Recent Problems related to the identification of Shallow Gas

Date/Time:

15 Mar 2012
10:00-10:30

Location:

South Galleries

Seminar Details
The presentation is divided into three parts with the main topic focused on how to avoid geohazard incidents in future wells. 1. Identification of Geohazards Shallow Gas has traditionally been predicted and identified by the use of High Resolution (HR) 2D seismic data. Identification of features like reflection amplitude anomalies, phase-reversal, gas chimneys etc. have all been relatively easy and quick to identify and map. However during recent years Statoil has experienced a number of cases where free shallow gas has caused serious incidents during drilling without leaving any seismic features to identify. Similarly a large number of very clear gas indicators have proven not to be gas filled at all. 2. Statistics Going through the records of wells drilled in the period 1986-2009 showed that results have been more, or less, the same over the years. The following categories were compared: 1. Gas Predicted but not shown during drilling 2. Gas not predicted but shown during drilling 3. Gas predicted and shown during drilling 4. Gas not predicted and not shown during drilling Problems with such statistics are demonstrated by the complete lack of any wells in category (3). The reason for this is that when gas is predicted, the well is very often moved and, even if it isn't, the countermeasures are such that it makes the identification of possible gas more or less impossible. 3. Mass-Prediction In addition, the presentation deals with the problem that the geophysical data acquired during a site survey very often are not linked to geology. Many of the produced maps are created automatically by seismic interpretation software and are not always thought through properly. A better understanding of the local geology is the key to the identification of potential shallow gas traps. The industry needs to allow time to carry out a proper shallow geological interpretation if incidents are to be avoided in the future.