A geohazard investigation in deep water presents a challenge to all existing investigative techniques This paper reports on the work performed over the last 4 years in developing wireline logging systems which add significantly to the quality and applicability of sample and seismic data gathered during such investigations The systems provide the enhanced geological and engineering detail required for such investigations, and are of great value in putting intermittent sampling into a full borehole context. They are capable of deployment m any deepwater drilling situation A number of innovative techniques and designs are required to cover both the practicality of deployment and the technical requirements for the systems to maximize the effectiveness of the wireline data. This paper shows how good vertical resolution can be achieved with geotechnical wireline logs and how significant this could be in assessing geohazards in deep water by providing geological information, input into seismic models and engineering data.
The assessment of the Geohazard risks in the deep-water environment presents a number of problems
The lack of knowledge of the deep-water depositional environment - Deep water deposits are rarely exposed at the earth's surface for geological inspection and, if they are, they have often gone through considerable alteration. The deep-water environment is thus very much a new frontier both to geological and engineering understandings.
Cost of assessment-Given our lack of knowledge and experience, the simple answer would be to gather as much data as possible before exploiting any reserve potential. Deep water is, of course, an expensive place to research and a considerable technical challenge. We must be Innovative and prepared to learn from other fields of geoscience to obtain cost effective investigations of a high enough standard.
Potential geohazards may be represented by just a single thin anomalous horizon-To add to the first two problems we may be searching for a needle in a haystack-the likely hazards such as slope failures and shallow flows may be the result of single thin anomalous horizons. They will occur at relatively shallow depths below mud-line in deposits well away from any potential reservoir evaluation, in deposits least like a potential reservoir and in the sector of any exploration or proving well least investigated. The area of geotechnical concern, in effect has to be ‘shut out’ from investigation to prevent any risk to the deeper well.
As we will normally have no useable wireline data to assist seismic evaluation the potential hazards also occur in the zones with least control from the seismic evaluation.
So what Informahon do we need?
We need detailed geological information to understand the depositional environment and determine whether conditions are susceptible to any of our deep water geohazards
We need good wireline log data to offer control to our shallow seismic data to fully model and evaluate structure and specify the limits of the potential geohazard by getting the best from that seismic data.
We need detailed engineering measurements to confirm that the geohazard is present and most importantly the limiting factors of that risk.