Explanation of Shallow Blowouts and Lost Time Incidents with Some Recommendations
Authors: Baird Ralph W. 
Published: October 2008

There have been several major blowouts in the marine environment due to shallow gas or shallow faults carrying gas to the surface. In addition numerous drilling sites have been abandoned due to sediment failure and lost circulation above 3000 feet subsea level. Millions of dollars have been spent on these problems. Each could have been avoided with prior knowledge of the hazard. Drilling engineers can design their drilling plan to compensate for potential hazards.

A potential hazard is anything that will endanger or will cause delays to the drilling operation. These are shallow gas near surface faults hard and soft sediments old river and glacier channels waterbottom anomalies and man-made objects. Loss of life loss of the rig, loss of materials, wasted rig time, adverse publicity and government control can be avoided. The geophysical prediction of potential hazards at depths above 3000 feet is the subject of this paper. Recent case histories are used as illustrations.

High resolution engineering geophysical methods can locates potential hazards mud lumps, acoustic voids, mud volcanoes, and gas-charged sediments do not blow rigs out of the water. Usually only digital common-depth-point, multifold, acoustic data delineates hazardous shallow gas. If careful attention is paid to the data collected by high resolution engineering surveys, the risk to offshore drilling operations will be reduced.

INTRODUCTION

The only way to forecast potential drilling problems besides drilling a well is by geophysical surveying. Conventional CDP exploration seismic surveys are focused below 2000 feet and cannot define the shallow drilling and foundation zones adequately. A combination of engineering geophysical tools are required to "see" drilling hazards.

Drilling is vulnerable at shallow depths without the interpretation of shallow potential hazards. The geophysicist sees such potential problems as shallow gas, near surface faults, old river and glacier channels, sediment strength, waterbottom anomalies, and man-made objects. The drilling engineer knows he has low overburden gradient, a low fracture gradient, no blowout preventors no casing, and low mud weight. He has to jack-up successfully the first time, anchor his floater and stabilize the wellhead BOP stack, or set his platform with precise judgment. Drillers are scared of shallow gas and uncontrollable situations. These problems are predictable from High Resolution Geophysical data; shallow gas and uncontrolled situations can be either planned for in the drill plan or simply avoided by a change of location.

Lack of adequate site investigations and proper interpretation of geophysical drilling data can lead to drilling failures. Drilling permits have been delayed by Government authorities due to the lack of adequate drilling forecast information. A number of jack-up rigs and floaters have been severely damaged or destroyed when the well blew out or when subsurface sediments gave way.

In recent years, we have been finding more and more applications for marine seismic technology that involve high resolution and relatively shallow depths of interest. The main purpose of this paper is to illustrate the capabilities of high frequency sources in this kind of work.