Shallow Geohazard Risk Mitigation; A Drilling Contractor's Perspective

Reed, David (Global Santa Fe Corporation)


Abstract The introduction of the Offshore Installations (Safety Case) Regulations 1992 and the Offshore Installations and Wells (Design and Construction, etc) Regulations 1996 to offshore operations in the UK North Sea require risks related to the drilling of wells to be kept As Low As Reasonably Practicable (ALARP). The responsibility of the rig owner in this regime is to identify safety critical elements associated with the rig and ensure that these risks are maintained to within ALARP principles. This responsibility also recognises that wellbores attached to the rig may also present a threat to the installation and those onboard. Additionally, the rig owner must take into account well-related safety critical elements. By using examples, this paper examines why and how a drilling contractor has implemented a Shallow Geohazard Review process in order to mitigate shallow geohazard risks. This paper reviews the operator - geotechnical contractor relationship and identifies where drilling contractor involvement has strengthened the risk analysis process. Finally, this paper summarises the positive outcomes of this process as well as challenges the drilling and geotechnical contractors to bridge the information gap. Introduction Geohazards encountered whilst drilling top-hole sections of a wellbore are varied and may include, shallow gas, shallow water flows, gas hydrates, mud volcanoes, faulting, and boulders. Although this paper relates to shallow gas geohazards, it could be equally applicable to other shallow geohazards, which may induce uncontrolled well flow. Due to their shallow nature and limited formation strength, those top-hole shallow events, that can cause the well to flow, cannot be closed in and killed by normal circulation methods. The options for maintaining well control once a flow occurs are restricted (during both riserless drilling and drilling with a riser plus diverter system.) Immediately the rig is in a condition of emergency. Therefore, shallow geohazards can present some of the most hazardous situations to a mobile drilling rig. One solution to avert such situations is to identify and avoid shallow geohazards of whatever nature wherever practical. During August 2000, two Santa Fe rigs experienced uncontrolled shallow gas blowouts, which had the potential for serious rig and personnel damage. In both cases, no one was injured and the rigs concerned did not sustain damage. Both blowouts occurred from jack-up drilling units whilst undertaking riserless operations in geographically diverse regions. The incidents were reviewed externally by the operator and internally within Santa Fe. The first incident occurred in the North Sea whilst drilling 26" size hole, with return to seabed. The first indication of the event was the observation of a plume in the sea around the rig. The visual estimation was an ariel extent of 500 m around the rig. After the initial high volume escape of gas, the well continued to percolate gas at an intermittent rate until after cement was pumped. There was also evidence of breaching away from the wellbore though at no time did this affect the stability of the legs. The rig was moved off location after the initial blowout. A high-resolution survey was undertaken of the area. A considerable period of time elapsed before the rig could be moved back to a shallow gas-free location nearby. The second incident occurred in the Gulf of Thailand whilst drilling in the 9 5/8" conductor casing. A shallow gas flow was encountered at 803' MDBRT. The flow was stopped within one hour with kill weight mud and plugged two days later after pumping cement. The well was abandoned by cutting the casing below seabed and setting further cement plugs. The reviews covered all aspects of well design and drilling operations and the conclusions were similar.

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