch, O., 2013. Offshore Well Blowout – Investigation Report. [online] Officer of the Watch. Available at: <https://officerofthewatch.com/2013/04/15/offshore-well-blowout/> [Accessed 8 August 2020].
