Recent hydrate assessments from the Ocean Drilling Programme (ODP) and the Mallik Test site have advanced the techniques of hydrate detection and evaluation. Transfer, and some modification, of these techniques to the routine evaluation of hydrates as a deepwater geohazard is now needed. Early detection of a hydrate geohazard can be achieved by the integration of seismic, wireline and sampling data before full evaluation proceeds. Proxy identifiers of hydrates are established and seismic and wireline data, particularly where used in tandem, are proving effective at locating hydrate risk. Recent infra-red spectral core logging has shown the potential for identifying a hydrate signature in non-pressurized cores. No single proxy method of measurement is totally effective at identification and none, as yet, appears reliable at quantifying the absolute values of hydrate concentration and critical porosity. At our present depth of knowledge, a pressurized core recovery programme is required to obtain this objective data.
Indications of hydrates have now been found worldwide in practically all deepwater with suitable temperature and pressure regimes. Production from hydrates has been achieved below permafrost caps in Siberia and at the test site at Mallik in the McKenzie Delta. Dealing with a potential geohazard starts with the decision of whether to mitigate or avoid the hazard. With the growing evidence for the widespread occurrence of hydrates, avoidance may not always be an option. Locating and evaluating a hydrate hazard will continue to depend initially on proxy methods of measurement and identification. The challenge now is to calibrate and integrate these proxy measurements with absolute values. The shallow gas risk of hydrates is present at all concentrations but the other major risk caused by the development of overpressures occurs when the hydrate dissociation increases the effective porosity of the formation above the critical porosity. The critical porosity is the level above which the pore fluid becomes load bearing. Porosity, ?, and the fraction of the pore space occupied by hydrate:- the hydrate concentration, ?, are thus the key measurements for assessing the hydrate hazard.
The primary proxy indicator of hydrates is the identification and recognition of a Bottom Simulating Reflector (BSR). Potentially a BSR indicates the interface of the gas hydrate stability zone (GHSZ) with the free gas zone beneath. For assessment of the hydrate hazard the BSR has four drawbacks. Firstly, the hydrate BSR must be separated from other potential BSRs. Secondly, a number of observations have confirmed that hydrates can be present without a BSR. Thirdly, the BSR is just the bottom limit of the GHSZ so the vertical extent of the hazard can only be inferred by estimating temperature/pressure conditions and from theoretical hydrate stability envelopes. Lastly, a travel time seismic survey provides no absolute values that can be used to approximate the hydrate content of the sediment. It has been observed that BSRs appear to be sharper in some areas than others but this need not reflect varying concentrations of hydrate.
